Related model uses parent connection when $connection is null

[lombervid]

Laravel Version

11.19.0

PHP Version

8.3.8

Database Driver & Version

MySQL 5.7.44

Description

Having a model, without $connection property specified, as a relation from another model with explicit $connection causes the child to use the parent's connection instead of the default connection.

I found out this behavior was implemented in #16103 but I think it might not be the desired behavior.
According with what the documentation says, it should use the default connection (and that was exactly what I was expecting):

By default, all Eloquent models will use the default database connection that is configured for your application. If you would like to specify a different connection that should be used when interacting with a particular model, you should define a $connection property on the model:

Proposed solution:

I think it could be implemented something like this:

Model.php

<?php

namespace Illuminate\Database\Eloquent;

abstract class Model
{
    // ...
    protected $modelShouldInheritConnection = false;

    // ...

    public function inheritsConnection(): bool
    {
        return $this->$modelShouldInheritConnection;
    }

    // ...
}

HasRelationships.php

<?php

namespace Illuminate\Database\Eloquent\Concerns;

trait HasRelationships
{
    // ...

    protected function newRelatedInstance($class)
    {
        return tap(new $class, function ($instance) {
            if ($instance->inheritsConnection() && ! $instance->getConnectionName()) {
                $instance->setConnection($this->connection);
            }
        });
    }

    // ...
}

Then in the model you want to inherit the parent's connection:

<?php

namespace App\Models;

use Illuminate\Database\Eloquent\Model;

class Child extends Model
{
    protected $modelShouldInheritConnection = true;
}

Or it could be set to true by default so as not to break the current behavior.

Kind regards!

Steps To Reproduce

Child Model:

<?php

namespace App\Models;

use Illuminate\Database\Eloquent\Model;

class Child extends Model
{
    //
}

Parent Model:

<?php

namespace App\Models;

use Illuminate\Database\Eloquent\Model;
use Illuminate\Database\Eloquent\Relations\BelongsTo;

class Parent extends Model
{
    protected $connection = 'another';

    public function child(): BelongsTo
    {
        // `Child` model will use `another` connection instead of the default
        return $this->belongsTo(Child::class);
    }
}