laravel-auditing: Can't get Audit Users when User Model uses non-default Primary Key

Q	A
Bug?	yes
New Feature?	no
Framework	Laravel
Framework version	5.4.25
Package version	4.0.7
PHP version	7.0.18

Actual Behaviour

It is not possible to get a Model Audit’s User if the User Model uses a non-default Primary Key (ie. a Primary Key different than id).

Expected Behaviour

It should be possible 😃

Steps to Reproduce

• Setup Laravel and Auditing.
• Change the App\User model’s Primary Key to something else (like some_id) both in the Model (via the protected $primaryKey property) and the database.
• Add an auditable model:

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;
use OwenIt\Auditing\Auditable;
use OwenIt\Auditing\Contracts\Auditable as AuditableContract;

class SomeModel extends Model implements AuditableContract
{
    use Auditable;

    public $timestamps = false;

    /**
     * The attributes that are mass assignable.
     *
     * @var array
     */
    protected $fillable = [
        'field1', 'field2', 'field3',
    ];
}

• Add a table for the new model with a migration with:

Schema::create('some_models', function (Blueprint $table) {
        $table->increments('id');
        $table->string('field1');
        $table->string('field2');
        $table->string('field3');
    });

• Run a simple test like:

public function testAudits()
{
    $modelFields = [
        'field1' => 'Field '.rand(),
        'field2' => 'mail'.rand().'@mail.com',
        'field3' => 'password'.rand(),
    ];

    $model = new SomeModel($modelFields);

    // set a proper ID in here
    Auth::onceUsingId(1);
    $model->save();

    // get model with audits and users
    $model = SomeModel::with('audits.user')->find($model->getKey());

    echo "model: \n";
    print_r($model->attributesToArray());

    $audit = $model->audits->first();

    echo "audit: \n";
    print_r($audit->attributesToArray());

    $user = $audit->user;

    echo "user: \n";
    print_r($user->attributesToArray());
}

Result:

1) Tests\Unit\SomeModelTest::testAudits
Error: Call to a member function attributesToArray() on null

(on the print_r($user->attributesToArray()); line)

Possible Solutions

I got it working by adding a 'user_id' parameter to the Audit::model() method so it looks like:

public function user()
{
    return $this->belongsTo(Config::get('audit.user.model'), 'user_id');
}