DATAMONGO-1327 - Added support for $stdDevSamp and $stdDevPop to aggregation $group stage.

Original Pull Request: #360
CLA: 171720160409030719 (Gustavo de Geus)

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

@@ -27,6 +27,9 @@ import org.springframework.data.mongodb.core.aggregation.ExposedFields.FieldRefe
 import org.springframework.util.Assert;
 import org.springframework.util.StringUtils;
 
+import com.mongodb.BasicDBObject;
+import com.mongodb.DBObject;
+
 /**
  * Encapsulates the aggregation framework {@code $group}-operation.
  * <p>
@@ -37,6 +40,7 @@ import org.springframework.util.StringUtils;
  * @author Sebastian Herold
  * @author Thomas Darimont
  * @author Oliver Gierke
+ * @author Gustavo de Geus
  * @author Christoph Strobl
  * @since 1.3
  */
@@ -311,6 +315,27 @@ public class GroupOperation implements FieldsExposingAggregationOperation {
 		return newBuilder(GroupOps.MAX, null, expr);
 	}
 
+	/**
+	 * Generates an {@link GroupOperationBuilder} for an {@code $stdDevSamp}-expression that for the given
+	 * field-reference.
+	 *
+	 * @param reference
+	 * @return
+	 */
+	public GroupOperationBuilder stdDevSamp(String reference) {
+		return newBuilder(GroupOps.STD_DEV_SAMP, reference, null);
+	}
+
+	/**
+	 * Generates an {@link GroupOperationBuilder} for an {@code $stdDevPop}-expression that for the given field-reference.
+	 *
+	 * @param reference
+	 * @return
+	 */
+	public GroupOperationBuilder stdDevPop(String reference) {
+		return newBuilder(GroupOps.STD_DEV_POP, reference, null);
+	}
+
 	private GroupOperationBuilder newBuilder(Keyword keyword, String reference, Object value) {
 		return new GroupOperationBuilder(this, new Operation(keyword, null, reference, value));
 	}
@@ -375,7 +400,7 @@ public class GroupOperation implements FieldsExposingAggregationOperation {
 
 	private static enum GroupOps implements Keyword {
 
-		SUM, LAST, FIRST, PUSH, AVG, MIN, MAX, ADD_TO_SET, COUNT;
+		SUM, LAST, FIRST, PUSH, AVG, MIN, MAX, ADD_TO_SET, COUNT, STD_DEV_SAMP, STD_DEV_POP;
 
 		@Override
 		public String toString() {

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

@@ -31,6 +31,7 @@ import org.springframework.data.mongodb.core.DocumentTestUtils;
  * 
  * @author Oliver Gierke
  * @author Thomas Darimont
+ * @author Gustavo de Geus
  */
 public class GroupOperationUnitTests {
 
@@ -202,6 +203,34 @@ public class GroupOperationUnitTests {
 		assertThat(tagsCount.get("$first"), is((Object) new Document("$size", Arrays.asList("$tags"))));
 	}
 
+	/**
+	 * @see DATAMONGO-1327
+	 */
+	@Test
+	public void groupOperationStdDevSampWithValue() {
+
+		GroupOperation groupOperation = Aggregation.group("a", "b").stdDevSamp("field").as("fieldStdDevSamp");
+
+		Document groupClause = extractDocumentFromGroupOperation(groupOperation);
+		Document push = DocumentTestUtils.getAsDocument(groupClause, "fieldStdDevSamp");
+
+		assertThat(push, is(new Document("$stdDevSamp", "$field")));
+	}
+
+	/**
+	 * @see DATAMONGO-1327
+	 */
+	@Test
+	public void groupOperationStdDevPopWithValue() {
+
+		GroupOperation groupOperation = Aggregation.group("a", "b").stdDevPop("field").as("fieldStdDevPop");
+
+		Document groupClause = extractDocumentFromGroupOperation(groupOperation);
+		Document push = DocumentTestUtils.getAsDocument(groupClause, "fieldStdDevPop");
+
+		assertThat(push, is(new Document("$stdDevPop", "$field")));
+	}
+
 	private Document extractDocumentFromGroupOperation(GroupOperation groupOperation) {
 		Document document = groupOperation.toDocument(Aggregation.DEFAULT_CONTEXT);
 		Document groupClause = DocumentTestUtils.getAsDocument(document, "$group");