title: Allow update on specific collection property
body: <p>I am trying to add a "ratings" property to a collection and want to enable ANY user (not just the owner) to be able to add a rating to the ratings set within a collection.  My problem is that I have allow/deny rules set up so that only the owner can perform updates to collections that they own.  Is there a way to allow any user to update a collection only if they are updating a specific property (the "ratings" set), and deny them update access if they are trying to update any other property.</p>
  
  <p>My allow/deny rules on the server are as follows...</p>
  
  <pre><code>Playlists.allow({
    insert: function(userId, doc) {
      return (userId &amp;&amp; doc.owner === userId);
    },
    update: function (userId, docs, fields, modifier) {
      return _.all(docs, function(doc) {
        return doc.owner === userId;
      });
    },
    remove: function (userId, docs) {
      return _.all(docs, function(doc) {
        return doc.owner === userId;
      });
    }
  });
  
  Playlists.deny({
    update: function (userId, docs, fields, modifier) {
      return _.contains(fields, 'owner');
    },
    remove: function (userId, docs) {
      return _.any(docs, function (doc) {
        return doc.locked;
      });
    },
    fetch: ['locked']
  });
  </code></pre>