/**
  * find an access token.
  *
  * @param UserEntity $user
  *
  * @return array|null
  */
 public function findAccessTokensByUser(UserEntity $user)
 {
     $accessTokenModel = $this->modelResolver->getModel('AccessTokenModel');
     /** @var Collection $accessTokens */
     $accessTokens = $accessTokenModel::where('user_id', $user->getIdentifier())->get();
     if ($accessTokens->isEmpty()) {
         return;
     }
     return $accessTokens->map(function ($accessToken) {
         return $this->getAccessTokenEntity($accessToken);
     })->toArray();
 }