title: Updating old add-on with portal_tool from Plone 2.1 (Archetypes) to 4.3 (Dexterity)
body: <p>I'm trying to update an plone 2.1 add-on with 2 content types (Archetypes) to Plone 4 (Dexterity).</p>
  
  <p>One of this content types (foo) acts like a portal_tool, which is a unique 'container' where the another content type (bar) with some fields must be added (and just on this 'container').</p>
  
  <p>I'm not sure if there's a way to do this on Dexterity, as before on Archetypes. Also, is a custom portal_tools, in this case, a viable way in Plone 4.x+ or is deprecated?</p>
  
  <p>Here some excerpts (from AT):</p>
  
  <p>foo.py:</p>
  
  <pre><code>class foo(UniqueObject, BaseFolder):
      ...
      __implements__ = (getattr(UniqueObject,'__implements__',()),) + (getattr(BaseFolder,'__implements__',()),)
      ...
      allowed_content_types = ['bar']
      ...
      def __init__(self, id=None):
          BaseFolder.__init__(self,'portal_foo')
          self.setTitle('Foo')
      ...
  </code></pre>
  
  <p>bar.py: </p>
  
  <pre><code>class bar(BaseContent):
      ...
  </code></pre>