Fix references to pages that are to be deleted

[stefanw]

Models (e.g. CMS plugins) that use cms.models.fields.PageField or other references to cms.Page can break with IntegrityError when trying to delete the Pages in the migration_cleanup step.

Before deleting a page:

• find the remaining equivalent Page via same tree node
• find all models that have references to the Page model
• for each model, determine the kind of reference:
  • one-to-one: delete all objects referencing the page as likely the one-to-one to remaining page is already present
  • one-to-many or many-to-many: replace references to page with remaining page

I'm not sure this is the right approach depending on your models, e.g. this will also re-assign CMS Page Permissions which might not be desired. However, this was blocking the deletion of the superfluous Page objects and this code unblocked that step.

[Will-Hoey]

@stefanw has there been any further movement on this?

[stefanw]

@Will-Hoey The code is slightly specific to your site's requirements. It works for us and if it works for more people it might be worth adding as an option.

[Will-Hoey]

@stefanw thanks for the reply! I ended up forking your branch and extending it slightly to fit my case