_addForeignKeyToRegistry(type, fk) {
    this._registry[type] = this._registry[type] || {
      class: null,
      foreignKeys: [],
    };

    let fks = this._registry[type].foreignKeys;
    if (!fks.includes(fk)) {
      fks.push(fk);
    }
  }
@method _addForeignKeyToRegistry
    @param type
    @param fk
    @private
    @hide