Problem with backbone change callback, with different types for the same attribute.

changed-attributes-nonsense-backbone-js

Order = Backbone.Model.extend({  
  initialize: function() {
      this.bind("change:shipping_method_id", this.persist);
  },

...

// If the Order is valid, it tries to persist it.
persist: function(model, name) {
  if (this.vanillaValid("#checkout_form_confirm")) {
    this.updateTotal(true);
  }
}, 

... 

// Console
> model.changedAttributes()["shipping_method_id"]
19
> model.previousAttributes()["shipping_method_id"]
"19"

Problem was in my Backbone.js view. It was grabbing a value from the DOM (a string) and setting it to the Model.

It should always set a number when possible.