title: Inverse of ._meta to get Class of Reverse FK
body: <p>If I have a class, to get the FK class, I can do:</p>
  
  <pre><code>MasterModel = Model._meta.get_field('master').rel.to
  </code></pre>
  
  <p>Given a MasterModel, how would I get the reverse FK? Something like:</p>
  
  <pre><code>Model = MasterModel.???
  </code></pre>
  
  <p>Currently, to get around this, I am doing:</p>
  
  <pre><code>    if item_type == 'TV Episode' or item_type == 'Movie':
          InstanceModel = Catalog
      elif item_type == 'TV Season':
          InstanceModel = CollectionInstance
      elif item_type == 'TV Series':
          InstanceModel = TVSeriesInstance
  </code></pre>