Refactor follow activities and test data previously

[aaronjae22]

Refactor FollowActivity

Following what we did with the LikeActivity, we got the following refactor for FollowActivity:

• Added target_actor_url and target_actor_data fields for remote actors
• Updated the clean() method to validate that either local or remote data is provided
• get_json_ld handle both local and remote actors

The first one is a Remote Follow Activity and the second one is a Local Follow Activity.

We are able to represent both scenarios, when an Actor follows another Actor within the same server and the other one in which an Actor follows another one from other server.

Closes #72

Adding test data in previously field

previously field in the Actor model should contain historical identifiers or data about the account if it was moved/migrated from another server.

In a real federation scenario, it might look like:

"previously": [
    {
        "type": "Move",
        "object": "https://mastodon.social/users/alice",
        "published": "2023-07-20T00:00:00Z"
    },
    {
        "type": "Move",
        "object": "https://pixelfed.social/users/alice",
        "published": "2022-12-01T00:00:00Z"
    }
]

• First, we extend the Actor model with a method to handle moves
• Then, modify the seed command to create some actors with history

Thoughts

During seeding I thought that it doesn't make sense for every Actor to have a previously field filled. The previously field should only be populated for actors that have actually moved or migrated from another server.

So in our seed data, it would be more realistic to:

1. Have most actors with empty previously (new accounts)
2. Only give some actors a migration history

With no move:

With move history:

previously field being display in Actors outboxes when referencing FollowActivities and so on?

I figured that when a FollowActivity or other activities reference an Actor with a migration history, that history should be part of the Actor's JSON-LD representation in those activities.

So when we see a Follow activity in the outbox, if it's following a local actor who has migration history, we’ll see something like the following scenario:

For example, if Bob moved from mastodon.social to our server, and Alice follows Bob, the activity should show:

{
    "type": "Follow",
    "actor": "https://example.com/users/alice",
    "object": {
        "type": "Person",
        "id": "https://example.com/users/bob",
        "preferredUsername": "bob",
        "previously": [
            {
                "type": "Move",
                "object": "https://old-server.com/users/bob",
                "published": "2025-03-20T00:00:00Z"
            }
        ]
    }
}

I believe this is good because it maintains the portability information throughout the activity stream.

This is only for Actors who has moved or migrated from one server to another. I am sure we'll need to add previously data to each Activity when doing migration.

Closes #52

[alexeyqu]

Do we still need a for here then? Perhaps we are planning to change this param later?
If so, maybe we can move this to a const or some parameter (if we want to set this dynamically during runtime)?

[aaronjae22]

The seeding in here is just trying to recreate the FollowActivities; as you said, I agree if we move it to a constant.

Maybe we could work in some other way to create the seeding. Any idea is good.

[alexeyqu]

ditto + in similar for loops across the file

[alexeyqu]

IIUC previously is semantically similar to the stack behaviour?
https://swicg.github.io/activitypub-data-portability/lola.html#adding-breadcrumbs

[aaronjae22]

Yes, it is

[alexeyqu]

since we explicitly convert it to an empty list in several places (i.e. self.previously or [] on L77) why don't we set null=False here?

[aaronjae22]

I agree. Makes sense.

[alexeyqu]

Left some comments -- overall looks good to me, however I'll defer the "does the model fit the proposal?" to someone else 😉