DATAMONGO-784 - Add support for comparison aggregation operators to group & project.

We now directly support comparison aggregation operators ($cmp, $eq, $gt, $gte, $lt, $lte and $ne) on both group and project stages.

Original pull request: #414.

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

@@ -33,7 +33,7 @@ import com.mongodb.DBObject;
  */
 public enum AggregationFunctionExpressions {
 
-	SIZE, GTE;
+	SIZE, CMP, EQ, GT, GTE, LT, LTE, NE;
 
 	/**
 	 * Returns an {@link AggregationExpression} build from the current {@link Enum} name and the given parameters.

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

@@ -616,6 +616,78 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 			return project("size");
 		}
 
+		/**
+		 * Generates a {@code $cmp} expression (compare to) that compares the value of the field to a given value or field.
+		 *
+		 * @return never {@literal null}.
+		 * @since 1.10
+		 */
+		public ProjectionOperationBuilder cmp(Object compareValue) {
+			return project("cmp", compareValue);
+		}
+
+		/**
+		 * Generates a {@code $eq} expression (equal) that compares the value of the field to a given value or field.
+		 *
+		 * @return never {@literal null}.
+		 * @since 1.10
+		 */
+		public ProjectionOperationBuilder eq(Object compareValue) {
+			return project("eq", compareValue);
+		}
+
+		/**
+		 * Generates a {@code $gt} expression (greater than) that compares the value of the field to a given value or field.
+		 *
+		 * @return never {@literal null}.
+		 * @since 1.10
+		 */
+		public ProjectionOperationBuilder gt(Object compareValue) {
+			return project("gt", compareValue);
+		}
+
+		/**
+		 * Generates a {@code $gte} expression (greater than equal) that compares the value of the field to a given value or
+		 * field.
+		 *
+		 * @return never {@literal null}.
+		 * @since 1.10
+		 */
+		public ProjectionOperationBuilder gte(Object compareValue) {
+			return project("gte", compareValue);
+		}
+
+		/**
+		 * Generates a {@code $lt} expression (less than) that compares the value of the field to a given value or field.
+		 *
+		 * @return never {@literal null}.
+		 * @since 1.10
+		 */
+		public ProjectionOperationBuilder lt(Object compareValue) {
+			return project("lt", compareValue);
+		}
+
+		/**
+		 * Generates a {@code $lte} expression (less than equal) that compares the value of the field to a given value or
+		 * field.
+		 *
+		 * @return never {@literal null}.
+		 * @since 1.10
+		 */
+		public ProjectionOperationBuilder lte(Object compareValue) {
+			return project("lte", compareValue);
+		}
+
+		/**
+		 * Generates a {@code $ne} expression (not equal) that compares the value of the field to a given value or field.
+		 *
+		 * @return never {@literal null}.
+		 * @since 1.10
+		 */
+		public ProjectionOperationBuilder ne(Object compareValue) {
+			return project("ne", compareValue);
+		}
+
 		/**
 		 * Generates a {@code $slice} expression that returns a subset of the array held by the given field. <br />
 		 * If {@literal n} is positive, $slice returns up to the first n elements in the array. <br />

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

@@ -19,6 +19,7 @@ import static org.hamcrest.Matchers.*;
 import static org.junit.Assert.*;
 import static org.springframework.data.mongodb.core.aggregation.AggregationFunctionExpressions.*;
 import static org.springframework.data.mongodb.core.aggregation.Fields.*;
+import static org.springframework.data.mongodb.test.util.IsBsonObject.*;
 import static org.springframework.data.mongodb.util.DBObjectUtils.*;
 
 import java.util.Arrays;
@@ -402,6 +403,90 @@ public class ProjectionOperationUnitTests {
 				is((Object) new BasicDBObject("$slice", Arrays.<Object> asList("$field", 5, 10))));
 	}
 
+	/**
+	 * @see DATAMONGO-784
+	 */
+	@Test
+	public void shouldRenderCmpCorrectly() {
+
+		ProjectionOperation operation = Aggregation.project().and("field").cmp(10).as("cmp10");
+
+		assertThat(operation.toDBObject(Aggregation.DEFAULT_CONTEXT),
+				isBsonObject().containing("$project.cmp10.$cmp.[0]", "$field").containing("$project.cmp10.$cmp.[1]", 10));
+	}
+
+	/**
+	 * @see DATAMONGO-784
+	 */
+	@Test
+	public void shouldRenderEqCorrectly() {
+
+		ProjectionOperation operation = Aggregation.project().and("field").eq(10).as("eq10");
+
+		assertThat(operation.toDBObject(Aggregation.DEFAULT_CONTEXT),
+				isBsonObject().containing("$project.eq10.$eq.[0]", "$field").containing("$project.eq10.$eq.[1]", 10));
+	}
+
+	/**
+	 * @see DATAMONGO-784
+	 */
+	@Test
+	public void shouldRenderGtCorrectly() {
+
+		ProjectionOperation operation = Aggregation.project().and("field").gt(10).as("gt10");
+
+		assertThat(operation.toDBObject(Aggregation.DEFAULT_CONTEXT),
+				isBsonObject().containing("$project.gt10.$gt.[0]", "$field").containing("$project.gt10.$gt.[1]", 10));
+	}
+
+	/**
+	 * @see DATAMONGO-784
+	 */
+	@Test
+	public void shouldRenderGteCorrectly() {
+
+		ProjectionOperation operation = Aggregation.project().and("field").gte(10).as("gte10");
+
+		assertThat(operation.toDBObject(Aggregation.DEFAULT_CONTEXT),
+				isBsonObject().containing("$project.gte10.$gte.[0]", "$field").containing("$project.gte10.$gte.[1]", 10));
+	}
+
+	/**
+	 * @see DATAMONGO-784
+	 */
+	@Test
+	public void shouldRenderLtCorrectly() {
+
+		ProjectionOperation operation = Aggregation.project().and("field").lt(10).as("lt10");
+
+		assertThat(operation.toDBObject(Aggregation.DEFAULT_CONTEXT),
+				isBsonObject().containing("$project.lt10.$lt.[0]", "$field").containing("$project.lt10.$lt.[1]", 10));
+	}
+
+	/**
+	 * @see DATAMONGO-784
+	 */
+	@Test
+	public void shouldRenderLteCorrectly() {
+
+		ProjectionOperation operation = Aggregation.project().and("field").lte(10).as("lte10");
+
+		assertThat(operation.toDBObject(Aggregation.DEFAULT_CONTEXT),
+				isBsonObject().containing("$project.lte10.$lte.[0]", "$field").containing("$project.lte10.$lte.[1]", 10));
+	}
+
+	/**
+	 * @see DATAMONGO-784
+	 */
+	@Test
+	public void shouldRenderNeCorrectly() {
+
+		ProjectionOperation operation = Aggregation.project().and("field").ne(10).as("ne10");
+
+		assertThat(operation.toDBObject(Aggregation.DEFAULT_CONTEXT),
+				isBsonObject().containing("$project.ne10.$ne.[0]", "$field").containing("$project.ne10.$ne.[1]", 10));
+	}
+
 	private static DBObject exctractOperation(String field, DBObject fromProjectClause) {
 		return (DBObject) fromProjectClause.get(field);
 	}