DATAMONGO-861 - Add support for $cond and $ifNull operators in aggregation operations.

We now support $cond and $ifNull operators for projection and grouping operations. ConditionalOperator and IfNullOperators are AggregationExpressions that can be applied to transform or generate values during aggregation.

TypedAggregation<InventoryItem> agg = newAggregation(InventoryItem.class,
  project().and("discount")
    .transform(ConditionalOperator.newBuilder().when(Criteria.where("qty").gte(250))
      .then(30)
      .otherwise(20))
    .and(ifNull("description", "Unspecified")).as("description")
);

corresponds to

{ "$project": { "discount": { "$cond": { "if": { "$gte": [ "$qty", 250 ] },
        "then": 30, "else": 20 } },
    "description": { "$ifNull": [ "$description", "Unspecified"] }
  }
}

Original Pull Request: #385

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/Aggregation.java

@@ -372,6 +372,68 @@ public class Aggregation {
 		return new LookupOperation(from, localField, foreignField, as);
 	}
 
+	/**
+	 * Creates a new {@link IfNullOperator} for the given {@code field} and {@code replacement} value.
+	 *
+	 * @param field must not be {@literal null}.
+	 * @param replacement must not be {@literal null}.
+	 * @return never {@literal null}.
+	 * @since 1.10
+	 */
+	public static IfNullOperator ifNull(String field, Object replacement) {
+		return IfNullOperator.newBuilder().ifNull(field).thenReplaceWith(replacement);
+	}
+
+	/**
+	 * Creates a new {@link IfNullOperator} for the given {@link Field} and {@link Field} to obtain a value from.
+	 *
+	 * @param field must not be {@literal null}.
+	 * @param replacement must not be {@literal null}.
+	 * @return never {@literal null}.
+	 * @since 1.10
+	 */
+	public static IfNullOperator ifNull(Field field, Field replacement) {
+		return IfNullOperator.newBuilder().ifNull(field).thenReplaceWith(replacement);
+	}
+
+	/**
+	 * Creates a new {@link IfNullOperator} for the given {@link Field} and {@code replacement} value.
+	 *
+	 * @param field must not be {@literal null}.
+	 * @param replacement must not be {@literal null}.
+	 * @return never {@literal null}.
+	 * @since 1.10
+	 */
+	public static IfNullOperator ifNull(Field field, Object replacement) {
+		return IfNullOperator.newBuilder().ifNull(field).thenReplaceWith(replacement);
+	}
+
+	/**
+	 * Creates a new {@link ConditionalOperator} for the given {@link Field} that holds a {@literal boolean} value.
+	 *
+	 * @param booleanField must not be {@literal null}.
+	 * @param then must not be {@literal null}.
+	 * @param otherwise must not be {@literal null}.
+	 * @return never {@literal null}.
+	 * @since 1.10
+	 */
+	public static ConditionalOperator conditional(Field booleanField, Object then, Object otherwise) {
+		return ConditionalOperator.newBuilder().when(booleanField).then(then).otherwise(otherwise);
+	}
+
+	/**
+	 * Creates a new {@link ConditionalOperator} for the given {@link Criteria}.
+	 *
+	 * @param criteria must not be {@literal null}.
+	 * @param then must not be {@literal null}.
+	 * @param otherwise must not be {@literal null}.
+	 * @return never {@literal null}.
+	 * @since 1.10
+	 */
+	public static ConditionalOperator conditional(Criteria criteria, Object then, Object otherwise) {
+		return ConditionalOperator.newBuilder().when(criteria).then(then).otherwise(otherwise);
+	}
+
 	/**
 	 * Creates a new {@link Fields} instance for the given field names.
 	 *

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/ConditionalOperator.java

@@ -0,0 +1,381 @@
+/*
+ * Copyright 2016 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.mongodb.core.aggregation;
+
+import org.springframework.dao.InvalidDataAccessApiUsageException;
+import org.springframework.data.mongodb.core.query.CriteriaDefinition;
+import org.springframework.util.Assert;
+import org.springframework.util.ClassUtils;
+
+import com.mongodb.BasicDBList;
+import com.mongodb.BasicDBObject;
+import com.mongodb.DBObject;
+
+/**
+ * Encapsulates the aggregation framework {@code $cond} operator. A {@link ConditionalOperator} allows nested conditions
+ * {@code if-then[if-then-else]-else} using {@link Field}, {@link CriteriaDefinition} or a {@link DBObject custom}
+ * condition. Replacement values can be either {@link Field field references}, values of simple MongoDB types or values
+ * that can be converted to a simple MongoDB type.
+ * 
+ * @see http://docs.mongodb.com/manual/reference/operator/aggregation/cond/
+ * @author Mark Paluch
+ * @since 1.10
+ */
+public class ConditionalOperator implements AggregationExpression {
+
+	private final Object condition;
+	private final Object thenValue;
+	private final Object otherwiseValue;
+
+	/**
+	 * Creates a new {@link ConditionalOperator} for a given {@link Field} and {@code then}/{@code otherwise} values.
+	 * 
+	 * @param condition must not be {@literal null}.
+	 * @param thenValue must not be {@literal null}.
+	 * @param otherwiseValue must not be {@literal null}.
+	 */
+	public ConditionalOperator(Field condition, Object thenValue, Object otherwiseValue) {
+		this((Object) condition, thenValue, otherwiseValue);
+	}
+
+	/**
+	 * Creates a new {@link ConditionalOperator} for a given {@link CriteriaDefinition} and {@code then}/{@code otherwise}
+	 * values.
+	 *
+	 * @param condition must not be {@literal null}.
+	 * @param thenValue must not be {@literal null}.
+	 * @param otherwiseValue must not be {@literal null}.
+	 */
+	public ConditionalOperator(CriteriaDefinition condition, Object thenValue, Object otherwiseValue) {
+		this((Object) condition, thenValue, otherwiseValue);
+	}
+
+	/**
+	 * Creates a new {@link ConditionalOperator} for a given {@link DBObject criteria} and {@code then}/{@code otherwise}
+	 * values.
+	 *
+	 * @param condition must not be {@literal null}.
+	 * @param thenValue must not be {@literal null}.
+	 * @param otherwiseValue must not be {@literal null}.
+	 */
+	public ConditionalOperator(DBObject condition, Object thenValue, Object otherwiseValue) {
+		this((Object) condition, thenValue, otherwiseValue);
+	}
+
+	private ConditionalOperator(Object condition, Object thenValue, Object otherwiseValue) {
+
+		Assert.notNull(condition, "Condition must not be null!");
+		Assert.notNull(thenValue, "'Then value' must not be null!");
+		Assert.notNull(otherwiseValue, "'Otherwise value' must not be null!");
+
+		assertNotBuilder(condition, "Condition");
+		assertNotBuilder(thenValue, "'Then value'");
+		assertNotBuilder(otherwiseValue, "'Otherwise value'");
+
+		this.condition = condition;
+		this.thenValue = thenValue;
+		this.otherwiseValue = otherwiseValue;
+	}
+
+	/* 
+	 * (non-Javadoc)
+	 * @see org.springframework.data.mongodb.core.aggregation.AggregationExpression#toDbObject(org.springframework.data.mongodb.core.aggregation.AggregationOperationContext)
+	 */
+	@Override
+	public DBObject toDbObject(AggregationOperationContext context) {
+
+		BasicDBObject condObject = new BasicDBObject();
+
+		condObject.append("if", resolveCriteria(context, condition));
+		condObject.append("then", resolveValue(context, thenValue));
+		condObject.append("else", resolveValue(context, otherwiseValue));
+
+		return new BasicDBObject("$cond", condObject);
+	}
+
+	private Object resolveValue(AggregationOperationContext context, Object value) {
+
+		if (value instanceof DBObject || value instanceof Field) {
+			return resolve(context, value);
+		}
+
+		if (value instanceof ConditionalOperator) {
+			return ((ConditionalOperator) value).toDbObject(context);
+		}
+
+		DBObject toMap = context.getMappedObject(new BasicDBObject("$set", value));
+		return toMap.get("$set");
+	}
+
+	private Object resolveCriteria(AggregationOperationContext context, Object value) {
+
+		if (value instanceof DBObject || value instanceof Field) {
+			return resolve(context, value);
+		}
+
+		if (value instanceof CriteriaDefinition) {
+
+			DBObject mappedObject = context.getMappedObject(((CriteriaDefinition) value).getCriteriaObject());
+			BasicDBList clauses = new BasicDBList();
+
+			clauses.addAll(getClauses(context, mappedObject));
+
+			if (clauses.size() == 1) {
+				return clauses.get(0);
+			}
+
+			return clauses;
+		}
+
+		throw new InvalidDataAccessApiUsageException(
+				String.format("Invalid value in condition. Supported: DBObject, Field references, Criteria, got: %s", value));
+	}
+
+	private BasicDBList getClauses(AggregationOperationContext context, DBObject mappedObject) {
+
+		BasicDBList clauses = new BasicDBList();
+
+		for (String key : mappedObject.keySet()) {
+
+			Object predicate = mappedObject.get(key);
+			clauses.addAll(getClauses(context, key, predicate));
+		}
+
+		return clauses;
+	}
+
+	private BasicDBList getClauses(AggregationOperationContext context, String key, Object predicate) {
+
+		BasicDBList clauses = new BasicDBList();
+
+		if (predicate instanceof BasicDBList) {
+
+			BasicDBList args = new BasicDBList();
+			for (Object clause : (BasicDBList) predicate) {
+				args.addAll(getClauses(context, (BasicDBObject) clause));
+			}
+
+			clauses.add(new BasicDBObject(key, args));
+
+		} else if (predicate instanceof DBObject) {
+
+			DBObject nested = (DBObject) predicate;
+
+			for (String s : nested.keySet()) {
+
+				if (!isKeyword(s)) {
+					continue;
+				}
+
+				BasicDBList args = new BasicDBList();
+				args.add("$" + key);
+				args.add(nested.get(s));
+				clauses.add(new BasicDBObject(s, args));
+			}
+
+		} else if (!isKeyword(key)) {
+
+			BasicDBList args = new BasicDBList();
+			args.add("$" + key);
+			args.add(predicate);
+			clauses.add(new BasicDBObject("$eq", args));
+		}
+
+		return clauses;
+	}
+
+	/**
+	 * Returns whether the given {@link String} is a MongoDB keyword.
+	 *
+	 * @param candidate
+	 * @return
+	 */
+	private boolean isKeyword(String candidate) {
+		return candidate.startsWith("$");
+	}
+
+	private Object resolve(AggregationOperationContext context, Object value) {
+
+		if (value instanceof DBObject) {
+			return context.getMappedObject((DBObject) value);
+		}
+
+		return context.getReference((Field) value).toString();
+	}
+
+	private void assertNotBuilder(Object toCheck, String name) {
+		Assert.isTrue(!ClassUtils.isAssignableValue(ConditionalExpressionBuilder.class, toCheck),
+				String.format("%s must not be of type %s", name, ConditionalExpressionBuilder.class.getSimpleName()));
+	}
+
+	/**
+	 * Get a builder that allows fluent creation of {@link ConditionalOperator}.
+	 *
+	 * @return a new {@link ConditionalExpressionBuilder}.
+	 */
+	public static ConditionalExpressionBuilder newBuilder() {
+		return ConditionalExpressionBuilder.newBuilder();
+	}
+
+	public static interface WhenBuilder {
+
+		/**
+		 * @param booleanExpression expression that yields in a boolean result, must not be {@literal null}.
+		 * @return the {@link ThenBuilder}
+		 */
+		ThenBuilder when(DBObject booleanExpression);
+
+		/**
+		 * @param booleanField reference to a field holding a boolean value, must not be {@literal null}.
+		 * @return the {@link ThenBuilder}
+		 */
+		ThenBuilder when(Field booleanField);
+
+		/**
+		 * @param booleanField name of a field holding a boolean value, must not be {@literal null}.
+		 * @return the {@link ThenBuilder}
+		 */
+		ThenBuilder when(String booleanField);
+
+		/**
+		 * @param criteria criteria to evaluate, must not be {@literal null}.
+		 * @return the {@link ThenBuilder}
+		 */
+		ThenBuilder when(CriteriaDefinition criteria);
+	}
+
+	public static interface ThenBuilder {
+
+		/**
+		 * @param value the value to be used if the condition evaluates {@literal true}. Can be a {@link DBObject}, a value
+		 *          that is supported by MongoDB or a value that can be converted to a MongoDB representation but must not
+		 *          be {@literal null}.
+		 * @return the {@link OtherwiseBuilder}
+		 */
+		OtherwiseBuilder then(Object value);
+	}
+
+	public static interface OtherwiseBuilder {
+
+		/**
+		 * @param value the value to be used if the condition evaluates {@literal false}. Can be a {@link DBObject}, a value
+		 *          that is supported by MongoDB or a value that can be converted to a MongoDB representation but must not
+		 *          be {@literal null}.
+		 * @return the {@link ConditionalOperator}
+		 */
+		ConditionalOperator otherwise(Object value);
+	}
+
+	/**
+	 * Builder for fluent {@link ConditionalOperator} creation.
+	 *
+	 * @author Mark Paluch
+	 * @since 1.10
+	 */
+	public static final class ConditionalExpressionBuilder implements WhenBuilder, ThenBuilder, OtherwiseBuilder {
+
+		private Object condition;
+		private Object thenValue;
+
+		private ConditionalExpressionBuilder() {}
+
+		/**
+		 * Creates a new builder for {@link ConditionalOperator}.
+		 *
+		 * @return never {@literal null}.
+		 */
+		public static ConditionalExpressionBuilder newBuilder() {
+			return new ConditionalExpressionBuilder();
+		}
+
+		/* 
+		 * (non-Javadoc)
+		 * @see org.springframework.data.mongodb.core.aggregation.ConditionalOperator.WhenBuilder#when(com.mongodb.DBObject)
+		 */
+		@Override
+		public ConditionalExpressionBuilder when(DBObject booleanExpression) {
+
+			Assert.notNull(booleanExpression, "'Boolean expression' must not be null!");
+
+			this.condition = booleanExpression;
+			return this;
+		}
+
+		/* 
+		 * (non-Javadoc)
+		 * @see org.springframework.data.mongodb.core.aggregation.ConditionalOperator.WhenBuilder#when(org.springframework.data.mongodb.core.query.CriteriaDefinition)
+		 */
+		@Override
+		public ThenBuilder when(CriteriaDefinition criteria) {
+
+			Assert.notNull(criteria, "Criteria must not be null!");
+
+			this.condition = criteria;
+			return this;
+		}
+
+		/* 
+		 * (non-Javadoc)
+		 * @see org.springframework.data.mongodb.core.aggregation.ConditionalOperator.WhenBuilder#when(org.springframework.data.mongodb.core.aggregation.Field)
+		 */
+		@Override
+		public ThenBuilder when(Field booleanField) {
+
+			Assert.notNull(booleanField, "Boolean field must not be null!");
+
+			this.condition = booleanField;
+			return this;
+		}
+
+		/* 
+		 * (non-Javadoc)
+		 * @see org.springframework.data.mongodb.core.aggregation.ConditionalOperator.WhenBuilder#when(java.lang.String)
+		 */
+		@Override
+		public ThenBuilder when(String booleanField) {
+
+			Assert.hasText(booleanField, "Boolean field name must not be null or empty!");
+
+			this.condition = Fields.field(booleanField);
+			return this;
+		}
+
+		/* 
+		 * (non-Javadoc)
+		 * @see org.springframework.data.mongodb.core.aggregation.ConditionalOperator.ThenBuilder#then(java.lang.Object)
+		 */
+		@Override
+		public OtherwiseBuilder then(Object thenValue) {
+
+			Assert.notNull(thenValue, "'Then-value' must not be null!");
+
+			this.thenValue = thenValue;
+			return this;
+		}
+
+		/* 
+		 * (non-Javadoc)
+		 * @see org.springframework.data.mongodb.core.aggregation.ConditionalOperator.OtherwiseBuilder#otherwise(java.lang.Object)
+		 */
+		@Override
+		public ConditionalOperator otherwise(Object otherwiseValue) {
+
+			Assert.notNull(otherwiseValue, "'Otherwise-value' must not be null!");
+
+			return new ConditionalOperator(condition, thenValue, otherwiseValue);
+		}
+	}
+}

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/IfNullOperator.java

@@ -0,0 +1,186 @@
+/*
+ * Copyright 2016 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.data.mongodb.core.aggregation;
+
+import org.springframework.util.Assert;
+
+import com.mongodb.BasicDBList;
+import com.mongodb.BasicDBObject;
+import com.mongodb.DBObject;
+
+/**
+ * Encapsulates the aggregation framework {@code $ifNull} operator. Replacement values can be either {@link Field field
+ * references}, values of simple MongoDB types or values that can be converted to a simple MongoDB type.
+ * 
+ * @see http://docs.mongodb.com/manual/reference/operator/aggregation/ifNull/
+ * @author Mark Paluch
+ * @since 1.10
+ */
+public class IfNullOperator implements AggregationExpression {
+
+	private final Field field;
+	private final Object value;
+
+	/**
+	 * Creates a new {@link IfNullOperator} for the given {@link Field} and replacement {@code value}.
+	 * 
+	 * @param field must not be {@literal null}.
+	 * @param value must not be {@literal null}.
+	 */
+	public IfNullOperator(Field field, Object value) {
+
+		Assert.notNull(field, "Field must not be null!");
+		Assert.notNull(value, "'Replacement-value' must not be null!");
+
+		this.field = field;
+		this.value = value;
+	}
+
+	/* 
+	 * (non-Javadoc)
+	 * @see org.springframework.data.mongodb.core.aggregation.AggregationExpression#toDbObject(org.springframework.data.mongodb.core.aggregation.AggregationOperationContext)
+	 */
+	@Override
+	public DBObject toDbObject(AggregationOperationContext context) {
+
+		BasicDBList list = new BasicDBList();
+
+		list.add(context.getReference(field).toString());
+
+		if (value instanceof Field) {
+			list.add(context.getReference((Field) value).toString());
+		} else {
+
+			DBObject toMap = context.getMappedObject(new BasicDBObject("$set", value));
+			list.add(toMap.get("$set"));
+		}
+
+		return new BasicDBObject("$ifNull", list);
+	}
+
+	/**
+	 * Get a builder that allows fluent creation of {@link IfNullOperator}.
+	 *
+	 * @return a new {@link IfNullBuilder}.
+	 */
+	public static IfNullBuilder newBuilder() {
+		return IfNullOperatorBuilder.newBuilder();
+	}
+
+	public static interface IfNullBuilder {
+
+		/**
+		 * @param field the field to check for a {@literal null} value, field reference must not be {@literal null}.
+		 * @return the {@link ThenBuilder}
+		 */
+		ThenBuilder ifNull(Field field);
+
+		/**
+		 * @param field the field to check for a {@literal null} value, field name must not be {@literal null} or empty.
+		 * @return the {@link ThenBuilder}
+		 */
+		ThenBuilder ifNull(String field);
+	}
+
+	public static interface ThenBuilder {
+
+		/**
+		 * @param field the field holding the replacement value, must not be {@literal null}.
+		 * @return the {@link IfNullOperator}
+		 */
+		IfNullOperator thenReplaceWith(Field field);
+
+		/**
+		 * @param value the value to be used if the {@code $ifNull }condition evaluates {@literal true}. Can be a
+		 *          {@link DBObject}, a value that is supported by MongoDB or a value that can be converted to a MongoDB
+		 *          representation but must not be {@literal null}.
+		 * @return the {@link IfNullOperator}
+		 */
+		IfNullOperator thenReplaceWith(Object value);
+	}
+
+	/**
+	 * Builder for fluent {@link IfNullOperator} creation.
+	 *
+	 * @author Mark Paluch
+	 * @since 1.10
+	 */
+	public static final class IfNullOperatorBuilder implements IfNullBuilder, ThenBuilder {
+
+		private Field field;
+
+		private IfNullOperatorBuilder() {}
+
+		/**
+		 * Creates a new builder for {@link IfNullOperator}.
+		 *
+		 * @return never {@literal null}.
+		 */
+		public static IfNullOperatorBuilder newBuilder() {
+			return new IfNullOperatorBuilder();
+		}
+
+		/* 
+		 * (non-Javadoc)
+		 * @see org.springframework.data.mongodb.core.aggregation.IfNullOperator.IfNullBuilder#ifNull(org.springframework.data.mongodb.core.aggregation.Field)
+		 */
+
+		public ThenBuilder ifNull(Field field) {
+
+			Assert.notNull(field, "Field must not be null!");
+
+			this.field = field;
+			return this;
+		}
+
+		/* 
+		 * (non-Javadoc)
+		 * @see org.springframework.data.mongodb.core.aggregation.IfNullOperator.IfNullBuilder#ifNull(java.lang.String)
+		 */
+		public ThenBuilder ifNull(String name) {
+
+			Assert.hasText(name, "Field name must not be null or empty!");
+
+			this.field = Fields.field(name);
+			return this;
+		}
+
+		/* 
+		 * (non-Javadoc)
+		 * @see org.springframework.data.mongodb.core.aggregation.IfNullOperator.ThenReplaceBuilder#thenReplaceWith(org.springframework.data.mongodb.core.aggregation.Field)
+		 */
+		@Override
+		public IfNullOperator thenReplaceWith(Field replacementField) {
+
+			Assert.notNull(replacementField, "Replacement field must not be null!");
+
+			return new IfNullOperator(this.field, replacementField);
+		}
+
+		/* 
+		 * (non-Javadoc)
+		 * @see org.springframework.data.mongodb.core.aggregation.IfNullOperator.ThenReplaceBuilder#thenReplaceWith(java.lang.Object)
+		 */
+		@Override
+		public IfNullOperator thenReplaceWith(Object value) {
+
+			Assert.notNull(value, "'Replacement-value' must not be null!");
+
+			return new IfNullOperator(this.field, value);
+		}
+	}
+}

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/ProjectionOperation.java

@@ -104,8 +104,8 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 	 */
 	private ProjectionOperation andReplaceLastOneWith(Projection projection) {
 
-		List<Projection> projections = this.projections.isEmpty() ? Collections.<Projection> emptyList() : this.projections
-				.subList(0, this.projections.size() - 1);
+		List<Projection> projections = this.projections.isEmpty() ? Collections.<Projection> emptyList()
+				: this.projections.subList(0, this.projections.size() - 1);
 		return new ProjectionOperation(projections, Arrays.asList(projection));
 	}
 
@@ -240,6 +240,24 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 		 * @return
 		 */
 		public abstract ProjectionOperation as(String alias);
+
+		/**
+		 * Apply a conditional projection using {@link ConditionalOperator}.
+		 *
+		 * @param conditional must not be {@literal null}.
+		 * @return never {@literal null}.
+		 * @since 1.10
+		 */
+		public abstract ProjectionOperation transform(ConditionalOperator conditional);
+
+		/**
+		 * Apply a conditional value replacement for {@literal null} values using {@link IfNullOperator}.
+		 *
+		 * @param ifNull must not be {@literal null}.
+		 * @return never {@literal null}.
+		 * @since 1.10
+		 */
+		public abstract ProjectionOperation transform(IfNullOperator ifNull);
 	}
 
 	/**
@@ -370,7 +388,8 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 		 * @param operation must not be {@literal null}.
 		 * @param previousProjection the previous operation projection, may be {@literal null}.
 		 */
-		public ProjectionOperationBuilder(String name, ProjectionOperation operation, OperationProjection previousProjection) {
+		public ProjectionOperationBuilder(String name, ProjectionOperation operation,
+				OperationProjection previousProjection) {
 			super(name, operation);
 
 			this.name = name;
@@ -419,7 +438,7 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 		/**
 		 * Allows to specify an alias for the previous projection operation.
 		 * 
-		 * @param string
+		 * @param alias
 		 * @return
 		 */
 		@Override
@@ -436,6 +455,24 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 			return this.operation.and(new FieldProjection(Fields.field(alias, name), null));
 		}
 
+		/* 
+		 * (non-Javadoc)
+		 * @see org.springframework.data.mongodb.core.aggregation.ProjectionOperation.AbstractProjectionOperationBuilder#transform(org.springframework.data.mongodb.core.aggregation.ConditionalOperator)
+		 */
+		@Override
+		public ProjectionOperation transform(ConditionalOperator conditional) {
+			return this.operation.and(new ExpressionProjection(Fields.field(name), conditional));
+		}
+
+		/* 
+		 * (non-Javadoc)
+		 * @see org.springframework.data.mongodb.core.aggregation.ProjectionOperation.AbstractProjectionOperationBuilder#transform(org.springframework.data.mongodb.core.aggregation.IfNullOperator)
+		 */
+		@Override
+		public ProjectionOperation transform(IfNullOperator ifNull) {
+			return this.operation.and(new ExpressionProjection(Fields.field(name), ifNull));
+		}
+
 		/**
 		 * Generates an {@code $add} expression that adds the given number to the previously mentioned field.
 		 * 
@@ -764,7 +801,7 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 				this.values = Arrays.asList(values);
 			}
 
-			/* 
+			/*
 			 * (non-Javadoc)
 			 * @see org.springframework.data.mongodb.core.aggregation.ProjectionOperation.Projection#toDBObject(org.springframework.data.mongodb.core.aggregation.AggregationOperationContext)
 			 */

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/AggregationTests.java

@@ -20,6 +20,7 @@ import static org.junit.Assert.*;
 import static org.junit.Assume.*;
 import static org.springframework.data.domain.Sort.Direction.*;
 import static org.springframework.data.mongodb.core.aggregation.Aggregation.*;
+import static org.springframework.data.mongodb.core.aggregation.Fields.*;
 import static org.springframework.data.mongodb.core.query.Criteria.*;
 import static org.springframework.data.mongodb.test.util.IsBsonObject.*;
 
@@ -55,6 +56,7 @@ import org.springframework.data.mongodb.core.MongoTemplate;
 import org.springframework.data.mongodb.core.Venue;
 import org.springframework.data.mongodb.core.aggregation.AggregationTests.CarDescriptor.Entry;
 import org.springframework.data.mongodb.core.index.GeospatialIndex;
+import org.springframework.data.mongodb.core.mapping.Document;
 import org.springframework.data.mongodb.core.query.Criteria;
 import org.springframework.data.mongodb.core.query.NearQuery;
 import org.springframework.data.mongodb.core.query.Query;
@@ -131,6 +133,8 @@ public class AggregationTests {
 		mongoTemplate.dropCollection(Reservation.class);
 		mongoTemplate.dropCollection(Venue.class);
 		mongoTemplate.dropCollection(MeterData.class);
+		mongoTemplate.dropCollection(LineItem.class);
+		mongoTemplate.dropCollection(InventoryItem.class);
 	}
 
 	/**
@@ -484,6 +488,250 @@ public class AggregationTests {
 		assertThat(stateStats.totalPopulation, is(29760021));
 	}
 
+	/**
+	 * @see DATAMONGO-861
+	 * @see https://docs.mongodb.com/manual/reference/operator/aggregation/cond/#example
+	 */
+	@Test
+	public void aggregationUsingConditionalProjectionToCalculateDiscount() {
+
+		/*
+		db.inventory.aggregate(
+		[
+		  {
+		     $project:
+		       {
+		         item: 1,
+		         discount:
+		           {
+		             $cond: { if: { $gte: [ "$qty", 250 ] }, then: 30, else: 20 }
+		           }
+		       }
+		  }
+		]
+		)
+		 */
+
+		mongoTemplate.insert(new InventoryItem(1, "abc1", 300));
+		mongoTemplate.insert(new InventoryItem(2, "abc2", 200));
+		mongoTemplate.insert(new InventoryItem(3, "xyz1", 250));
+
+		TypedAggregation<InventoryItem> aggregation = newAggregation(InventoryItem.class, //
+				project("item") //
+						.and("discount")//
+						.transform(ConditionalOperator.newBuilder().when(Criteria.where("qty").gte(250)) //
+								.then(30) //
+								.otherwise(20)));
+
+		assertThat(aggregation.toString(), is(notNullValue()));
+
+		AggregationResults<DBObject> result = mongoTemplate.aggregate(aggregation, DBObject.class);
+		assertThat(result.getMappedResults().size(), is(3));
+
+		DBObject first = result.getMappedResults().get(0);
+		assertThat(first.get("_id"), is((Object) 1));
+		assertThat(first.get("discount"), is((Object) 30));
+
+		DBObject second = result.getMappedResults().get(1);
+		assertThat(second.get("_id"), is((Object) 2));
+		assertThat(second.get("discount"), is((Object) 20));
+
+		DBObject third = result.getMappedResults().get(2);
+		assertThat(third.get("_id"), is((Object) 3));
+		assertThat(third.get("discount"), is((Object) 30));
+	}
+
+	/**
+	 * @see DATAMONGO-861
+	 * @see https://docs.mongodb.com/manual/reference/operator/aggregation/ifNull/#example
+	 */
+	@Test
+	public void aggregationUsingIfNullToProjectSaneDefaults() {
+
+		/*
+		db.inventory.aggregate(
+		[
+		  {
+		     $project: {
+		        item: 1,
+		        description: { $ifNull: [ "$description", "Unspecified" ] }
+		     }
+		  }
+		]
+		)
+		 */
+
+		mongoTemplate.insert(new InventoryItem(1, "abc1", "product 1", 300));
+		mongoTemplate.insert(new InventoryItem(2, "abc2", 200));
+		mongoTemplate.insert(new InventoryItem(3, "xyz1", 250));
+
+		TypedAggregation<InventoryItem> aggregation = newAggregation(InventoryItem.class, //
+				project("item") //
+						.and(ifNull("description", "Unspecified")) //
+						.as("description")//
+		);
+
+		assertThat(aggregation.toString(), is(notNullValue()));
+
+		AggregationResults<DBObject> result = mongoTemplate.aggregate(aggregation, DBObject.class);
+		assertThat(result.getMappedResults().size(), is(3));
+
+		DBObject first = result.getMappedResults().get(0);
+		assertThat(first.get("_id"), is((Object) 1));
+		assertThat(first.get("description"), is((Object) "product 1"));
+
+		DBObject second = result.getMappedResults().get(1);
+		assertThat(second.get("_id"), is((Object) 2));
+		assertThat(second.get("description"), is((Object) "Unspecified"));
+	}
+
+	/**
+	 * @see DATAMONGO-861
+	 */
+	@Test
+	public void aggregationUsingConditionalProjection() {
+
+		TypedAggregation<ZipInfo> aggregation = newAggregation(ZipInfo.class, //
+				project() //
+						.and("largePopulation")//
+						.transform(ConditionalOperator.newBuilder().when(Criteria.where("population").gte(20000)) //
+								.then(true) //
+								.otherwise(false)) //
+						.and("population").as("population"));
+
+		assertThat(aggregation, is(notNullValue()));
+		assertThat(aggregation.toString(), is(notNullValue()));
+
+		AggregationResults<DBObject> result = mongoTemplate.aggregate(aggregation, DBObject.class);
+		assertThat(result.getMappedResults().size(), is(29467));
+
+		DBObject firstZipInfoStats = result.getMappedResults().get(0);
+		assertThat(firstZipInfoStats.get("largePopulation"), is((Object) false));
+		assertThat(firstZipInfoStats.get("population"), is((Object) 6055));
+	}
+
+	/**
+	 * @see DATAMONGO-861
+	 */
+	@Test
+	public void aggregationUsingNestedConditionalProjection() {
+
+		TypedAggregation<ZipInfo> aggregation = newAggregation(ZipInfo.class, //
+				project() //
+						.and("size")//
+						.transform(ConditionalOperator.newBuilder().when(Criteria.where("population").gte(20000)) //
+								.then(ConditionalOperator.newBuilder().when(Criteria.where("population").gte(200000)).then("huge")
+										.otherwise("small")) //
+								.otherwise("small")) //
+						.and("population").as("population"));
+
+		assertThat(aggregation, is(notNullValue()));
+		assertThat(aggregation.toString(), is(notNullValue()));
+
+		AggregationResults<DBObject> result = mongoTemplate.aggregate(aggregation, DBObject.class);
+		assertThat(result.getMappedResults().size(), is(29467));
+
+		DBObject firstZipInfoStats = result.getMappedResults().get(0);
+		assertThat(firstZipInfoStats.get("size"), is((Object) "small"));
+		assertThat(firstZipInfoStats.get("population"), is((Object) 6055));
+	}
+
+	/**
+	 * @see DATAMONGO-861
+	 */
+	@Test
+	public void aggregationUsingIfNullProjection() {
+
+		mongoTemplate.insert(new LineItem("id", "caption", 0));
+		mongoTemplate.insert(new LineItem("idonly", null, 0));
+
+		TypedAggregation<LineItem> aggregation = newAggregation(LineItem.class, //
+				project("id") //
+						.and("caption")//
+						.transform(ifNull(field("caption"), "unknown")),
+				sort(ASC, "id"));
+
+		assertThat(aggregation.toString(), is(notNullValue()));
+
+		AggregationResults<DBObject> result = mongoTemplate.aggregate(aggregation, DBObject.class);
+		assertThat(result.getMappedResults().size(), is(2));
+
+		DBObject id = result.getMappedResults().get(0);
+		assertThat((String) id.get("caption"), is(equalTo("caption")));
+
+		DBObject idonly = result.getMappedResults().get(1);
+		assertThat((String) idonly.get("caption"), is(equalTo("unknown")));
+	}
+
+	/**
+	 * @see DATAMONGO-861
+	 */
+	@Test
+	public void aggregationUsingIfNullReplaceWithFieldReferenceProjection() {
+
+		mongoTemplate.insert(new LineItem("id", "caption", 0));
+		mongoTemplate.insert(new LineItem("idonly", null, 0));
+
+		TypedAggregation<LineItem> aggregation = newAggregation(LineItem.class, //
+				project("id") //
+						.and("caption")//
+						.transform(ifNull(field("caption"), field("id"))),
+				sort(ASC, "id"));
+
+		assertThat(aggregation.toString(), is(notNullValue()));
+
+		AggregationResults<DBObject> result = mongoTemplate.aggregate(aggregation, DBObject.class);
+		assertThat(result.getMappedResults().size(), is(2));
+
+		DBObject id = result.getMappedResults().get(0);
+		assertThat((String) id.get("caption"), is(equalTo("caption")));
+
+		DBObject idonly = result.getMappedResults().get(1);
+		assertThat((String) idonly.get("caption"), is(equalTo("idonly")));
+	}
+
+	/**
+	 * @see DATAMONGO-861
+	 */
+	@Test
+	public void shouldAllowGroupingUsingConditionalExpressions() {
+
+		mongoTemplate.dropCollection(CarPerson.class);
+
+		CarPerson person1 = new CarPerson("first1", "last1", new CarDescriptor.Entry("MAKE1", "MODEL1", 2000),
+				new CarDescriptor.Entry("MAKE1", "MODEL2", 2001));
+
+		CarPerson person2 = new CarPerson("first2", "last2", new CarDescriptor.Entry("MAKE3", "MODEL4", 2014));
+		CarPerson person3 = new CarPerson("first3", "last3", new CarDescriptor.Entry("MAKE2", "MODEL5", 2015));
+
+		mongoTemplate.save(person1);
+		mongoTemplate.save(person2);
+		mongoTemplate.save(person3);
+
+		TypedAggregation<CarPerson> agg = Aggregation.newAggregation(CarPerson.class,
+				unwind("descriptors.carDescriptor.entries"), //
+				project() //
+						.and(new ConditionalOperator(Criteria.where("descriptors.carDescriptor.entries.make").is("MAKE1"), "good",
+								"meh"))
+						.as("make") //
+						.and("descriptors.carDescriptor.entries.model").as("model") //
+						.and("descriptors.carDescriptor.entries.year").as("year"), //
+				group("make").avg(new ConditionalOperator(Criteria.where("year").gte(2012), 1, 9000)).as("score"),
+				sort(ASC, "make"));
+
+		AggregationResults<DBObject> result = mongoTemplate.aggregate(agg, DBObject.class);
+
+		assertThat(result.getMappedResults(), hasSize(2));
+
+		DBObject meh = result.getMappedResults().get(0);
+		assertThat((String) meh.get("_id"), is(equalTo("meh")));
+		assertThat(((Number) meh.get("score")).longValue(), is(equalTo(1L)));
+
+		DBObject good = result.getMappedResults().get(1);
+		assertThat((String) good.get("_id"), is(equalTo("good")));
+		assertThat(((Number) good.get("score")).longValue(), is(equalTo(9000L)));
+	}
+
 	/**
 	 * @see http://docs.mongodb.org/manual/tutorial/aggregation-examples/#return-the-five-most-common-likes
 	 */
@@ -1460,4 +1708,31 @@ public class AggregationTests {
 			this.dateValue = dateValue;
 		}
 	}
+
+	/**
+	 * @see DATAMONGO-861
+	 */
+	@Document(collection = "inventory")
+	static class InventoryItem {
+
+		int id;
+		String item;
+		String description;
+		int qty;
+
+		public InventoryItem() {}
+
+		public InventoryItem(int id, String item, int qty) {
+			this.id = id;
+			this.item = item;
+			this.qty = qty;
+		}
+
+		public InventoryItem(int id, String item, String description, int qty) {
+			this.id = id;
+			this.item = item;
+			this.description = description;
+			this.qty = qty;
+		}
+	}
 }

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/AggregationUnitTests.java

@@ -19,6 +19,7 @@ import static org.hamcrest.CoreMatchers.*;
 import static org.junit.Assert.*;
 import static org.springframework.data.mongodb.core.DBObjectTestUtils.*;
 import static org.springframework.data.mongodb.core.aggregation.Aggregation.*;
+import static org.springframework.data.mongodb.core.aggregation.Fields.*;
 import static org.springframework.data.mongodb.core.query.Criteria.*;
 import static org.springframework.data.mongodb.test.util.IsBsonObject.*;
 
@@ -29,7 +30,10 @@ import org.junit.Rule;
 import org.junit.Test;
 import org.junit.rules.ExpectedException;
 import org.springframework.data.domain.Sort.Direction;
+import org.springframework.data.mongodb.core.query.Criteria;
+import org.springframework.data.mongodb.test.util.BasicDbListBuilder;
 
+import com.mongodb.BasicDBList;
 import com.mongodb.BasicDBObject;
 import com.mongodb.BasicDBObjectBuilder;
 import com.mongodb.DBObject;
@@ -243,6 +247,25 @@ public class AggregationUnitTests {
 		assertThat(fields.get("foosum"), is((Object) new BasicDBObject("$sum", "$foo")));
 	}
 
+	/**
+	 * @see DATAMONGO-861
+	 */
+	@Test
+	public void conditionExpressionBasedFieldsShouldBeReferencableInFollowingOperations() {
+
+		DBObject agg = newAggregation( //
+				project("a"), //
+				group("a").first(conditional(Criteria.where("a").gte(42), "answer", "no-answer")).as("foosum") //
+		).toDbObject("foo", Aggregation.DEFAULT_CONTEXT);
+
+		@SuppressWarnings("unchecked")
+		DBObject secondProjection = ((List<DBObject>) agg.get("pipeline")).get(1);
+		DBObject fields = getAsDBObject(secondProjection, "$group");
+		assertThat(getAsDBObject(fields, "foosum"), isBsonObject().containing("$first"));
+		assertThat(getAsDBObject(fields, "foosum"), isBsonObject().containing("$first.$cond.then", "answer"));
+		assertThat(getAsDBObject(fields, "foosum"), isBsonObject().containing("$first.$cond.else", "no-answer"));
+	}
+
 	/**
 	 * @see DATAMONGO-908
 	 */
@@ -379,6 +402,160 @@ public class AggregationUnitTests {
 		assertThat(getAsDBObject(group, "count"), is(new BasicDBObjectBuilder().add("$sum", "$tags_count").get()));
 	}
 
+	/**
+	 * @see DATAMONGO-861
+	 */
+	@Test
+	public void shouldRenderProjectionConditionalExpressionCorrectly() {
+
+		DBObject agg = Aggregation.newAggregation(//
+				project().and(ConditionalOperator.newBuilder() //
+						.when("isYellow") //
+						.then("bright") //
+						.otherwise("dark")).as("color"))
+				.toDbObject("foo", Aggregation.DEFAULT_CONTEXT);
+
+		DBObject project = extractPipelineElement(agg, 0, "$project");
+		DBObject expectedCondition = new BasicDBObject() //
+				.append("if", "$isYellow") //
+				.append("then", "bright") //
+				.append("else", "dark");
+
+		assertThat(getAsDBObject(project, "color"), isBsonObject().containing("$cond", expectedCondition));
+	}
+
+	/**
+	 * @see DATAMONGO-861
+	 */
+	@Test
+	public void shouldRenderProjectionConditionalCorrectly() {
+
+		DBObject agg = Aggregation.newAggregation(//
+				project().and("color")
+						.transform(ConditionalOperator.newBuilder() //
+								.when("isYellow") //
+								.then("bright") //
+								.otherwise("dark")))
+				.toDbObject("foo", Aggregation.DEFAULT_CONTEXT);
+
+		DBObject project = extractPipelineElement(agg, 0, "$project");
+		DBObject expectedCondition = new BasicDBObject() //
+				.append("if", "$isYellow") //
+				.append("then", "bright") //
+				.append("else", "dark");
+
+		assertThat(getAsDBObject(project, "color"), isBsonObject().containing("$cond", expectedCondition));
+	}
+
+	/**
+	 * @see DATAMONGO-861
+	 */
+	@Test
+	public void shouldRenderProjectionConditionalWithCriteriaCorrectly() {
+
+		DBObject agg = Aggregation
+				.newAggregation(project()//
+						.and("color")//
+						.transform(conditional(Criteria.where("key").gt(5), "bright", "dark"))) //
+				.toDbObject("foo", Aggregation.DEFAULT_CONTEXT);
+
+		DBObject project = extractPipelineElement(agg, 0, "$project");
+		DBObject expectedCondition = new BasicDBObject() //
+				.append("if", new BasicDBObject("$gt", new BasicDbListBuilder().add("$key").add(5).get())) //
+				.append("then", "bright") //
+				.append("else", "dark");
+
+		assertThat(getAsDBObject(project, "color"), isBsonObject().containing("$cond", expectedCondition));
+	}
+
+	/**
+	 * @see DATAMONGO-861
+	 */
+	@Test
+	public void referencingProjectionAliasesShouldRenderProjectionConditionalWithFieldReferenceCorrectly() {
+
+		DBObject agg = Aggregation
+				.newAggregation(//
+						project().and("color").as("chroma"),
+						project().and("luminosity") //
+								.transform(conditional(field("chroma"), "bright", "dark"))) //
+				.toDbObject("foo", Aggregation.DEFAULT_CONTEXT);
+
+		DBObject project = extractPipelineElement(agg, 1, "$project");
+		DBObject expectedCondition = new BasicDBObject() //
+				.append("if", "$chroma") //
+				.append("then", "bright") //
+				.append("else", "dark");
+
+		assertThat(getAsDBObject(project, "luminosity"), isBsonObject().containing("$cond", expectedCondition));
+	}
+
+	/**
+	 * @see DATAMONGO-861
+	 */
+	@Test
+	public void referencingProjectionAliasesShouldRenderProjectionConditionalWithCriteriaReferenceCorrectly() {
+
+		DBObject agg = Aggregation
+				.newAggregation(//
+						project().and("color").as("chroma"),
+						project().and("luminosity") //
+								.transform(conditional(Criteria.where("chroma").is(100), "bright", "dark"))) //
+				.toDbObject("foo", Aggregation.DEFAULT_CONTEXT);
+
+		DBObject project = extractPipelineElement(agg, 1, "$project");
+		DBObject expectedCondition = new BasicDBObject() //
+				.append("if", new BasicDBObject("$eq", new BasicDbListBuilder().add("$chroma").add(100).get())) //
+				.append("then", "bright") //
+				.append("else", "dark");
+
+		assertThat(getAsDBObject(project, "luminosity"), isBsonObject().containing("$cond", expectedCondition));
+	}
+
+	/**
+	 * @see DATAMONGO-861
+	 */
+	@Test
+	public void shouldRenderProjectionIfNullWithFieldReferenceCorrectly() {
+
+		DBObject agg = Aggregation
+				.newAggregation(//
+						project().and("color"), //
+						project().and("luminosity") //
+								.transform(ifNull(field("chroma"), "unknown"))) //
+				.toDbObject("foo", Aggregation.DEFAULT_CONTEXT);
+
+		DBObject project = extractPipelineElement(agg, 1, "$project");
+		BasicDBList expectedCondition = new BasicDbListBuilder() //
+				.add("$chroma") //
+				.add("unknown")//
+				.get();
+
+		assertThat(getAsDBObject(project, "luminosity"), isBsonObject().containing("$ifNull", expectedCondition));
+	}
+
+	/**
+	 * @see DATAMONGO-861
+	 */
+	@Test
+	public void shouldRenderProjectionIfNullWithFallbackFieldReferenceCorrectly() {
+
+		DBObject agg = Aggregation
+				.newAggregation(//
+						project("fallback").and("color").as("chroma"),
+						project().and("luminosity") //
+								.transform(ifNull(field("chroma"), field("fallback")))) //
+				.toDbObject("foo", Aggregation.DEFAULT_CONTEXT);
+
+		DBObject project = extractPipelineElement(agg, 1, "$project");
+		BasicDBList expectedCondition = new BasicDbListBuilder() //
+				.add("$chroma") //
+				.add("$fallback")//
+				.get();
+
+		assertThat(getAsDBObject(project, "luminosity"), isBsonObject().containing("$ifNull", expectedCondition));
+	}
+
 	private DBObject extractPipelineElement(DBObject agg, int index, String operation) {
 
 		List<DBObject> pipeline = (List<DBObject>) agg.get("pipeline");

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/ConditionalOperatorUnitTests.java

@@ -0,0 +1,210 @@
+/*
+ * Copyright 2016 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.mongodb.core.aggregation;
+
+import static org.junit.Assert.*;
+import static org.springframework.data.mongodb.core.aggregation.ConditionalOperator.*;
+import static org.springframework.data.mongodb.test.util.IsBsonObject.*;
+
+import org.junit.Test;
+import org.springframework.data.mongodb.core.query.Criteria;
+import org.springframework.data.mongodb.test.util.BasicDbListBuilder;
+
+import com.mongodb.BasicDBObject;
+import com.mongodb.DBObject;
+
+/**
+ * Unit tests for {@link ConditionalOperator}.
+ *
+ * @author Mark Paluch
+ */
+public class ConditionalOperatorUnitTests {
+
+	/**
+	 * @see DATAMONGO-861
+	 */
+	@Test(expected = IllegalArgumentException.class)
+	public void shouldRejectNullCondition() {
+		new ConditionalOperator((Field) null, "", "");
+	}
+
+	/**
+	 * @see DATAMONGO-861
+	 */
+	@Test(expected = IllegalArgumentException.class)
+	public void shouldRejectThenValue() {
+		new ConditionalOperator(Fields.field("field"), null, "");
+	}
+
+	/**
+	 * @see DATAMONGO-861
+	 */
+	@Test(expected = IllegalArgumentException.class)
+	public void shouldRejectOtherwiseValue() {
+		new ConditionalOperator(Fields.field("field"), "", null);
+	}
+
+	/**
+	 * @see DATAMONGO-861
+	 */
+	@Test(expected = IllegalArgumentException.class)
+	public void builderRejectsEmptyFieldName() {
+		newBuilder().when("");
+	}
+
+	/**
+	 * @see DATAMONGO-861
+	 */
+	@Test(expected = IllegalArgumentException.class)
+	public void builderRejectsNullFieldName() {
+		newBuilder().when((DBObject) null);
+	}
+
+	/**
+	 * @see DATAMONGO-861
+	 */
+	@Test(expected = IllegalArgumentException.class)
+	public void builderRejectsNullCriteriaName() {
+		newBuilder().when((Criteria) null);
+	}
+
+	/**
+	 * @see DATAMONGO-861
+	 */
+	@Test(expected = IllegalArgumentException.class)
+	public void builderRejectsBuilderAsThenValue() {
+		newBuilder().when("isYellow").then(newBuilder().when("field").then("then-value")).otherwise("otherwise");
+	}
+
+	/**
+	 * @see DATAMONGO-861
+	 */
+	@Test
+	public void simpleBuilderShouldRenderCorrectly() {
+
+		ConditionalOperator operator = newBuilder().when("isYellow").then("bright").otherwise("dark");
+		DBObject dbObject = operator.toDbObject(Aggregation.DEFAULT_CONTEXT);
+
+		DBObject expectedCondition = new BasicDBObject() //
+				.append("if", "$isYellow") //
+				.append("then", "bright") //
+				.append("else", "dark");
+
+		assertThat(dbObject, isBsonObject().containing("$cond", expectedCondition));
+	}
+
+	/**
+	 * @see DATAMONGO-861
+	 */
+	@Test
+	public void simpleCriteriaShouldRenderCorrectly() {
+
+		ConditionalOperator operator = newBuilder().when(Criteria.where("luminosity").gte(100)).then("bright")
+				.otherwise("dark");
+		DBObject dbObject = operator.toDbObject(Aggregation.DEFAULT_CONTEXT);
+
+		DBObject expectedCondition = new BasicDBObject() //
+				.append("if", new BasicDBObject("$gte", new BasicDbListBuilder().add("$luminosity").add(100).get())) //
+				.append("then", "bright") //
+				.append("else", "dark");
+
+		assertThat(dbObject, isBsonObject().containing("$cond", expectedCondition));
+	}
+
+	/**
+	 * @see DATAMONGO-861
+	 */
+	@Test
+	public void andCriteriaShouldRenderCorrectly() {
+
+		ConditionalOperator operator = newBuilder() //
+				.when(Criteria.where("luminosity").gte(100) //
+						.andOperator(Criteria.where("hue").is(50), //
+								Criteria.where("saturation").lt(11)))
+				.then("bright").otherwise("dark");
+
+		DBObject dbObject = operator.toDbObject(Aggregation.DEFAULT_CONTEXT);
+
+		BasicDBObject luminosity = new BasicDBObject("$gte", new BasicDbListBuilder().add("$luminosity").add(100).get());
+		BasicDBObject hue = new BasicDBObject("$eq", new BasicDbListBuilder().add("$hue").add(50).get());
+		BasicDBObject saturation = new BasicDBObject("$lt", new BasicDbListBuilder().add("$saturation").add(11).get());
+
+		DBObject expectedCondition = new BasicDBObject() //
+				.append("if",
+						new BasicDbListBuilder().add(luminosity)
+								.add(new BasicDBObject("$and", new BasicDbListBuilder().add(hue).add(saturation).get())).get()) //
+				.append("then", "bright") //
+				.append("else", "dark");
+
+		assertThat(dbObject, isBsonObject().containing("$cond", expectedCondition));
+	}
+
+	/**
+	 * @see DATAMONGO-861
+	 */
+	@Test
+	public void twoArgsCriteriaShouldRenderCorrectly() {
+
+		Criteria criteria = Criteria.where("luminosity").gte(100) //
+				.and("saturation").and("chroma").is(200);
+		ConditionalOperator operator = newBuilder().when(criteria).then("bright").otherwise("dark");
+
+		DBObject dbObject = operator.toDbObject(Aggregation.DEFAULT_CONTEXT);
+
+		BasicDBObject gte = new BasicDBObject("$gte", new BasicDbListBuilder().add("$luminosity").add(100).get());
+		BasicDBObject is = new BasicDBObject("$eq", new BasicDbListBuilder().add("$chroma").add(200).get());
+
+		DBObject expectedCondition = new BasicDBObject() //
+				.append("if", new BasicDbListBuilder().add(gte).add(is).get()) //
+				.append("then", "bright") //
+				.append("else", "dark");
+
+		assertThat(dbObject, isBsonObject().containing("$cond", expectedCondition));
+	}
+
+	/**
+	 * @see DATAMONGO-861
+	 */
+	@Test
+	public void nestedCriteriaShouldRenderCorrectly() {
+
+		ConditionalOperator operator = newBuilder() //
+				.when(Criteria.where("luminosity").gte(100)) //
+				.then(newBuilder() //
+						.when(Criteria.where("luminosity").gte(200)) //
+						.then("verybright") //
+						.otherwise("not-so-bright")) //
+				.otherwise(newBuilder() //
+						.when(Criteria.where("luminosity").lt(50)) //
+						.then("very-dark") //
+						.otherwise("not-so-dark"));
+
+		DBObject dbObject = operator.toDbObject(Aggregation.DEFAULT_CONTEXT);
+
+		DBObject trueCondition = new BasicDBObject() //
+				.append("if", new BasicDBObject("$gte", new BasicDbListBuilder().add("$luminosity").add(200).get())) //
+				.append("then", "verybright") //
+				.append("else", "not-so-bright");
+
+		DBObject falseCondition = new BasicDBObject() //
+				.append("if", new BasicDBObject("$lt", new BasicDbListBuilder().add("$luminosity").add(50).get())) //
+				.append("then", "very-dark") //
+				.append("else", "not-so-dark");
+
+		assertThat(dbObject, isBsonObject().containing("$cond.then.$cond", trueCondition));
+		assertThat(dbObject, isBsonObject().containing("$cond.else.$cond", falseCondition));
+	}
+}

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/IfNullOperatorUnitTests.java

@@ -0,0 +1,90 @@
+/*
+ * Copyright 2016 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.data.mongodb.core.aggregation;
+
+import static org.junit.Assert.*;
+import static org.springframework.data.mongodb.test.util.IsBsonObject.*;
+
+import org.junit.Test;
+import org.springframework.data.mongodb.test.util.BasicDbListBuilder;
+
+import com.mongodb.BasicDBList;
+import com.mongodb.DBObject;
+
+/**
+ * Unit tests for {@link IfNullOperator}.
+ *
+ * @author Mark Paluch
+ */
+public class IfNullOperatorUnitTests {
+
+	/**
+	 * @see DATAMONGO-861
+	 */
+	@Test(expected = IllegalArgumentException.class)
+	public void shouldRejectNullCondition() {
+		new IfNullOperator(null, "");
+	}
+
+	/**
+	 * @see DATAMONGO-861
+	 */
+	@Test(expected = IllegalArgumentException.class)
+	public void shouldRejectThenValue() {
+		new IfNullOperator(Fields.field("aa"), null);
+	}
+
+	/**
+	 * @see DATAMONGO-861
+	 */
+	@Test
+	public void simpleIfNullShouldRenderCorrectly() {
+
+		IfNullOperator operator = IfNullOperator.newBuilder() //
+				.ifNull("optional") //
+				.thenReplaceWith("a more sophisticated value");
+
+		DBObject dbObject = operator.toDbObject(Aggregation.DEFAULT_CONTEXT);
+
+		BasicDBList expected = new BasicDbListBuilder() //
+				.add("$optional") //
+				.add("a more sophisticated value")//
+				.get();
+
+		assertThat(dbObject, isBsonObject().containing("$ifNull", expected));
+	}
+
+	/**
+	 * @see DATAMONGO-861
+	 */
+	@Test
+	public void fieldReplacementIfNullShouldRenderCorrectly() {
+
+		IfNullOperator operator = IfNullOperator.newBuilder() //
+				.ifNull(Fields.field("optional")) //
+				.thenReplaceWith(Fields.field("never-null"));
+
+		DBObject dbObject = operator.toDbObject(Aggregation.DEFAULT_CONTEXT);
+
+		BasicDBList expected = new BasicDbListBuilder() //
+				.add("$optional") //
+				.add("$never-null")//
+				.get();
+
+		assertThat(dbObject, isBsonObject().containing("$ifNull", expected));
+	}
+}

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/TypeBasedAggregationOperationContextUnitTests.java

@@ -18,6 +18,8 @@ package org.springframework.data.mongodb.core.aggregation;
 import static org.hamcrest.CoreMatchers.*;
 import static org.junit.Assert.*;
 import static org.springframework.data.mongodb.core.aggregation.Aggregation.*;
+import static org.springframework.data.mongodb.core.aggregation.Fields.*;
+import static org.springframework.data.mongodb.test.util.IsBsonObject.*;
 
 import java.util.Arrays;
 import java.util.List;
@@ -89,7 +91,7 @@ public class TypeBasedAggregationOperationContextUnitTests {
 
 		AggregationOperationContext context = getContext(Foo.class);
 
-		Field field = Fields.field("bar.name");
+		Field field = field("bar.name");
 
 		assertThat(context.getReference("bar.name"), is(notNullValue()));
 		assertThat(context.getReference(field), is(notNullValue()));
@@ -103,7 +105,7 @@ public class TypeBasedAggregationOperationContextUnitTests {
 	public void aliasesIdFieldCorrectly() {
 
 		AggregationOperationContext context = getContext(Foo.class);
-		assertThat(context.getReference("id"), is(new FieldReference(new ExposedField(Fields.field("id", "_id"), true))));
+		assertThat(context.getReference("id"), is(new FieldReference(new ExposedField(field("id", "_id"), true))));
 	}
 
 	/**
@@ -279,6 +281,58 @@ public class TypeBasedAggregationOperationContextUnitTests {
 		agg.toDbObject("meterData", context);
 	}
 
+	/**
+	 * @see DATAMONGO-861
+	 */
+	@Test
+	public void rendersAggregationConditionalInTypedAggregationContextCorrectly() {
+
+		AggregationOperationContext context = getContext(FooPerson.class);
+		TypedAggregation<FooPerson> agg = newAggregation(FooPerson.class,
+				project("name") //
+						.and("age") //
+						.transform(conditional(Criteria.where("age.value").lt(10), new Age(0), field("age"))) //
+		);
+
+		DBObject dbo = agg.toDbObject("person", context);
+
+		DBObject projection = getPipelineElementFromAggregationAt(dbo, 0);
+		assertThat(projection.containsField("$project"), is(true));
+
+		DBObject project = getValue(projection, "$project");
+		DBObject age = getValue(project, "age");
+
+		assertThat((DBObject) getValue(age, "$cond"), isBsonObject().containing("then.value", 0));
+		assertThat((DBObject) getValue(age, "$cond"), isBsonObject().containing("then._class", Age.class.getName()));
+		assertThat((DBObject) getValue(age, "$cond"), isBsonObject().containing("else", "$age"));
+	}
+
+	/**
+	 * @see DATAMONGO-861
+	 */
+	@Test
+	public void rendersAggregationIfNullInTypedAggregationContextCorrectly() {
+
+		AggregationOperationContext context = getContext(FooPerson.class);
+		TypedAggregation<FooPerson> agg = newAggregation(FooPerson.class,
+				project("name") //
+						.and("age") //
+						.transform(ifNull("age", new Age(0))) //
+		);
+
+		DBObject dbo = agg.toDbObject("person", context);
+
+		DBObject projection = getPipelineElementFromAggregationAt(dbo, 0);
+		assertThat(projection.containsField("$project"), is(true));
+
+		DBObject project = getValue(projection, "$project");
+		DBObject age = getValue(project, "age");
+
+		assertThat(age, isBsonObject().containing("$ifNull.[0]", "$age"));
+		assertThat(age, isBsonObject().containing("$ifNull.[1].value", 0));
+		assertThat(age, isBsonObject().containing("$ifNull.[1]._class", Age.class.getName()));
+	}
+
 	@Document(collection = "person")
 	public static class FooPerson {
 

src/main/asciidoc/new-features.adoc

@@ -4,6 +4,7 @@
 [[new-features.1-10-0]]
 == What's new in Spring Data MongoDB 1.10
 * Support for `$min`, `$max` and `$slice` operators via `Update`.
+* Support for `$cond` and `$ifNull` operators via `Aggregation`.
 
 [[new-features.1-9-0]]
 == What's new in Spring Data MongoDB 1.9

src/main/asciidoc/reference/mongodb.adoc

@@ -1688,6 +1688,9 @@ At the time of this writing we provide support for the following Aggregation Ope
 | Array Aggregation Operators
 | size, slice
 
+| Conditional Aggregation Operators
+| cond, ifNull
+
 |===
 
 Note that the aggregation operations not listed here are currently not supported by Spring Data MongoDB. Comparison aggregation operators are expressed as `Criteria` expressions.
@@ -1985,6 +1988,50 @@ List<DBObject> resultList = result.getMappedResults();
 
 Note that we can also refer to other fields of the document within the SpEL expression.
 
+[[mongo.aggregation.examples.example6]]
+.Aggregation Framework Example 6
+
+This example uses conditional projection. It's derived from the https://docs.mongodb.com/manual/reference/operator/aggregation/cond/[$cond reference documentation].
+
+[source,java]
+----
+public class InventoryItem {
+
+  @Id int id;
+  String item;
+  String description;
+  int qty;
+}
+
+public class InventoryItemProjection {
+
+  @Id int id;
+  String item;
+  String description;
+  int qty;
+  int discount
+}
+----
+
+[source,java]
+----
+import static org.springframework.data.mongodb.core.aggregation.Aggregation.*;
+
+TypedAggregation<InventoryItem> agg = newAggregation(InventoryItem.class,
+  project("item").and("discount")
+    .transform(ConditionalOperator.newBuilder().when(Criteria.where("qty").gte(250))
+      .then(30)
+      .otherwise(20))
+    .and(ifNull("description", "Unspecified")).as("description")
+);
+
+AggregationResults<InventoryItemProjection> result = mongoTemplate.aggregate(agg, "inventory", InventoryItemProjection.class);
+List<InventoryItemProjection> stateStatsList = result.getMappedResults();
+----
+
+* This one-step aggregation uses a projection operation with the `inventory` collection. We project the `discount` field using a conditional transformation for all inventory items that have a `qty` greater or equal to `250`. An second conditional projection is performed for the `description` field. We apply the description `Unspecified` to all items that either do not have a `description` field of items that have a `null` description.
+
+
 [[mongo.custom-converters]]
 == Overriding default mapping with custom converters