[SOLVED] How to restrict the creation of an object?

I need to restrict the creation of a custom (Dexterity) content type to only one instance (per site). How? It should be trivial...

Add an event subscriber for zope.lifecycleevent.interfaces.IObjectAddedEvent that would implement such a check e.g. by performing a catalog search. Raise an exception in case ...
Disclaimer: exceptions from event handlers are possibly swallowed somewhere in Plone

https://docs.plone.org/external/plone.app.dexterity/docs/advanced/event-handlers.html

And there is also some "magic" in Zope/OFS/ObjectManager ... setting __replaceable__ = UNIQUE on an object may help, perhaps.

I had a similar requirement which had some edge cases: do test the validator that the catalog search is not finding the singleton item you just created and therefore blocks.

(Or for example when moving the Item, which would trigger a new ObjectAddedEvent.)

Try and see if disabling "Implicitly allowed" (or global allow, or something) for the content type is good enough.

This can be set in the ( /@@content-controlpanel ) control panel

Replying to myself:

I added a init to the content type class and used portal_catalog to check the number of indexed objects of that type. Something like:

from plone import api
from plone. dexterity.content import Item

class MyItem(Item):
    def __init__(self, id=None, **kwargs):
        catalog = api.portal.get_tool('portal_catalog')
        brains = catalog(portal_type='mytype')
        if brains:
            raise Exception('Only one "mytype" object is allowed per Plone instance')

    super(MyItem, self).__init__(id, **kwargs)

Thanks to all.

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