DATAMONGO-2200 - Derive fields for aggregation $project stage from a given type.

We now allow to derive field names for a $project stage from a given type by including all top level fields.

    // $project : { title : 1, author : 1 }
    Aggregation.project(Book.class)

Original pull request: #748.

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

@@ -244,6 +244,19 @@ public class Aggregation {
 		return new ProjectionOperation(fields);
 	}
 
+	/**
+	 * Creates a new {@link ProjectionOperation} including all top level fields of the given given {@link Class}.
+	 *
+	 * @param type must not be {@literal null}.
+	 * @return new instance of {@link ProjectionOperation}.
+	 * @since 2.2
+	 */
+	public static ProjectionOperation project(Class<?> type) {
+
+		Assert.notNull(type, "Type must not be null!");
+		return new ProjectionOperation(type);
+	}
+
 	/**
 	 * Factory method to create a new {@link UnwindOperation} for the field with the given name.
 	 *

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

@@ -30,6 +30,7 @@ import org.springframework.data.mongodb.core.aggregation.ProjectionOperation.Pro
 import org.springframework.data.mongodb.core.aggregation.VariableOperators.Let.ExpressionVariable;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
+import org.springframework.util.ReflectionUtils;
 
 /**
  * Encapsulates the aggregation framework {@code $project}-operation.
@@ -73,6 +74,16 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 		this(NONE, ProjectionOperationBuilder.FieldProjection.from(fields));
 	}
 
+	/**
+	 * Creates a new {@link ProjectionOperation} including all top level fields of the given {@link Class type}.
+	 *
+	 * @param type must not be {@literal null}.
+	 * @since 2.2
+	 */
+	public ProjectionOperation(Class<?> type) {
+		this(NONE, Collections.singletonList(new TypeProjection(type)));
+	}
+
 	/**
 	 * Copy constructor to allow building up {@link ProjectionOperation} instances from already existing
 	 * {@link Projection}s.
@@ -1700,4 +1711,35 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 			return new Document(field.getName(), expression.toDocument(context));
 		}
 	}
+
+	/**
+	 * A {@link Projection} including all top level fields of the given target type mapped to include potentially
+	 * deviating field names.
+	 *
+	 * @since 2.2
+	 * @author Christoph Strobl
+	 */
+	static class TypeProjection extends Projection {
+
+		private final Class<?> type;
+
+		TypeProjection(Class<?> type) {
+
+			super(Fields.field(type.getSimpleName()));
+			this.type = type;
+		}
+
+		/*
+		 * (non-Javadoc)
+		 * @see org.springframework.data.mongodb.core.aggregation.ProjectionOperation.Projection#toDocument(org.springframework.data.mongodb.core.aggregation.AggregationOperationContext)
+		 */
+		@Override
+		public Document toDocument(AggregationOperationContext context) {
+
+			Document projections = new Document();
+			ReflectionUtils.doWithFields(type, it -> projections.append(it.getName(), 1));
+
+			return context.getMappedObject(projections, type);
+		}
+	}
 }

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

@@ -27,6 +27,7 @@ import lombok.Data;
 import java.util.Arrays;
 import java.util.List;
 
+import org.assertj.core.api.Assertions;
 import org.bson.Document;
 import org.junit.Test;
 import org.springframework.data.domain.Range;
@@ -41,6 +42,12 @@ import org.springframework.data.mongodb.core.aggregation.DateOperators.Timezone;
 import org.springframework.data.mongodb.core.aggregation.ProjectionOperation.ProjectionOperationBuilder;
 import org.springframework.data.mongodb.core.aggregation.StringOperators.Concat;
 import org.springframework.data.mongodb.core.aggregation.VariableOperators.Let.ExpressionVariable;
+import org.springframework.data.mongodb.core.convert.MappingMongoConverter;
+import org.springframework.data.mongodb.core.convert.MongoConverter;
+import org.springframework.data.mongodb.core.convert.NoOpDbRefResolver;
+import org.springframework.data.mongodb.core.convert.QueryMapper;
+import org.springframework.data.mongodb.core.mapping.Field;
+import org.springframework.data.mongodb.core.mapping.MongoMappingContext;
 
 /**
  * Unit tests for {@link ProjectionOperation}.
@@ -61,7 +68,7 @@ public class ProjectionOperationUnitTests {
 
 	@Test(expected = IllegalArgumentException.class) // DATAMONGO-586
 	public void rejectsNullFields() {
-		new ProjectionOperation(null);
+		new ProjectionOperation((Fields) null);
 	}
 
 	@Test // DATAMONGO-586
@@ -2099,6 +2106,36 @@ public class ProjectionOperationUnitTests {
 				"{ $project : { newDate: { $dateFromString: { dateString : \"2017-02-08T12:10:40.787\", format : \"dd/mm/yyyy\" } } } }"));
 	}
 
+	@Test // DATAMONGO-2200
+	public void typeProjectionShouldIncludeTopLevelFieldsOfType() {
+
+		ProjectionOperation operation = Aggregation.project(Book.class);
+
+		Document document = operation.toDocument(Aggregation.DEFAULT_CONTEXT);
+		Document projectClause = DocumentTestUtils.getAsDocument(document, PROJECT);
+
+		Assertions.assertThat(projectClause) //
+				.hasSize(2) //
+				.containsEntry("title", 1) //
+				.containsEntry("author", 1);
+	}
+
+	@Test // DATAMONGO-2200
+	public void typeProjectionShouldMapFieldNames() {
+
+		MongoMappingContext mappingContext = new MongoMappingContext();
+		MongoConverter converter = new MappingMongoConverter(NoOpDbRefResolver.INSTANCE, mappingContext);
+
+		Document document = Aggregation.project(BookRenamed.class)
+				.toDocument(new TypeBasedAggregationOperationContext(Book.class, mappingContext, new QueryMapper(converter)));
+		Document projectClause = DocumentTestUtils.getAsDocument(document, PROJECT);
+
+		Assertions.assertThat(projectClause) //
+				.hasSize(2) //
+				.containsEntry("ti_tl_e", 1) //
+				.containsEntry("author", 1);
+	}
+
 	private static Document exctractOperation(String field, Document fromProjectClause) {
 		return (Document) fromProjectClause.get(field);
 	}
@@ -2109,6 +2146,12 @@ public class ProjectionOperationUnitTests {
 		Author author;
 	}
 
+	@Data
+	static class BookRenamed {
+		@Field("ti_tl_e") String title;
+		Author author;
+	}
+
 	@Data
 	static class Author {
 		String first;