да

balance.go

func (s balanceStorage) ChangeBalance(ctx context.Context, userID int64, amount int64, desc string) error {
	// acquire connection
	c, err := s.pool.Acquire(ctx)
	if err != nil {
		return errors.NewInternal(err, "acquire connection")
	}
	defer c.Release()
	// begin transaction
	tx, err := c.Begin(ctx)
	if err != nil {
		return errors.NewInternal(err, "begin transaction")
	}
	defer tx.Rollback(ctx) // ignore error on rollback
	//
	var balance, balanceID int64
	// get balance
	err = tx.QueryRow(ctx, "SELECT balance_id,balance FROM balances WHERE user_id = $1 FOR UPDATE", userID).Scan(&balanceID, &balance)
	if err != nil {
		if err == pgx.ErrNoRows { // no rows -> balance not found
			if amount < 0 { // we can't create new balance with negative amount
				return fmt.Errorf("balance with user id %d not found", userID)
			}

			// create new balance
			err = tx.QueryRow(ctx,
				"INSERT INTO balances (user_id,balance) VALUES ($1,$2) RETURNING balance_id", userID, amount,
			).Scan(&balanceID)
		} else { // internal error
			return errors.NewInternal(err, "query: get balance for update")
		}
	} else { // if balance found
		balance += amount
		if balance < 0 { // check whether there is enough money to proceed change
			return fmt.Errorf("insufficient funds: missing %.2f", float64(-balance)/100)
		}
		// update existing balance
		_, err = tx.Exec(ctx, "UPDATE balances SET balance = $1 WHERE balance_id = $2", balance, balanceID)
		if err != nil {
			return errors.NewInternal(err, "exec: update balance")
		}
		//
	}
	// create record
	_, err = tx.Exec(ctx, `INSERT INTO transactions (balance_id,action,description)
	VALUES ($1,$2,$3,$4)`, balanceID, amount, desc)
	if err != nil {
		return errors.NewInternal(err, "exec: create record")
	}
	// commit transaction
	err = tx.Commit(ctx)
	if err != nil {
		return errors.NewInternal(err, "commit transaction")
	}
	//
	return nil
}