DATAMONGO-788 - Projection operations do not render synthetic fields properly.

Introduced boolean isSynthetic() attribute to FieldReference to determine if the given reference points to a synthetic field, as this controls whether we render a simple include (1) or a concrete reference ($fieldName) E.g. isSynthetic() would be true for a field reference to _id.

Original pull request: #90.

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

@@ -334,6 +334,13 @@ public class ExposedFields implements Iterable<ExposedField> {
 			this.field = field;
 		}
 
+		/**
+		 * @return
+		 */
+		public boolean isSynthetic() {
+			return field.synthetic;
+		}
+
 		/**
 		 * Returns the raw, unqualified reference, i.e. the field reference without a {@literal $} prefix.
 		 * 

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

@@ -508,7 +508,7 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 					// check whether referenced field exists in the context
 					FieldReference reference = context.getReference(field.getTarget());
 
-					if (field.getName().equals(field.getTarget())) {
+					if (field.getName().equals(field.getTarget()) && reference.isSynthetic()) {
 
 						// render field as included
 						return 1;

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

@@ -85,6 +85,7 @@ public class AggregationTests {
 		mongoTemplate.dropCollection(Product.class);
 		mongoTemplate.dropCollection(UserWithLikes.class);
 		mongoTemplate.dropCollection(DATAMONGO753.class);
+		mongoTemplate.dropCollection(DATAMONGO788.class);
 	}
 
 	/**
@@ -520,6 +521,36 @@ public class AggregationTests {
 		}
 	}
 
+	/**
+	 * @see DATAMONGO-788
+	 */
+	@Test
+	public void referencesToGroupIdsShouldBeRenderedProperly() {
+
+		mongoTemplate.insert(new DATAMONGO788(1, 1));
+		mongoTemplate.insert(new DATAMONGO788(1, 1));
+		mongoTemplate.insert(new DATAMONGO788(1, 1));
+		mongoTemplate.insert(new DATAMONGO788(2, 1));
+		mongoTemplate.insert(new DATAMONGO788(2, 1));
+
+		AggregationOperation projectFirst = Aggregation.project("x", "y").and("xField").as("x").and("yField").as("y");
+		AggregationOperation group = Aggregation.group("x", "y").count().as("xPerY");
+		AggregationOperation project = Aggregation.project("xPerY", "x", "y").andExclude("_id");
+
+		TypedAggregation<DATAMONGO788> aggregation = Aggregation.newAggregation(DATAMONGO788.class, projectFirst, group,
+				project);
+		AggregationResults<DBObject> aggResults = mongoTemplate.aggregate(aggregation, DBObject.class);
+		List<DBObject> items = aggResults.getMappedResults();
+
+		assertThat(items.size(), is(2));
+		assertThat((Integer) items.get(0).get("xPerY"), is(2));
+		assertThat((Integer) items.get(0).get("x"), is(2));
+		assertThat((Integer) items.get(0).get("y"), is(1));
+		assertThat((Integer) items.get(1).get("xPerY"), is(3));
+		assertThat((Integer) items.get(1).get("x"), is(1));
+		assertThat((Integer) items.get(1).get("y"), is(1));
+	}
+
 	private void assertLikeStats(LikeStats like, String id, long count) {
 
 		assertThat(like, is(notNullValue()));
@@ -585,4 +616,22 @@ public class AggregationTests {
 			this.up = up;
 		}
 	}
+
+	static class DATAMONGO788 {
+
+		int x;
+		int y;
+		int xField;
+		int yField;
+
+		public DATAMONGO788() {}
+
+		public DATAMONGO788(int x, int y) {
+			this.x = x;
+			this.xField = x;
+			this.y = y;
+			this.yField = y;
+		}
+	}
+
 }

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

@@ -15,12 +15,19 @@
  */
 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.query.Criteria.*;
 
+import java.util.List;
+
 import org.junit.Rule;
 import org.junit.Test;
 import org.junit.rules.ExpectedException;
+import org.springframework.data.mongodb.core.DBObjectUtils;
+
+import com.mongodb.DBObject;
 
 /**
  * Unit tests for {@link Aggregation}.
@@ -94,4 +101,23 @@ public class AggregationUnitTests {
 				project("a", "b") // b should still be available
 		).toDbObject("foo", Aggregation.DEFAULT_CONTEXT);
 	}
+
+	/**
+	 * @see DATAMONGO-788
+	 */
+	@Test
+	public void referencesToGroupIdsShouldBeRenderedAsReferences() {
+
+		DBObject agg = newAggregation( //
+				project("a"), //
+				group("a").count().as("aCnt"), //
+				project("aCnt", "a") //
+		).toDbObject("foo", Aggregation.DEFAULT_CONTEXT);
+
+		@SuppressWarnings("unchecked")
+		DBObject secondProjection = ((List<DBObject>) agg.get("pipeline")).get(2);
+		DBObject fields = DBObjectUtils.getAsDBObject(secondProjection, "$project");
+		assertThat((Integer) fields.get("aCnt"), is(1));
+		assertThat((String) fields.get("a"), is("$_id.a"));
+	}
 }