`has_many` field doesn't let inline creation if foreign key is not nullable

[tognee]

Description

I have two models, let's say Folder and File.
Folder has many Files, and File belongs to folder.

I've set these so that the file must have a folder (folder_id foreign key set as non nullable).

I go and create a folder, then with the has_many fieldtype I go and create the file. When I try to save the file I get the error folder_id doesn't have a default value because it's null.

The current flow as I understood it is:

• When you save the child you save it as is without the reference to the parent.
• The reference to the parent gets saved in the child element only when you save the parent as well.
• When creating the child you cannot save both, hence the parent id must be nullable to save it.

The current workaround I found is to make the foreign keys nullable, but then you cannot have the option to delete the children, as the only option that's there now is to unlink them.

Steps to reproduce

1. Create some models with the same relationship as the example (with non nullable foreign kets)
2. Add them to as a runway resource
3. Create a blueprint for both, with an has_many fieldtype in the parent and a belongs_to fieldtype on the child.
4. Try to create a child from a saved parent.

Environment

Environment
Laravel Version: 12.56.0
PHP Version: 8.4.19
Composer Version: 2.8.4
Environment: local
Debug Mode: ENABLED
Maintenance Mode: OFF
Timezone: UTC
Locale: it

Cache
Config: NOT CACHED
Events: NOT CACHED
Routes: NOT CACHED
Views: CACHED

Drivers
Broadcasting: log
Cache: file
Database: mysql
Logs: stack / single
Mail: smtp
Queue: sync
Session: file

Storage
public/storage: NOT LINKED

Statamic
Addons: 2
Sites: 1
Stache Watcher: Enabled (auto)
Static Caching: Disabled
Version: 6.11.0 Solo

Statamic Addons
statamic-rad-pack/runway: 9.3.3
statamic/eloquent-driver: 5.6.2

Statamic Eloquent Driver
Addon Settings: file
Asset Containers: file
Assets: eloquent
Blueprints: file
Collection Trees: file
Collections: file
Entries: eloquent
Fieldsets: file
Form Submissions: eloquent
Forms: file
Global Sets: file
Global Variables: file
Navigation Trees: file
Navigations: file
Revisions: file
Sites: file
Taxonomies: file
Terms: file
Tokens: eloquent