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Howto override the enable_conversation method.
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plone.app.discussion way to decide if commenting is enabled on a content
object can be quite complex and cumbersome due to the need to be backward
compatible with the way the old commenting system in Plone (<4.1) worked.

The comments viewlet calls the enabled method of the ConversationView to
decide if the add comment form should show up:

.. literalinclude:: ../../../plone/app/discussion/browser/conversation.py
   :language: python
   :pyobject: ConversationView

If you want to override this behavior, you just have to create your own enabled method. To do so, we first have to register our custom
ConversationView by overriding the existing one in our configure.zcml::

    <configure
      xmlns="http://namespaces.zope.org/zope"
      xmlns:browser="http://namespaces.zope.org/browser"
      i18n_domain="mycustompackage.discussion">

      <!-- Override plone.app.discussion's conversation view -->
      <browser:page
        name="conversation_view"
        for="plone.dexterity.interfaces.IDexterityContent"
        class=".conversation.ConversationView"
        permission="zope2.View"
        />

    </configure>

Now we implement the conversation view with a single "enable" method in
conversation.py::

    from zope.component import queryUtility

    from plone.registry.interfaces import IRegistry

    from Acquisition import aq_inner

    from Products.CMFCore.utils import getToolByName

    from plone.app.discussion.interfaces import IDiscussionSettings


    class ConversationView(object):

        def enabled(self):
            context = aq_inner(self.context)

            # Fetch discussion registry
            registry = queryUtility(IRegistry)
            settings = registry.forInterface(IDiscussionSettings, check=False)

            # Check if discussion is allowed globally
            if not settings.globally_enabled:
                return False

            # Check if discussion is allowed on the content object
            if context.allow_discussion is not None:
                return context.allow_discussion

            # Check if discussion is allowed on the content type
            portal_types = getToolByName(self, 'portal_types')
            document_fti = getattr(portal_types, context.portal_type)
            return document_fti.getProperty('allow_discussion')