title: Firebase + backbone: difference between collection.create() and collection.push()?
body: <p>I see a difference using create() and push() on collections using Backfire and wonder if this is a misunderstanding on my part, or a bug.</p>
  
  <p>I have an Animal model and Animals collection as below. Normally, the collection is created with an options object containing a zoo_id which is then used to populate the zoo_id in new models. It's a fixed value for this example.</p>
  
  <pre><code>var Animal = Backbone.Model.extend({
        initialize: function(attributes, options) {
          console.log("model initializing", attributes, options)
        }
      }),
      Animals = Backbone.Firebase.Collection.extend({
        firebase: myFirebaseUrl + "/animal",
        initialize: function(models, options) {
          this.model = function(attrs, opts) {
            return new Animal(_.extend(attrs, {zoo_id: 4}))
          };
          this.on("add", function(model, collection, options) {
            console.log("adding", model, collection, options, model.attributes)
          })
        }
      })
  var a= new Animals()
  </code></pre>
  
  <p>If there's data in Firebase, all of the retrieved animal models in a[] have zoo_id = 4, as expected.</p>
  
  <p>When I <strong>push</strong> a new model</p>
  
  <pre><code>a.push({name: "racoon"})
  </code></pre>
  
  <p>all of the attribute objects logged to the console have zoo_id = 4.  However, the returned object does not have a zoo_id, nor is zoo_id present for the new entry in the Forge.</p>
  
  <p>When I <strong>create</strong> a new model</p>
  
  <pre><code>a.create({name: "ape"})
  </code></pre>
  
  <p>all of the attribute objects logged to the console have zoo_id = 4, the returned object has zoo_id = 4, and the new entry has zoo_id = 4 is in the Forge.</p>
  
  <p>If I remove the Firebase extensions and just use a regular Backbone model and collection in the same manner, <strong>push</strong> returns an object with a zoo_id, and <strong>create</strong> fails as there's no url set up (as expected).</p>
  
  <p>thanks in advance for clarification!</p>