jashkenas/backbone — pull request #2764 by lmc981 (2013-09-18 14:15:16+00:00)
Title: Managing collection with model with complex ids

Using a data model that has multiple fields as keys for the objects, I faced an issue when such objects are added to a Backbone.Collection.
If the id field of the model is an object, it is mapped as "[object Object]", so the following models update the first one, instead of being added to the collection.
Stringifying the model id, eliminates the issue.