Backbone User Model with multiple update paths

I have built a simple user model in Backbone

define([
  'underscore',
  'backbone'
], function(_, Backbone) {

   var UserModel = Backbone.Model.extend({
      urlRoot: '/api/user',
      idAttribute: '_id',
      defaults: {
         userName: '',
         password: ''
         personalDetails: {
         title: '',
         name: {
           firstName: '',
           lastName: ''
         },
         gender: '',
         dob: ''
       }
    }
 });

 return UserModel;
});

Calling save() on the model will issue a PUT request to /api/user/:id which is fine.

However, in the front end I want 3 forms - updatePersonalDetails, updateUserName, updatePassword.

With my current implementation these will all use the same model and all save() to the same endpoint /api/user/:id. My problem is that I wont know which form the user posted.

Whats the best solution to this issue whilst still conforming to RESTful standards? Having 3 separate Models - UserDetailsModel, UserPasswordModel etc? Or to just do a bulk UPDATE on the whole model regardless of which form the user posted.

Allot been discussed regarding model separation vs model BULK in the question comments, I have nothing to add there.

Regarding your need to understand the UPDATE request form source, you can use query string parameters:

Just POST /api/user/1?form=my_special_form

In backbone you just need to save the model with url option

model.save({}, {
  url: model.url() + "?form=my_special_form"
});