[Feature] Support using Models as pivot tables

[tabacitu]

Bug report

What I did

Added Hero to the demo, which should be using monsters as the pivot table. The idea was that... if Monster were a pivot table... we would be able to test ALL field types, if they work as pivot fields.

That would help us fill in the last column in this table.

What I expected to happen

Create / Update pages to load.

What happened

What I've already tried to fix it

Nothing yet, since this is not exactly urgent. We can add this later, as a feature. I think what fields we support right now as pivot subfields are already MORE than enough for most projects.

Basically, what I think we need to do is:

• create a new MonsterPivot model, that extends Illuminate\Database\Eloquent\Relations\Pivot.. or better yet, just has those few methods and properties on it and extends Monster;
• make sure Monster has all relationships it needs;
• test all relationship fields, that they work as pivot subfields;

Is it a bug in the latest version of Backpack?

v4.2/v5 - whatever you wanna call it

[tabacitu]

Note - until this is done, people can still use fields that do NOT represent a relationship on a Model. Like... select_from_array and select2_from_array.

[tabacitu]

@pxpm this has come up in https://stackoverflow.com/questions/72793952/many-to-many-column-not-found-issue-in-subfield-relationships

How difficult do you think this would be to implement - where would we need changes?

[pxpm]

@tabacitu What does not work is repeatable over repeatable, but using selects it works: https://recordit.co/SuEWKUAAoR

[kasperbjerby]

If we could use a model as a pivot table, would that also allow us to add "RevisionableTrait" to it and set things like "touches"? Cause as of right now, i don't see how i can update the parents "updated_at" when something changes in the pivot table and i also can't see the changes in the revisions view.. Would this maybe allow for that?