Log pre-existing object state when creating an ObjectChange instance

[jeremystretch]

Environment

• Python version: 3.7.9
• NetBox version: 2.10.5

Proposed Functionality

When creating a new ObjectChange instance to log a change to a NetBox object, capture its pre-change state as well as its current state. This may take a bit of cleverness to implement, but it should be possible by using a custom signal to register a pre-change copy of data on an instance immediately prior to modifying it.

Use Case

Storing both the pre- and post-change copies of an object allows us to present a more accurate depiction of that changes attributable to a specific action, and not worry about indirect changes not being accounted for. Related issues:

• Provide a diff showing the changes between two logged changes #2511: Provide a diff showing the changes between two logged changes (implemented)
• WebHook should provide old data together with new data #3451: WebHook should provide old data together with new data
• Unnecessary changelog records for tag add/change/remove #5583: Unnecessary changelog records for tag add/change/remove

Database Changes

Add a field to the ObjectChange model for storing pre-change data.

External Dependencies

None expected

[jeremystretch]

This is what I've come up with so far: The ObjectChange model has been extended to store both pre- and post-change data. When a request is processed, the responsible view calls snapshot() on the object being modified or deleted, which saves a serialized representation of the object on the object itself generated by the serialize_object() utility. After the change has been made, serialize_object() is called on the object once more to catch the post-change data. Both of these serialized representations are then stored in the new ObjectChange instance.

I'd like to add some GitHub-style diff highlighting to the UI, but the important bits are all there. I also still need to write changelog tests for bulk edit and delete views.

[jeremystretch]

Now with diff highlighting!

I expect we'll put a bit more polish on it in conjunction with the ongoing UI overhaul (#5893) prior to release.