Merge pull request #2350 from hshoff/polymorphic

add documentation about collections containing polymorphic models. Fixes...

index.html

@@ -1493,6 +1493,25 @@ bill.set({name : "Bill Jones"});
 var Library = Backbone.Collection.extend({
   model: Book
 });
+</pre>
+
+    <p>
+      A collection can also contain polymorphic models by overriding this property
+      with a function that returns a model.
+    </p>
+
+<pre>
+var Library = Backbone.Collection.extend({
+
+  model: function(attrs, options) {
+    if (condition) {
+      return new PublicDocument(attrs, options);
+    } else {
+      return new PrivateDocument(attrs, options);
+    }
+  }
+
+});
 </pre>
 
     <p id="Collection-constructor">