DATAMONGO-1893 - Allow exclusion of other fields than _id in aggregation $project.

As of MongoDB 3.4 exclusion of fields other than _id is allowed so we removed the limitation in our code.

Original pull request: #538.

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

@@ -140,11 +140,6 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 	 */
 	public ProjectionOperation andExclude(String... fieldNames) {
 
-		for (String fieldName : fieldNames) {
-			Assert.isTrue(Fields.UNDERSCORE_ID.equals(fieldName),
-					String.format(EXCLUSION_ERROR, fieldName, Fields.UNDERSCORE_ID));
-		}
-
 		List<FieldProjection> excludeProjections = FieldProjection.from(Fields.fields(fieldNames), false);
 		return new ProjectionOperation(this.projections, excludeProjections);
 	}

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

@@ -200,10 +200,13 @@ public class ProjectionOperationUnitTests {
 		assertThat(oper.get(MOD)).isEqualTo((Object) Arrays.<Object> asList("$a", 3));
 	}
 
-	@Test(expected = IllegalArgumentException.class) // DATAMONGO-758
+	@Test // DATAMONGO-758, DATAMONGO-1893
-	public void excludeShouldThrowExceptionForFieldsOtherThanUnderscoreId() {
+	public void excludeShouldAllowExclusionOfFieldsOtherThanUnderscoreId/* since MongoDB 3.4 */() {
 
-		new ProjectionOperation().andExclude("foo");
+		ProjectionOperation projectionOp = new ProjectionOperation().andExclude("foo");
+		Document document = projectionOp.toDocument(Aggregation.DEFAULT_CONTEXT);
+		Document projectClause = DocumentTestUtils.getAsDocument(document, PROJECT);
+		assertThat((Integer) projectClause.get("foo")).isEqualTo(0);
 	}
 
 	@Test // DATAMONGO-758