ruby-on-rails - 如何连接表以仅获取 has-many 关联的最后一条记录？

Question

我有两个模型:公司和交易(有很多关系)。事务模型具有 balance 属性。我有一个加入模型的查询:

scope :joined_transactions, (lambda do
  select('transactions.balance as current_balance')
  .joins('LEFT OUTER JOIN transactions ON transactions.company_id = companies.id')
end)

但是，我只想将最后一笔交易包含在此查询中。结果 Company.joined_transactions.first.current_balance == Company.first.transactions.last.balance 应该为真。

Answer 1

子查询 .joins('left join transactions t on t.company_id = companies.id AND t.id = (SELECT MAX(id) FROM transactions WHERE transactions.company_id = t.company_id)')