DATAMONGO-2055 - Allow position modifier to be negative using push at position on Update.

Original pull request: #600.

Author: christophstrobl

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/query/Update.java

@@ -805,18 +805,14 @@ public PushOperatorBuilder sort(Sort sort) {
 		/**
 		 * Forces values to be added at the given {@literal position}.
 		 *
-		 * @param position needs to be greater than or equal to zero.
+		 * @param position the position offset. As of MongoDB 3.6 use a negative value to indicate starting from the end,
+		 *          counting (but not including) the last element of the array.
 		 * @return never {@literal null}.
 		 * @since 1.7
 		 */
 		public PushOperatorBuilder atPosition(int position) {
 
-			if (position < 0) {
-				throw new IllegalArgumentException("Position must be greater than or equal to zero.");
-			}
-
 			this.modifiers.addModifier(new PositionModifier(position));
-
 			return this;
 		}
 

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/query/UpdateTests.java

@@ -399,9 +399,11 @@ public void getUpdateObjectShouldReturnCorrectRepresentationForBitwiseXor() {
 				equalTo(new BasicDBObjectBuilder().add("$bit", new BasicDBObject("key", new BasicDBObject("xor", 10L))).get()));
 	}
 
-	@Test(expected = IllegalArgumentException.class) // DATAMONGO-943
-	public void pushShouldThrowExceptionWhenGivenNegativePosition() {
-		new Update().push("foo").atPosition(-1).each("booh");
+	@Test // DATAMONGO-943, // DATAMONGO-2055
+	public void pushShouldAllowNegativePosition() {
+
+		assertThat(new Update().push("foo").atPosition(-1).each("booh").toString()).isEqualTo(
+				"{ \"$push\" : { \"foo\" : { \"$java\" : { \"$position\" : { \"$java\" : { \"$position\" : -1} }, \"$each\" : { \"$java\" : { \"$each\" : [ \"booh\"]} } } } } }");
 	}
 
 	@Test // DATAMONGO-1346