title: FIELDS_CHANGE mutation with connection field
body: <p>I'm trying to understand how Relay works.</p>
  
  <p>1.) Let's say I have <strong>UpdateProductMutation</strong> (FIELDS_CHANGE type) that updates fields of single product i.e. 'title', 'description', etc. I can send mutation with those fields changes and it works. (Say the product here is <code>Banana</code>)</p>
  
  <p>2.) Now I add <strong>product_categories</strong> field of type <em>ProductCategoryConnection</em> to the Product type. And in the <strong>UpdateProductMutation</strong> mutation, I send "catIds" array as additional inputField of the mutation. (i.e. <code>catIds: {type: new GraphQLList(GraphQLID)}</code>) It also successfully mutates the Product with the <em>product_categories</em> field.</p>
  
  <p>3.) To make it easier to follow, let's say <code>Banana</code> product had <code>Fruit</code> and <code>Yellow</code> categories. And I've added <code>Healthy</code> in step 2.) </p>
  
  <p>4.) Problem is, if I've queried <code>Healthy</code> page before step 2. and Relay already has the cache in store, visiting <code>Healthy</code> page again after step 2. doesn't show newly associated product, <code>Banana</code>.</p>
  
  <p>This is my mutation config and fat query.</p>
  
  <pre><code>getFatQuery() {
    return Relay.QL`
      fragment on UpdateProductPayload {
        updatedProduct,
      }
    `;
  }
  
  getConfigs() {
    console.warn('getConfigs', this.props);
    return [{
      type: 'FIELDS_CHANGE',
      fieldIDs: {updatedProduct: this.props.id},
    }];
  }
  </code></pre>
  
  <p>I'm not sure if I understand it totally wrong. It seems like FIELDS_CHANGE is not the correct type to use here because the mutation works fine and the mutation payload gives the changed product category edge. The problem occurs only when I go to the previously fetched related product category page and don't see the updated product there. (Relay doesn't even send the query again). From my understanding, RANGE_ADD or RANGE_DELETE don't allow you to change the normal field either (i.e. 'title', 'description', etc.) Also in my case, catIds array could either cause RANGE_ADD or RANGE_DELETE, depends on the selection on the page.</p>
  
  <p>Any advise on which direction I should take?</p>