How do I insert a model into a backbone.js collection in a specific index?

Question

I need to insert a model into a Collection at position Collection.length-2. The last model in the collection should always stay the last model in the collection.

What I tried so far:

I added one "page" model to the Collection "Pages" and then tried to swap them around by changing their sequence:

var insertedpage = Pages.at(Pages.length-1);
var lastpage = Pages.at(Pages.length-2);
insertedpage.set({sequence: Pages.length-1});
lastpage.set({sequence: Pages.length});

I also tried to remove the last page, then add a new page and then add the last page back in.

var lastpage =  Pages.pop();
Pages.add({example1: example2});
Pages.push(lastpage);

neither of these worked. The newly added page still appears as last model in the Collection. Do I need to call some kind of order function after this?

Answer 1

Backbone.Collection.add() takes an options object that supports an at key for specifying the index.

Pass {at: index} to splice the model into the collection at the specified index.

Example:

Pages.add({ foo: bar }, { at: Pages.length - 2 })

Answer 2

Along the same suggestion as Rob Hruska, use Backbone.Collection.add() with at in the options object.

Pages = new Backbone.Collection([
  {id:1, foo:'bar'},
  {id:2, foo:'barista'}  /* Last model should remain last */
]);

/* Insert new "page" not at the end (default) but length minus 1 */
Pages.add({id:3, foo:'bartender'}, { at: Pages.length - 1 });

Pages.at(0).id === 1; // true
Pages.at(Pages.length - 2).id === 3; // true
Pages.at(Pages.length - 1).id === 2; // true

You mentioned that Pages seems to be sorted by the attribute sequence; do you happen to have a comparator function defined on the Pages collection?

Another question, did you want to update this attribute sequence on ALL existing page models currently in the collection when a new page is added to the 2nd to the last position? Or was that attribute an attempt to accomplish your original question?