[Feature Request] Allow sequences in factories Belongs To relations

[dmohns]

Problem description:

When seeding data with a factory that implements a Belongs To relation to another entity we sometimes encounter a situation where we would like to create the entity belong to any, random parent.

For example, in case of the Users/Posts example from the Laravel documentation.

Assume we have already created Users in a previous steps, and all Users are available in $users (for example $users = User:all() or via output from a different factory).

Now, I want to create X number of Posts and have them assigned to distinct users, but I don't really care which.

Proposed solutions:

$posts = Post::factory()
            ->count(3)
            ->for(new Sequence(User::all()))
            ->create();

Work arounds:

Currently, to achieve the above we have to do some work arounds, for example

1. Looping around: We can loop over $users and create Posts inside this loop and assign them to the current loop's user.
2. Manually setting parent_id: Using state we can "manually" set user_id as state works with sequences. But this code can get really messy and unintuitive, especially when using polymorphic relations.