DATAMONGO-1549 - Add $count aggregation stage.

We now support the $count stage in aggregation pipelines.

newAggregation(
	match(where("hotelCode").is("0360")),
	count().as("documents"));

Original Pull Request: #422

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

@@ -23,6 +23,7 @@ import java.util.List;
 
 import org.springframework.data.domain.Sort;
 import org.springframework.data.domain.Sort.Direction;
+import org.springframework.data.mongodb.core.aggregation.CountOperation.CountOperationBuilder;
 import org.springframework.data.mongodb.core.aggregation.ExposedFields.DirectFieldReference;
 import org.springframework.data.mongodb.core.aggregation.ExposedFields.ExposedField;
 import org.springframework.data.mongodb.core.aggregation.ExposedFields.FieldReference;
@@ -396,6 +397,16 @@ public class Aggregation {
 		return new LookupOperation(from, localField, foreignField, as);
 	}
 
+	/**
+	 * Creates a new {@link CountOperationBuilder}.
+	 *
+	 * @return never {@literal null}.
+	 * @since 1.10
+	 */
+	public static CountOperationBuilder count() {
+		return new CountOperationBuilder();
+	}
+
 	/**
 	 * Creates a new {@link Fields} instance for the given field names.
 	 *

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

@@ -0,0 +1,82 @@
+/*
+ * 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.data.mongodb.core.aggregation.ExposedFields.ExposedField;
+import org.springframework.util.Assert;
+
+import com.mongodb.BasicDBObject;
+import com.mongodb.DBObject;
+
+/**
+ * Encapsulates the aggregation framework {@code $count}-operation.
+ * <p>
+ * We recommend to use the static factory method {@link Aggregation#count()} instead of creating instances of this
+ * class directly.
+ * 
+ * @see https://docs.mongodb.com/manual/reference/operator/aggregation/count/#pipe._S_count
+ * @author Mark Paluch
+ * @since 1.10
+ */
+public class CountOperation implements FieldsExposingAggregationOperation {
+
+	private final String fieldName;
+
+	/**
+	 * Creates a new {@link CountOperation} given the {@link fieldName} field name.
+	 *
+	 * @param asFieldName must not be {@literal null} or empty.
+	 */
+	public CountOperation(String fieldName) {
+
+		Assert.hasText(fieldName, "Field name must not be null or empty!");
+		this.fieldName = fieldName;
+	}
+
+	/* (non-Javadoc)
+	 * @see org.springframework.data.mongodb.core.aggregation.AggregationOperation#toDBObject(org.springframework.data.mongodb.core.aggregation.AggregationOperationContext)
+	 */
+	@Override
+	public DBObject toDBObject(AggregationOperationContext context) {
+		return new BasicDBObject("$count", fieldName);
+	}
+
+	/* (non-Javadoc)
+	 * @see org.springframework.data.mongodb.core.aggregation.FieldsExposingAggregationOperation#getFields()
+	 */
+	@Override
+	public ExposedFields getFields() {
+		return ExposedFields.from(new ExposedField(fieldName, true));
+	}
+
+	/**
+	 * Builder for {@link CountOperation}.
+	 *
+	 * @author Mark Paluch
+	 */
+	public static class CountOperationBuilder {
+
+		/**
+		 * Returns the finally to be applied {@link CountOperation} with the given alias.
+		 *
+		 * @param fieldName must not be {@literal null} or empty.
+		 * @return
+		 */
+		public CountOperation as(String fieldName) {
+			return new CountOperation(fieldName);
+		}
+	}
+}

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

@@ -100,6 +100,7 @@ public class AggregationTests {
 	private static final Version TWO_DOT_FOUR = new Version(2, 4);
 	private static final Version TWO_DOT_SIX = new Version(2, 6);
 	private static final Version THREE_DOT_TWO = new Version(3, 2);
+	private static final Version THREE_DOT_FOUR = new Version(3, 4);
 
 	private static boolean initialized = false;
 
@@ -1298,6 +1299,30 @@ public class AggregationTests {
 		assertThat(result.getMappedResults(), hasSize(2));
 	}
 
+	/**
+	 * @see DATAMONGO-1549
+	 */
+	@Test
+	public void shouldApplyCountCorrectly() {
+
+		assumeTrue(mongoVersion.isGreaterThanOrEqualTo(THREE_DOT_FOUR));
+
+		mongoTemplate.save(new Reservation("0123", "42", 100));
+		mongoTemplate.save(new Reservation("0360", "43", 200));
+		mongoTemplate.save(new Reservation("0360", "44", 300));
+
+		Aggregation agg = newAggregation( //
+				count().as("documents"), //
+				project("documents") //
+						.andExpression("documents * 2").as("twice"));
+		AggregationResults<DBObject> result = mongoTemplate.aggregate(agg, Reservation.class, DBObject.class);
+
+		assertThat(result.getMappedResults(), hasSize(1));
+
+		DBObject dbObject = result.getMappedResults().get(0);
+		assertThat(dbObject, isBsonObject().containing("documents", 3).containing("twice", 6));
+	}
+
 	/**
 	 * @see DATAMONGO-975
 	 */

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

@@ -0,0 +1,65 @@
+/*
+ * 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.hamcrest.CoreMatchers.*;
+import static org.junit.Assert.*;
+
+import org.junit.Test;
+
+import com.mongodb.DBObject;
+import com.mongodb.util.JSON;
+
+/**
+ * Unit tests for {@link CountOperation}.
+ * 
+ * @author Mark Paluch
+ */
+public class CountOperationUnitTests {
+
+	/**
+	 * @see DATAMONGO-1549
+	 */
+	@Test(expected = IllegalArgumentException.class)
+	public void rejectsEmptyFieldName() {
+		new CountOperation("");
+	}
+
+	/**
+	 * @see DATAMONGO-1549
+	 */
+	@Test
+	public void shouldRenderCorrectly() {
+
+		CountOperation countOperation = new CountOperation("field");
+		DBObject dbObject = countOperation.toDBObject(Aggregation.DEFAULT_CONTEXT);
+
+		assertThat(dbObject, is(JSON.parse("{$count : \"field\" }")));
+	}
+
+	/**
+	 * @see DATAMONGO-1549
+	 */
+	@Test
+	public void countExposesFields() {
+
+		CountOperation countOperation = new CountOperation("field");
+
+		assertThat(countOperation.getFields().exposesNoFields(), is(false));
+		assertThat(countOperation.getFields().exposesSingleFieldOnly(), is(true));
+		assertThat(countOperation.getFields().getField("field"), notNullValue());
+	}
+}