protected function normalizeRelationsData()
    {
        if (is_null($this->data['relationships'])) {
            $this->data['relationships'] = [];
        }

        foreach ([ 'normal', 'reverse', 'image', 'file', 'checkbox', 'category' ] as $key) {

            if (    ! array_key_exists($key, $this->data['relationships'])
                ||  ! is_array($this->data['relationships'][ $key ])
            ) {
                $this->data->relationships[ $key ] = [];
            }
        }

        return $this;
    }