title: Zope z3c.relationfield RelationList and wrong permissions for custom content type
body: <p>I have a simple content type with a few fields one of which uses RelationList to relate to items of another content type. As a simple user I can create the content type, and select the related items. This is saved ok, however if I edit the content type the list is empty . If I assign the role 'manager' to the user, then if I edit the content type I can see the items (as expected). 
  How can I set the correct permission so that editing as a user would contain the items? I tried to find anything related inside the ZMI and sections as security/workflow/permissions etc but this hasn't worked for me.</p>
  
  <p>Any suggestions highly welcome.</p>
  
  <pre><code>from z3c.relationfield.schema import RelationList
  
  class ILecture(form.Schema):
  
      form.widget(relatedItems=QuerySourceFieldCheckboxWidget)
      relatedItems = RelationList(
          title=u"Clips",
          description=,
          default=[],
          value_type=RelationChoice(title=_(u"Related"),
                     source=ObjPathSourceBinder(object_provides=IPlumiVideo.__identifier__)),
          required=True,
     )
  </code></pre>
  
  <p>YG. I've <a href="https://github.com/zopefoundation/z3c.relationfield/issues/5" rel="nofollow">posted</a> that but since I'm not sure how active the project is I post it here too. </p>