Allow referencing the $id field of dbrefs within an aggregation pipeline.

Closes: #4123
Original pull request: #4125.

Author: christophstrobl

Diff for: spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/Fields.java

@@ -255,7 +255,7 @@ public String getName() {
 
 		public String getTarget() {
 
-			if (isLocalVar()) {
+			if (isLocalVar() || pointsToDBRefId()) {
 				return this.getRaw();
 			}
 
@@ -280,6 +280,10 @@ public boolean isLocalVar() {
 			return raw.startsWith("$$") && !raw.startsWith("$$$");
 		}
 
+		protected boolean pointsToDBRefId() { // see https://jira.mongodb.org/browse/SERVER-14466
+			return raw.endsWith(".$id");
+		}
+
 		/**
 		 * @return
 		 * @since 1.10

Diff for: spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/FieldsUnitTests.java

@@ -117,6 +117,13 @@ public void stripsLeadingDollarsFromTarget() {
 		assertThat(Fields.field("$$$$target").getTarget()).isEqualTo("target");
 	}
 
+	@Test // GH-4123
+	public void keepsRawMappingToDbRefId() {
+
+		assertThat(Fields.field("$id").getName()).isEqualTo("id");
+		assertThat(Fields.field("person.$id").getTarget()).isEqualTo("person.$id");
+	}
+
 	private static void verify(Field field, String name, String target) {
 
 		assertThat(field).isNotNull();