Promise as argument of Collection add, remove, reset, set methods

[redexp]

What would you say if add ability to pass to Collection methods not only plain objects, arrays and models, but also promise. Just add code like this to each method

add: function(models, options) {
  //...
  if (typeof models.then === 'function') {
    var self = this;
    return models.then(function (models) {
      return self.set(models, options);
    });
  }
  //..
}

and so we can do

users.add($.get('/users'));

very useful if you decided to change some of your methods from sync to async like

User = Model.extend({
  getProducts: function () {
    // was
    return products.where({user_id: this.get('id')});
    // become
    return $.get('/products', {user_id: this.get('id')});
  }
});

// but code same for sync and async
bag.add(user.getProducts());

[akre54]

How about user.getProducts().then(products => bag.add(products))?

I think if we're going to add Promises to Backbone we should do so holistically (like #3729, #3651), not in fits and starts.