DATAMONGO-1254 - Grouping after projection in aggregation now uses correct aliased field name.

We now push the aliased field name down the aggregation pipeline for projections including operations. This allows to reference them in a later stage. Prior to this change the field reference was potentially resolved to the target field of the operation which did not result in an error but lead to false results.

Original pull request: #311.

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

@@ -21,6 +21,7 @@ import java.util.Collections;
 import java.util.List;
 
 import org.springframework.data.mongodb.core.aggregation.ExposedFields.ExposedField;
+import org.springframework.data.mongodb.core.aggregation.Fields.AggregationField;
 import org.springframework.data.mongodb.core.aggregation.ProjectionOperation.ProjectionOperationBuilder.FieldProjection;
 import org.springframework.util.Assert;
 
@@ -40,6 +41,7 @@ import com.mongodb.DBObject;
  * @author Tobias Trelle
  * @author Thomas Darimont
  * @author Oliver Gierke
+ * @author Christoph Strobl
  * @since 1.3
  */
 public class ProjectionOperation implements FieldsExposingAggregationOperation {
@@ -763,6 +765,20 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 				return field;
 			}
 
+			/*
+			 * (non-Javadoc)
+			 * @see org.springframework.data.mongodb.core.aggregation.ProjectionOperation.Projection#getExposedField()
+			 */
+			@Override
+			public ExposedField getExposedField() {
+
+				if (!getField().isAliased()) {
+					return super.getExposedField();
+				}
+
+				return new ExposedField(new AggregationField(getField().getName()), true);
+			}
+
 			/**
 			 * Creates a new instance of this {@link OperationProjection} with the given alias.
 			 * 

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2013-2014 the original author or authors.
+ * Copyright 2013-2015 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.
@@ -30,6 +30,7 @@ import org.junit.rules.ExpectedException;
 import org.springframework.data.domain.Sort.Direction;
 
 import com.mongodb.BasicDBObject;
+import com.mongodb.BasicDBObjectBuilder;
 import com.mongodb.DBObject;
 
 /**
@@ -37,6 +38,7 @@ import com.mongodb.DBObject;
  * 
  * @author Oliver Gierke
  * @author Thomas Darimont
+ * @author Christoph Strobl
  */
 public class AggregationUnitTests {
 
@@ -283,6 +285,40 @@ public class AggregationUnitTests {
 				is((DBObject) new BasicDBObject("_id", "$someKey").append("doc", new BasicDBObject("$first", "$$ROOT"))));
 	}
 
+	/**
+	 * @see DATAMONGO-1254
+	 */
+	@Test
+	public void shouldExposeAliasedFieldnameForProjectionsIncludingOperationsDownThePipeline() {
+
+		DBObject agg = Aggregation.newAggregation(//
+				project("date") //
+						.and("tags").minus(10).as("tags_count")//
+				, group("date")//
+						.sum("tags_count").as("count")//
+				).toDbObject("foo", Aggregation.DEFAULT_CONTEXT);
+
+		DBObject group = extractPipelineElement(agg, 1, "$group");
+		assertThat(getAsDBObject(group, "count"), is(new BasicDBObjectBuilder().add("$sum", "$tags_count").get()));
+	}
+
+	/**
+	 * @see DATAMONGO-1254
+	 */
+	@Test
+	public void shouldUseAliasedFieldnameForProjectionsIncludingOperationsDownThePipelineWhenUsingSpEL() {
+
+		DBObject agg = Aggregation.newAggregation(//
+				project("date") //
+						.andExpression("tags-10")//
+				, group("date")//
+						.sum("tags_count").as("count")//
+				).toDbObject("foo", Aggregation.DEFAULT_CONTEXT);
+
+		DBObject group = extractPipelineElement(agg, 1, "$group");
+		assertThat(getAsDBObject(group, "count"), is(new BasicDBObjectBuilder().add("$sum", "$tags_count").get()));
+	}
+
 	private DBObject extractPipelineElement(DBObject agg, int index, String operation) {
 
 		List<DBObject> pipeline = (List<DBObject>) agg.get("pipeline");