cashier-stripe: Failure when using custom primary key for user model.

I am having an issue with the BIllable trait when using a custom primary key of user_id. The issue seems to have been introduced in #337. The getForeignKey method in Billable assumes that the primary key is id and prepends user_ to it. However if I use user_id I end up with user_user_id which fails.

For now I have overridden the method in the user model to just return the unmodified key.

QueryException
SQLSTATE[42703]: Undefined column: 7 ERROR: column subscriptions.user_user_id does not exist
LINE 1: select * from "subscriptions" where "subscriptions"."user_us...
^ (SQL: select * from "subscriptions" where "subscriptions"."user_user_id" = 1 and "subscriptions"."user_user_id" is not null order by "created_at" desc)
in Connection.php (line 647) at Connection->runQueryCallback('select * from "subscriptions" where "subscriptions"."user_user_id" = ? and "subscriptions"."user_user_id" is not null order by "created_at" desc', array(1), object(Closure))
in Connection.php (line 607) at Connection->run('select * from "subscriptions" where "subscriptions"."user_user_id" = ? and "subscriptions"."user_user_id" is not null order by "created_at" desc', array(1), object(Closure))
in Connection.php (line 326) at Connection->select('select * from "subscriptions" where "subscriptions"."user_user_id" = ? and "subscriptions"."user_user_id" is not null order by "created_at" desc', array(1), true)
in Builder.php (line 1710) at Builder->runSelect()
in Builder.php (line 1695) at Builder->get(array('*'))
in Builder.php (line 462) at Builder->getModels(array('*'))
in Builder.php (line 446) at Builder->get()
in HasMany.php (line 16) at HasMany->getResults()
in HasAttributes.php (line 407) at Model->getRelationshipFromMethod('subscriptions')
in HasAttributes.php (line 386) at Model->getRelationValue('subscriptions')
in HasAttributes.php (line 316) at Model->getAttribute('subscriptions')
in Model.php (line 1265) at Model->__get('subscriptions')
in Billable.php (line 202) at User->subscription('main')
in Billable.php (line 180) at User->subscribed('main')
in HomeController.php (line 33) at HomeController->index() at call_user_func_array(array(object(HomeController), 'index'), array())
in Controller.php (line 55) at Controller->callAction('index', array())
in ControllerDispatcher.php (line 44) at ControllerDispatcher->dispatch(object(Route), object(HomeController), 'index')
in Route.php (line 203) at Route->runController()
in Route.php (line 160) at Route->run()
in Router.php (line 574) at Router->Illuminate\Routing\{closure}(object(Request))
in Pipeline.php (line 30) at Pipeline->Illuminate\Routing\{closure}(object(Request))
in SubstituteBindings.php (line 41) at SubstituteBindings->handle(object(Request), object(Closure))
in Pipeline.php (line 148) at Pipeline->Illuminate\Pipeline\{closure}(object(Request))
in Pipeline.php (line 53) at Pipeline->Illuminate\Routing\{closure}(object(Request))
in Authenticate.php (line 43) at Authenticate->handle(object(Request), object(Closure))
in Pipeline.php (line 148) at Pipeline->Illuminate\Pipeline\{closure}(object(Request))
in Pipeline.php (line 53) at Pipeline->Illuminate\Routing\{closure}(object(Request))
in VerifyCsrfToken.php (line 65) at VerifyCsrfToken->handle(object(Request), object(Closure))
in Pipeline.php (line 148) at Pipeline->Illuminate\Pipeline\{closure}(object(Request))
in Pipeline.php (line 53) at Pipeline->Illuminate\Routing\{closure}(object(Request))
in ShareErrorsFromSession.php (line 49) at ShareErrorsFromSession->handle(object(Request), object(Closure))
in Pipeline.php (line 148) at Pipeline->Illuminate\Pipeline\{closure}(object(Request))
in Pipeline.php (line 53) at Pipeline->Illuminate\Routing\{closure}(object(Request))
in StartSession.php (line 64) at StartSession->handle(object(Request), object(Closure))
in Pipeline.php (line 148) at Pipeline->Illuminate\Pipeline\{closure}(object(Request))
in Pipeline.php (line 53) at Pipeline->Illuminate\Routing\{closure}(object(Request))
in AddQueuedCookiesToResponse.php (line 37) at AddQueuedCookiesToResponse->handle(object(Request), object(Closure))
in Pipeline.php (line 148) at Pipeline->Illuminate\Pipeline\{closure}(object(Request))
in Pipeline.php (line 53) at Pipeline->Illuminate\Routing\{closure}(object(Request))
in EncryptCookies.php (line 59) at EncryptCookies->handle(object(Request), object(Closure))
in Pipeline.php (line 148) at Pipeline->Illuminate\Pipeline\{closure}(object(Request))
in Pipeline.php (line 53) at Pipeline->Illuminate\Routing\{closure}(object(Request))
in Pipeline.php (line 102) at Pipeline->then(object(Closure))
in Router.php (line 576) at Router->runRouteWithinStack(object(Route), object(Request))
in Router.php (line 535) at Router->dispatchToRoute(object(Request))
in Router.php (line 513) at Router->dispatch(object(Request))
in Kernel.php (line 176) at Kernel->Illuminate\Foundation\Http\{closure}(object(Request))
in Pipeline.php (line 30) at Pipeline->Illuminate\Routing\{closure}(object(Request))
in Debugbar.php (line 51) at Debugbar->handle(object(Request), object(Closure))
in Pipeline.php (line 148) at Pipeline->Illuminate\Pipeline\{closure}(object(Request))
in Pipeline.php (line 53) at Pipeline->Illuminate\Routing\{closure}(object(Request))
in TransformsRequest.php (line 30) at TransformsRequest->handle(object(Request), object(Closure))
in Pipeline.php (line 148) at Pipeline->Illuminate\Pipeline\{closure}(object(Request))
in Pipeline.php (line 53) at Pipeline->Illuminate\Routing\{closure}(object(Request))
in TransformsRequest.php (line 30) at TransformsRequest->handle(object(Request), object(Closure))
in Pipeline.php (line 148) at Pipeline->Illuminate\Pipeline\{closure}(object(Request))
in Pipeline.php (line 53) at Pipeline->Illuminate\Routing\{closure}(object(Request))
in ValidatePostSize.php (line 27) at ValidatePostSize->handle(object(Request), object(Closure))
in Pipeline.php (line 148) at Pipeline->Illuminate\Pipeline\{closure}(object(Request))
in Pipeline.php (line 53) at Pipeline->Illuminate\Routing\{closure}(object(Request))
in CheckForMaintenanceMode.php (line 46) at CheckForMaintenanceMode->handle(object(Request), object(Closure))
in Pipeline.php (line 148) at Pipeline->Illuminate\Pipeline\{closure}(object(Request))
in Pipeline.php (line 53) at Pipeline->Illuminate\Routing\{closure}(object(Request))
in Pipeline.php (line 102) at Pipeline->then(object(Closure))
in Kernel.php (line 151) at Kernel->sendRequestThroughRouter(object(Request))
in Kernel.php (line 116) at Kernel->handle(object(Request))
in index.php (line 53) at require('/Users/user/laravel/project/public/index.php')
in server.php (line 133)