表与hasManyThrough之间的关系

Question

I got 3 models in Laravel (User, Payment, Purchase).

What I want is pretty simple but i've no idea how to do it with Laravel/Eloquent ; I would succeed with raw SQL.

I want to get the purchases of a user without much code.

The users table is linked with the payments table which is linked to the purchases table. The finale code would look like

$purchases = $user->purchases()->get();

My models look like this

So I tried a simple

public function purchases()
{

    return $this->hasManyThrough('Purchase', 'Payment');

}

But it doesn't work

SQLSTATE[42S22]: Column not found: 1054 Unknown column 'purchases.payment_id' in 'on clause' (SQL: select purchases.*, payments.user_id from purchases inner join payments on payments.id = purchases.payment_id where payments.user_id = 1)

Pretty simple here, it searches purchases.payment_id but my table organization needs a join with payment.purchase_id. How can you tell that to Laravel ? Is there another way to do what I want ?

Maybe I could do it in raw SQL but keep the object generated via Eloquent ? I'm open to any solution ^^

Detail : I cannot rewrite my tables.