DATAMONGO-1871 - Fix AggregationExpression aliasing.

We now make sure to allow a nested property alias by setting the target.

Original pull request: #533.

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

@@ -455,7 +455,7 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 			}
 
 			if (value instanceof AggregationExpression) {
-				return this.operation.and(new ExpressionProjection(Fields.field(alias), (AggregationExpression) value));
+				return this.operation.and(new ExpressionProjection(Fields.field(alias, alias), (AggregationExpression) value));
 			}
 
 			return this.operation.and(new FieldProjection(Fields.field(alias, name), null));

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

@@ -574,6 +574,23 @@ public class AggregationUnitTests {
 				new BasicDBObject("$add", new BasicDbListBuilder().add("$value1.value").add("$value2.value").get())))));
 	}
 
+	@Test // DATAMONGO-1871
+	public void providedAliasShouldAllowNestingExpressionWithAliasCorrectly() {
+
+		DBObject condition = new BasicDBObject("$and",
+				Arrays.asList(new BasicDBObject("$gte", new BasicDbListBuilder().add("$$est.dt").add("2015-12-29").get()), //
+						new BasicDBObject("$lte", new BasicDbListBuilder().add("$$est.dt").add("2017-12-29").get()) //
+				));
+
+		Aggregation agg = newAggregation(project("_id", "dId", "aId", "cty", "cat", "plts.plt")
+				.and(ArrayOperators.arrayOf("plts.ests").filter().as("est").by(condition)).as("plts.ests"));
+
+		DBObject $project = extractPipelineElement(agg.toDbObject("collection-1", Aggregation.DEFAULT_CONTEXT), 0,
+				"$project");
+
+		assertThat($project.containsField("plts.ests"), is(true));
+	}
+
 	private DBObject extractPipelineElement(DBObject agg, int index, String operation) {
 
 		List<DBObject> pipeline = (List<DBObject>) agg.get("pipeline");