Merge pull request #4279 from jgonggrijp/document-defaults-idAttribute

Add warning to documentation about setting the model id in the defaults

index.html

@@ -1404,6 +1404,16 @@ <h2 id="Model">Backbone.Model</h2>
  Instead, define <b>defaults</b> as a function.
  </p>
 
+ <p class="warning">
+ If you set a value for the model&rsquo;s
+ <a href="#Model-idAttribute"><tt>idAttribute</tt></a>, you should define
+ the defaults as a function that returns a different, <u>universally</u>
+ unique id on every invocation. Not doing so would likely prevent an
+ instance of <tt>Backbone.Collection</tt> from correctly identifying model
+ hashes and is almost certainly a mistake, unless you never add instances
+ of the model class to a collection.
+ </p>
+
  <p id="Model-toJSON">
  <b class="header">toJSON</b><code>model.toJSON([options])</code>
  <br />