How to clone models from Backbone collection to another

Question

I need to clone the models in one backbone collection and add then add them to another. (IOW all of the models in the new collection need to be unique, and have no connection to the models in the original collection.)

Here is my code:

collection1.each(function( model ) {
  var clone = new Backbone.Model( model.toJSON() );
  clone.set( this.idAttribute, null, { silent: true });
  collection2.add( clone );
});

This doesn't quite work. I can only add a model from collection1 to collection2 once. If I try to do it a second time, it fails. So somehow Backbone is detecting a dup.

Any suggestions on what I am doing wrong?

Thanks (in advance) for your help

Answer 1

In the code you provided, this.idAttribute is probably not what you think it is and thus won't create models without their ids, leading to collisions when you copy the models a second time.

Underscore is a hard dependency of Backbone, so you can use _.omit on the JSON representation of your collection to filter out the ids. For example,

function copyCollection(collection1, collection2){
    var idAttribute = collection1.model.prototype.idAttribute;

    var data = _.map(
        collection1.toJSON(), 
        function(obj){ return _.omit(obj, idAttribute); }
    );

    collection2.add(data);
}

var c1 = new Backbone.Collection([
    {id: 1, name: "N1"},
    {id: 2, name: "N2"}
]);
var c2 = new Backbone.Collection();

copyCollection(c1, c2);
copyCollection(c1, c2);
console.log(c2.toJSON());

And a Fiddle http://jsfiddle.net/jT59v/