ruby-on-rails - 如何按周对我的账户余额进行分组？

Question

我有一个模型账户，它将看到余额随着事件而增长。有没有办法分组并获得那一周的最终余额？这里的一个例子:

帐户

created_at: 31 April 2016, balance: 8000  # Saturday
created_at: 1 May 2016, balance: 7500     # Sunday
created_at: 1 May 2016, balance: 7000     # Sunday
created_at: 7 May 2016, balance: 5000     # Saturday
created_at: 7 May 2016, balance: 4000     #    "
created_at: 8 May 2016, balance: 3500     # Sunday
created_at: 12 May 2016, balance: 7000    # Thursday

假设一周的结束点是每周一。我的预期结果是:

{
  2 May 2016 => 7000,    # Monday
  9 May 2016 => 3500,    # Monday
  13 May 2016 => 7000    # Today
}

我想要每个星期一的余额，但加上最近的余额，即使它不是在星期一也是如此。

Answer 1

按一年中的星期和年份对结果进行分组:

.group("extract(dow from 'created_at'), extract(week from 'created_at')")

然后为此选择余额总和:

.select("sum(balance)")