Question

I have a Backbone collection model (with sub-models as elements)
and views to edit it.

I would like it that when the model is initially created, to “turn off”
sync, so the back end is never invoked until the user clicks on a
button, then I would like to “turn on” the sync, and invoke the save
method on the root model, in order to save it to the DB.
Once a model it saved, it should behave like a normal model.

The goal is to avoid saving until the user determines that he is happy with
what he has entered.

Answer 1

Backbone will initially look for a model’s local sync function before going to Backbone.sync.

Backbone.js Documentation: The sync function may be overriden globally as Backbone.sync, or at a finer-grained level, by adding a sync function to a Backbone collection or to an individual model.

Therefore you can do this:

var MyModel = Backbone.Model.extend({
    // New instances of this model will have a 'dud' sync function
    sync: function () { return false; }
});

var MyView = Backbone.View.extend({

   ...

   events : {
       'click #my-button' : 'enableSync',
       'click #my-save-button' : 'saveModel'
   },

   enableSync: function () {
       // If this view's model is still pointing to our fake sync function,
       // update it so that it references Backbone.sync going forward.
       if (this.model.sync !== Backbone.sync) {
          this.model.sync = Backbone.sync;
       }
   },

   saveModel: function () {
       // This won't actually do anything until we click '#my-button'
       this.model.save();
   }

   ...

});

var view = new MyView({ model: new MyModel() });