I've got a local environment setup and my code runs fine, however when I deploy to forge I get a BadMethodCallException. Output below:

BadMethodCallException in Builder.php line 1999: Call to undefined method Illuminate\Database\Query\Builder::getSlugModelOrFail() in Builder.php line 1999 at Builder->__call('getSlugModelOrFail', array('maths')) at Builder->getSlugModelOrFail('maths') at call_user_func_array(array(object(Builder), 'getSlugModelOrFail'), array('maths')) in Builder.php line 933 at Builder->__call('getSlugModelOrFail', array('maths')) at Builder->getSlugModelOrFail('maths') at call_user_func_array(array(object(Builder), 'getSlugModelOrFail'), array('maths')) in Model.php line 3362 at Model->__call('getSlugModelOrFail', array('maths')) at Subject->getSlugModelOrFail('maths') at call_user_func_array(array(object(Subject), 'getSlugModelOrFail'), array('maths')) in Model.php line 3376 at Model::__callStatic('getSlugModelOrFail', array('maths')) in SubjectBelongsToUser.php line 19 at Subject::getSlugModelOrFail('maths') in SubjectBelongsToUser.php line 19 at SubjectBelongsToUser->handle(object(Request), object(Closure)) in Pipeline.php line 125 at Pipeline->Illuminate\Pipeline{closure}(object(Request)) in Authenticate.php line 44 at Authenticate->handle(object(Request), object(Closure)) in Pipeline.php line 125 at Pipeline->Illuminate\Pipeline{closure}(object(Request)) at call_user_func(object(Closure), object(Request)) in Pipeline.php line 101 at Pipeline->then(object(Closure)) in ControllerDispatcher.php line 108 at ControllerDispatcher->callWithinStack(object(SubjectsController), object(Route), object(Request), 'show') in ControllerDispatcher.php line 67 at ControllerDispatcher->dispatch(object(Route), object(Request), 'App\Http\Controllers\SubjectsController', 'show') in Route.php line 204 at Route->runWithCustomDispatcher(object(Request)) in Route.php line 134 at Route->run(object(Request)) in Router.php line 701 at Router->Illuminate\Routing{closure}(object(Request)) at call_user_func(object(Closure), object(Request)) in Pipeline.php line 141 at Pipeline->Illuminate\Pipeline{closure}(object(Request)) at call_user_func(object(Closure), object(Request)) in Pipeline.php line 101 at Pipeline->then(object(Closure)) in Router.php line 703 at Router->runRouteWithinStack(object(Route), object(Request)) in Router.php line 670 at Router->dispatchToRoute(object(Request)) in Router.php line 628 at Router->dispatch(object(Request)) in Kernel.php line 214 at Kernel->Illuminate\Foundation\Http{closure}(object(Request)) at call_user_func(object(Closure), object(Request)) in Pipeline.php line 141 at Pipeline->Illuminate\Pipeline{closure}(object(Request)) in VerifyCsrfToken.php line 43 at VerifyCsrfToken->handle(object(Request), object(Closure)) in VerifyCsrfToken.php line 18 at VerifyCsrfToken->handle(object(Request), object(Closure)) in Pipeline.php line 125 at Pipeline->Illuminate\Pipeline{closure}(object(Request)) in ShareErrorsFromSession.php line 55 at ShareErrorsFromSession->handle(object(Request), object(Closure)) in Pipeline.php line 125 at Pipeline->Illuminate\Pipeline{closure}(object(Request)) in StartSession.php line 61 at StartSession->handle(object(Request), object(Closure)) in Pipeline.php line 125 at Pipeline->Illuminate\Pipeline{closure}(object(Request)) in AddQueuedCookiesToResponse.php line 36 at AddQueuedCookiesToResponse->handle(object(Request), object(Closure)) in Pipeline.php line 125 at Pipeline->Illuminate\Pipeline{closure}(object(Request)) in EncryptCookies.php line 40 at EncryptCookies->handle(object(Request), object(Closure)) in Pipeline.php line 125 at Pipeline->Illuminate\Pipeline{closure}(object(Request)) in CheckForMaintenanceMode.php line 42 at CheckForMaintenanceMode->handle(object(Request), object(Closure)) in Pipeline.php line 125 at Pipeline->Illuminate\Pipeline{closure}(object(Request)) at call_user_func(object(Closure), object(Request)) in Pipeline.php line 101 at Pipeline->then(object(Closure)) in Kernel.php line 115 at Kernel->sendRequestThroughRouter(object(Request)) in Kernel.php line 84 at Kernel->handle(object(Request)) in index.php line 53