DATAMONGO-758 - Current mongodb Versions only support to explicitly exclude the _id property in a projection.

Added guard to FieldProjection.from within ProjectionOption to prevent users from excluding fields other than "_id".

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

@@ -362,6 +362,7 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 		 * A {@link FieldProjection} to map a result of a previous {@link AggregationOperation} to a new field.
 		 * 
 		 * @author Oliver Gierke
+		 * @author Thomas Darimont
 		 */
 		static class FieldProjection extends Projection {
 
@@ -399,6 +400,17 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 				List<FieldProjection> projections = new ArrayList<FieldProjection>();
 
 				for (Field field : fields) {
+
+					if (!include) {
+						if (!Fields.UNDERSCORE_ID.equals(field.getName())) {
+							throw new IllegalArgumentException(
+									String
+											.format(
+													"Exclusion of field %s not allowed. Projections by the mongodb aggregation framework only support the exclusion of the %s field!",
+													field.getName(), Fields.UNDERSCORE_ID));
+						}
+					}
+
 					projections.add(new FieldProjection(field, include ? null : 0));
 				}
 

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

@@ -31,6 +31,7 @@ import com.mongodb.DBObject;
  * Unit tests for {@link ProjectionOperation}.
  * 
  * @author Oliver Gierke
+ * @author Thomas Darimont
  */
 public class ProjectionOperationUnitTests {
 
@@ -183,6 +184,27 @@ public class ProjectionOperationUnitTests {
 		assertThat(oper.get(MOD), is((Object) Arrays.<Object> asList("$a", 3)));
 	}
 
+	/**
+	 * @see DATAMONGO-758
+	 */
+	@Test(expected = IllegalArgumentException.class)
+	public void excludeShouldThrowExceptionForFieldsOtherThanUnderscoreId() {
+
+		new ProjectionOperation().andExclude("foo");
+	}
+
+	/**
+	 * @see DATAMONGO-758
+	 */
+	@Test
+	public void excludeShouldExclusionOfUnderscoreId() {
+
+		ProjectionOperation projectionOp = new ProjectionOperation().andExclude(Fields.UNDERSCORE_ID);
+		DBObject dbObject = projectionOp.toDBObject(Aggregation.DEFAULT_CONTEXT);
+		DBObject projectClause = DBObjectUtils.getAsDBObject(dbObject, PROJECT);
+		assertThat((Integer) projectClause.get(Fields.UNDERSCORE_ID), is(0));
+	}
+
 	@Test(expected = IllegalArgumentException.class)
 	public void arithmenticProjectionOperationModByZeroException() {