migrate to using django-popolo instead of pombola's core models

[mhl]

I'm creating this new ticket to more accurately represent the work involved in this. (We had been using https://github.com/mysociety/pombola/issues/1551 as a placeholder for this job.) This plan probably needs some work.

The great advantage in doing this is that SayIt, when embedded in Pombola, and the rest of Pombola's code would use to the same Person objects. This would remove complex layers of indirection and state, and be a good step in reducing the complexity of maintaining the Pombola ZA scrapers. It's also a good move in terms of code reuse and moving Pombola to be more like a collection of components.

• [x] Subclass the django-popolo models (multi-table inheritance) and add any missing fields which were in the Pombola core models.
• [x] Write migrations to migrate data from the core Pombola models to the new django-popolo models.
• [x] Migrate any other models that refer to the core Pombola models either through ForeignKey or GenericForeignKey
• [ ] Make any use of the original Pombola core models an error (e.g. by replacing the Person class with class Person: pass
• [ ] Update the models that Elasticsearch is indexing (via Haystack) to point to the new models
• [ ] Update all code that uses the original models until the tests pass
• [ ] Update all the rest of the code that uses the original models but isn't covered by tests (adding tests at the time if possible)
• [ ] Update Pombola ZA's SayIt integration to eliminate the SASpeakerRedirectView and the linking of people between saying and Pombola via PopIt IDs (see https://github.com/mysociety/pombola/issues/1551 )
• [ ] Drop the tables with the old core Pombola models
• [ ] :cocktail: :wine_glass: :fireworks:

[davea]

A summary of the progress so far (in 1594-django-popolo-migration):

• Fields required by Pombola and not provided by django-popolo have been identified
• Subclasses of popolo.models.{Organization,Person,Post} have been created as pombola.core.Popolo{Organization,Person,Post} with the missing fields added.
• core.Organisation.name needed 200 chars but popolo.Organization.name is defined with 128 max_length, and you can't redefine fields in model subclasses so we're using our own fork of django-popolo with max_length set to 4000 for a few fields.
• A WIP migration has been created that so far migrates the 'simple' fields for Organisations along with the Contact relations (which are migrated to popolo's ContactDetail model).

I have been keeping some notes whilst working through the process.

[davea]

@dracos points out that mysociety/sayit@46b4dc8bbc66741fbcf203e46dd735b80b52e089 might be handy, too.

[mhl]

@duncanparkes and I have done some more work on the data migration, adding migrations for the rest of Person's relationships as well as Position, Organisation, Identifier, and AlternativePersonName.

Those that still need attention include:

• OrganisationRelationship (might not be in django-popolo)
• Source (except for contact detail sources, which have been dealt with)
• Those models in other applications that reference the core models, e.g. speakers in hansard, budgets, members interests, etc. etc.

[mhl]

After a quick survey, the other models that refer to the core objects that are being changed in this migration are:

• [x] hansard.models.Entry.speaker is a foreign key to Person
• [x] hansard.models.Alias.person is a foreign key to Person
• [x] tasks.models - has a generic foreign key which would need to be checked
• [x] images.models - has a generic foreign key, but that should have been dealt with in the Person / Organisation migrations
• [x] budgets.models - has a generic foreign key which would need to be checked and a foreign key to Organisation
• [x] pombola.bills.models.Bill.sponsor is a foreign key to Person
• [x] spinner.models - has a generic foreign key which would need to be checked
• [x] pombola.members_interests.models.Entry.person is a foreign key to Person
• [x] pombola.scorecards.models.Entry has a generic foreign key.
• [x] pombola.slug_helpers.SlugRedirect has a generic foreign key which would need to be migrated
• [x] pombola.core.models.Contact - check that there are no Contacts that refer to anything other than a Person or Organisation
• [x] pombola.core.models.InformationSource has a generic foreign key which would need to be migrated
• [x] pombola.core.models.Place.organisation is a foreign key to Organisation
• [x] pombola.core.models.ParliamentarySession.house is a foreign key to Organisation
• [x] pombola.core.models.OrganisationRelationship has two foreign keys to Organisation

[mhl]

Things to check:

• [ ] Are the new Haystack search indices being used?
• [ ] We'll need to add Pombola's nice admin enhancement (e.g. inline adding of alternative names and positions on the person admin page) to django-popolo
• [ ] Can we extend the django-popolo admin classes in Pombola? (Presumably yes, they're just classes, but are there any multi-table inheritance gotchas?)