UuidMixin conflicts with BaseScopedIdMixin and BaseScopedIdNameMixin

[jace]

The conflict derives from conflicting uses of the url_id attribute. Both mixins provide it, but one provides a column while the other provides a rendering of a UUID.

In the migration from Talkfunnel.com to HasGeek.com, we want to use both simultaneously, with the scoped numbering on the existing Talkfunnel site and UUID-based URLs on the new site. This requires using both mixins at once, so the conflict needs to be removed somehow. One proposal: move UuidMixin's url_id into another mixin. We are unlikely to be using UUID url_id since the general pattern has been to move to SUUID. However, a survey of existing code is necessary.