Migrate source plugin: Column in on clause is ambiguous

If you get this error, you need to add alias for the field in function getIds().

  /**
  * {@inheritdoc}
  */
 public function getIds() {
   return [
     'fid' => [
       'type' => 'integer',
       'alias' => 'pf',
     ],
     'uid' => [
       'type' => 'integer',
     ],
   ];
 }