DATAMONGO-806 - Fixed invalid rendering of id field references.

Tweaked the rendering of projection operations to always use the field based reference lookup to make sure the reference gets rendered aliased. Moved value calculation logic into FieldReference.

Original pull request: #101.

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

@@ -343,13 +343,6 @@ 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.
 		 * 
@@ -360,6 +353,16 @@ public class ExposedFields implements Iterable<ExposedField> {
 			return field.synthetic ? target : String.format("%s.%s", Fields.UNDERSCORE_ID, target);
 		}
 
+		/**
+		 * Returns the referenve value for the given field reference. Will return 1 for a synthetic, unaliased field or the
+		 * raw rendering of the reference otherwise.
+		 * 
+		 * @return
+		 */
+		public Object getReferenceValue() {
+			return field.synthetic && !field.isAliased() ? 1 : toString();
+		}
+
 		/*
 		 * (non-Javadoc)
 		 * @see java.lang.Object#toString()

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

@@ -21,7 +21,6 @@ import java.util.Collections;
 import java.util.List;
 
 import org.springframework.data.mongodb.core.aggregation.ExposedFields.ExposedField;
-import org.springframework.data.mongodb.core.aggregation.ExposedFields.FieldReference;
 import org.springframework.data.mongodb.core.aggregation.ProjectionOperation.ProjectionOperationBuilder.FieldProjection;
 import org.springframework.util.Assert;
 
@@ -506,8 +505,7 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 				if (value == null || Boolean.TRUE.equals(value)) {
 
 					// check whether referenced field exists in the context
-					FieldReference reference = context.getReference(field.getTarget());
-					return reference.isSynthetic() && !field.isAliased() ? 1 : reference.toString();
+					return context.getReference(field).getReferenceValue();
 
 				} else if (Boolean.FALSE.equals(value)) {
 

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

@@ -88,15 +88,17 @@ public class TypeBasedAggregationOperationContext implements AggregationOperatio
 	 */
 	@Override
 	public FieldReference getReference(String name) {
-		PropertyPath path = PropertyPath.from(name, type);
-
-		PersistentPropertyPath<MongoPersistentProperty> propertyPath = mappingContext.getPersistentPropertyPath(path);
-
-		return getReferenceFor(field(path.getLeafProperty().getSegment(),
-				propertyPath.toDotPath(MongoPersistentProperty.PropertyToFieldNameConverter.INSTANCE)));
+		return getReferenceFor(field(name));
 	}
 
 	private FieldReference getReferenceFor(Field field) {
-		return new FieldReference(new ExposedField(field, true));
+
+		PropertyPath path = PropertyPath.from(field.getTarget(), type);
+
+		PersistentPropertyPath<MongoPersistentProperty> propertyPath = mappingContext.getPersistentPropertyPath(path);
+		Field mappedField = field(propertyPath.getLeafProperty().getName(),
+				propertyPath.toDotPath(MongoPersistentProperty.PropertyToFieldNameConverter.INSTANCE));
+
+		return new FieldReference(new ExposedField(mappedField, true));
 	}
 }

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

@@ -23,10 +23,12 @@ import static org.springframework.data.mongodb.core.query.Criteria.*;
 
 import java.io.BufferedInputStream;
 import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.Date;
 import java.util.List;
 import java.util.Scanner;
 
+import org.joda.time.LocalDateTime;
 import org.junit.After;
 import org.junit.Before;
 import org.junit.Test;
@@ -36,6 +38,7 @@ import org.slf4j.LoggerFactory;
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.core.io.ClassPathResource;
 import org.springframework.dao.DataAccessException;
+import org.springframework.data.annotation.Id;
 import org.springframework.data.mongodb.core.CollectionCallback;
 import org.springframework.data.mongodb.core.MongoTemplate;
 import org.springframework.data.mongodb.core.query.Query;
@@ -551,6 +554,40 @@ public class AggregationTests {
 		assertThat((Integer) items.get(1).get("y"), is(1));
 	}
 
+	/**
+	 * @see DATAMONGO-806
+	 */
+	@Test
+	public void shouldAllowGroupByIdFields() {
+
+		mongoTemplate.dropCollection(User.class);
+
+		LocalDateTime now = new LocalDateTime();
+
+		User user1 = new User("u1", new PushMessage("1", "aaa", now.toDate()));
+		User user2 = new User("u2", new PushMessage("2", "bbb", now.minusDays(2).toDate()));
+		User user3 = new User("u3", new PushMessage("3", "ccc", now.minusDays(1).toDate()));
+
+		mongoTemplate.save(user1);
+		mongoTemplate.save(user2);
+		mongoTemplate.save(user3);
+
+		Aggregation agg = newAggregation( //
+				project("id", "msgs"), //
+				unwind("msgs"), //
+				match(where("msgs.createDate").gt(now.minusDays(1).toDate())), //
+				group("id").push("msgs").as("msgs") //
+		);
+
+		AggregationResults<DBObject> results = mongoTemplate.aggregate(agg, User.class, DBObject.class);
+
+		List<DBObject> mappedResults = results.getMappedResults();
+
+		DBObject firstItem = mappedResults.get(0);
+		assertThat(firstItem.get("_id"), is(notNullValue()));
+		assertThat(String.valueOf(firstItem.get("_id")), is("u1"));
+	}
+
 	private void assertLikeStats(LikeStats like, String id, long count) {
 
 		assertThat(like, is(notNullValue()));
@@ -634,4 +671,37 @@ public class AggregationTests {
 		}
 	}
 
+	/**
+	 * @see DATAMONGO-806
+	 */
+	static class User {
+
+		@Id String id;
+		List<PushMessage> msgs;
+
+		public User() {}
+
+		public User(String id, PushMessage... msgs) {
+			this.id = id;
+			this.msgs = Arrays.asList(msgs);
+		}
+	}
+
+	/**
+	 * @see DATAMONGO-806
+	 */
+	static class PushMessage {
+
+		@Id String id;
+		String content;
+		Date createDate;
+
+		public PushMessage() {}
+
+		public PushMessage(String id, String content, Date createDate) {
+			this.id = id;
+			this.content = content;
+			this.createDate = createDate;
+		}
+	}
 }