transact(txn) {

    if (!txn) {
      return this;
    }

    if (!(txn instanceof Transaction)) {
      throw new Error('Must provide valid transaction to Composer#transact');
    }

    if (txn.adapter.db !== this.db) {
      throw new Error('Transaction must belong to Model Database');
    }

    this._transaction = txn;
    return this;

  }
Sets the top-level query as a transaction using provided txn
@param {Transaction} txn The desired transaction to use
@return self