saveMany refering to a wrong table

Hi there, I have three tables with relationships:

Event - id / title / etc...

Corporate - id / name / etc...

EventCorporate - id / event_id / corporate_id. ( the ID field might not be necessary but I'm keeping it for now )

And each model looks like the following:

class Event extends Model
{
    protected $fillable = [
        'title',
        ...
    ];

    public function corporates_attending_events(){
        return $this->hasMany(EventCorporate::class);
    }

    public function add_corporate( $corporate ){
        $method = ( $corporate instanceof Corporate ) ? 'save' : 'saveMany';
        $this->corporates_attending_events()->$method( $corporate );

    }

    // and more codes...
}


class Corporate extends Model
{
    protected $fillable = [
        'name',
        'description',
        'link',
        'thumbnail',
        'is_active'
    ];

    // and more codes...
}


class EventCorporate extends Model
{

    protected $table = 'event_corporate';

    protected $fillable = [
        'event_id',
        'corporate_id',
        'is_active'
    ];

    public function event(){
        return $this->hasOne(Event::class);
    }

    public function corporate(){
        return $this->hasOne(Corporate::class);
    }

    // and more codes...
}

When I run the following unit test for Event:

public function it_adds_a_coporate_to_the_event()
    {
        $event = create_an_event();
        $corporate = factory(Corporate::class)->create();
        $event->add_corporate($corporate);
        $this->assertEquals(1, $event->corporates_attending_events->count());
    }

The unit test fails with the following error message:

Illuminate\Database\QueryException: SQLSTATE[HY000]: General error: 1 no such column: event_id (SQL: update "corporates" set "event_id" = 1, "updated_at" = 2019-01-11 21:26:08 where "id" = 1)

The first thing I instantly noticed is that add_corporate(...) in Event motel is trying to update a wrong table - coporates. I thought corporates_attending_events() is pointing to event_corporate table due to the relationship created above... but it seems not.

Where is this thing coming from?

Did I set the relationships incorrectly?

The source code for save/saveMany is from Jeffrey's tutorial ( https://laracasts.com/series/phpunit-testing-in-laravel/episodes/6 ). For those who cannot access laracasts, the source files are here (https://github.com/laracasts/Hands-On-Testing-Workflow)

Thanks!