[12.x] Allow fillAndInsert() usage with HasMany relationships by automatically setting the foreign key

[KentarouTakeda]

This pull request is a follow-up to #55038 introduced in v12.6.0 by @cosmastech. While fillAndInsert() was successfully added to allow merging attributes before insertion, it can cause errors when used on a HasMany relationship if the related table enforces a NOT NULL constraint on the foreign key.

For instance, the following snippet will fail on certain table definitions:

• Repro code

  $user->posts()->fillAndInsert([
      [ /* omission */ ]
  ]);

• Error message

  Integrity constraint violation: 19
    NOT NULL constraint failed: posts_having_uuids.user_id
  

• SQL

  insert into "posts_having_uuids"
    ("created_at", "id", "name", "published_at", "status", "status_string", "updated_at", "uuid")
  values
    /* omission */

With the current implementation (without this PR), the user_id column is not automatically populated, resulting in a constraint violation. This PR ensures that fillAndInsert() can correctly handle the foreign key column for HasMany relationships, preventing such errors.

(Above snippet and error details are included within the tests to demonstrate the exact failure under real conditions.)

[taylorotwell]

I'll need @cosmastech's opinion, but I'm not certain this method was ever meant to be used with relationships.

[cosmastech]

I'll need @cosmastech's opinion, but I'm not certain this method was ever meant to be used with relationships.

@taylorotwell I see no reason not to extend it to relationships. I believe it currently would be available, just wouldn't work as a user might expect, since it's not explicitly setting the relationship values. (edit: insert() also doesn't really work with setting the default relationship values 🤷, so maybe that could be fixed too?)

I haven't tested this myself, so I can't confirm the functionality, but conceptually I think it makes sense.

[KentarouTakeda]

@cosmastech
It's easy to add to insert(), but I think it should be considered carefully for the following reasons:

• insert() without fill will automatically fill the parent_id
• It will change behavior that has existed for a long time

In my opinion, it would be better to postpone the fix in minor versions or discuss it in another forum.

[cosmastech]

@cosmastech It's easy to add to insert(), but I think it should be considered carefully for the following reasons:

• insert() without fill will automatically fill the parent_id
• It will change behavior that has existed for a long time

In my opinion, it would be better to postpone the fix in minor versions or discuss it in another forum.

@KentarouTakeda makes sense to me. 👍