WIP: Add default many-to-many support

[bmedx]

Work in progress. Needs code cleanup, documentation added, and further testing (especially in the admin site) but seems to work as expected.

Adds support for Django's default many-to-many models (ones without explicit an through)

Description

Allows m2m fields to be explicitly added to the history of a model which uses HistoricalRecords. It allows access to the m2m history via a historical_{m2m_field_name} attribute.

When the parent model adds a history row, all connected m2m fields store all existing connections with a FK back to the history row. When the m2m mapping is updated (add, remove, clear) a new history row is created, firing off the same process.

Related Issue

jazzband#399
jazzband#370
jazzband#16

Motivation and Context

We need to track changes in many-to-many relationships over time.

How Has This Been Tested?

Added unit tests to cover everything I could think of.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have run the make format command to format my code
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have added my name and/or github handle to AUTHORS.rst
• I have added my change to CHANGES.rst
• All new and existing tests passed.

[kb3dow]

I had a question: I am using the AdminModel, how do I see the m2m field corresponding to a historical object on the change object screen. I am using it as a non-admin user (though I have admin access too). Any historical object chosen from the admin list display screen still shows the most recent m2m values on the screen (showing details of a single object). On the list display screen I was however able to see historical m2m values by looking at the HistoricalXYZ object.

Doesn't the admin.py:history_form_view() also need a change ?