LoopbackJS，Angularjs sdk，model。$ save duplicates

I am using lopbackJS AngularJS sdk.I have a model for sites (id(pk) int, name varchar(50), url varchar(100)). When i try to update name and calling site.save(), record became duplicated. If i try to update other fields except name, update succeeded.I am using lopbackJS AngularJS sdk.I have a mod