Update relationship field to use standard error handling

Previously, Relationship fields were cleared by calls to updateItem if
not explicitly passed in. Now this and other undefined values will
leave them untouched.

This is consistent with how other fields work, however is a breaking
change to updateItem() with relationships

fields/types/relationship/Readme.md

@@ -53,6 +53,8 @@ Will return unexpected results if the relationship path has been populated.
 
 Updates with the provided value if it is different from the stored value.
 
+`undefined` values are ignored.
+
 When `null` is passed, mongoose will remove the path from the stored document and the value will be `undefined` when the item is next retrieved. This behaviour is different in `many` mode, when an empty array will be stored.
 
 ### `validateInput`

fields/types/relationship/RelationshipType.js

@@ -219,10 +219,15 @@ relationship.prototype.updateItem = function (item, data, callback) {
 	if (item.populated(this.path)) {
 		throw new Error('fieldTypes.relationship.updateItem() Error - You cannot update populated relationships.');
 	}
+
+	var value = this.getValueFromData(data);
+	if (value === undefined) {
+		return process.nextTick(callback);
+	}
 	if (this.many) {
 		var arr = item.get(this.path);
 		var _old = arr.map(function (i) { return String(i); });
-		var _new = data[this.path];
+		var _new = value;
 		if (!utils.isArray(_new)) {
 			_new = String(_new || '').split(',');
 		}
@@ -232,9 +237,9 @@ relationship.prototype.updateItem = function (item, data, callback) {
 			item.set(this.path, _new);
 		}
 	} else {
-		if (data[this.path]) {
-			if (data[this.path] !== item.get(this.path)) {
-				item.set(this.path, data[this.path]);
+		if (value) {
+			if (value !== item.get(this.path)) {
+				item.set(this.path, value);
 			}
 		} else if (item.get(this.path)) {
 			item.set(this.path, null);

fields/types/relationship/test/type.js

@@ -187,15 +187,15 @@ exports.testFieldType = function (List) {
 			});
 		});
 
-		it('should clear the current value when data object does not contain the field', function (done) {
+		it('should not clear the current value when data object does not contain the field', function (done) {
 			var testItem = new List.model({
 				single: relatedItem.id,
 			});
 			testItem.save(function (err) {
 				List.fields.single.updateItem(testItem, {}, function () {
 					testItem.save(function (err, updatedItem) {
 						List.model.findById(updatedItem.id, function (err, persistedData) {
-							demand(persistedData.single).be.null();
+							demand(String(persistedData.single)).equal(String(relatedItem.id));
 							done();
 						});
 					});
@@ -247,15 +247,17 @@ exports.testFieldType = function (List) {
 			});
 		});
 
-		it('should clear the current values when data object does not contain the field', function (done) {
+		it('should not clear the current values when data object does not contain the field', function (done) {
 			var testItem = new List.model({
 				many: [relatedItem.id, relatedItem.id],
 			});
 			testItem.save(function (err) {
 				List.fields.many.updateItem(testItem, {}, function () {
 					testItem.save(function (err, updatedItem) {
 						List.model.findById(updatedItem.id, function (err, persistedData) {
-							demand(persistedData.many).to.eql([]);
+							demand(persistedData.many.length).equal(2);
+							demand(String(persistedData.many[0])).equal(String(relatedItem.id));
+							demand(String(persistedData.many[1])).equal(String(relatedItem.id));
 							done();
 						});
 					});