ActiveRecord深层嵌套查询和计算列

Question

以下是关联，

Publication：has_many exchange_rates，books和authors

Book belongs_to author，exchange_rate

Author belongs_to exchange_rate

我正在尝试编写一个查询，以便在books之后找到computed_column。

这是我到目前为止尝试的查询

publication.books.joins(:exchange_rate, :author).select("books.*, (exchange_rates.exchange_ratio + (SELECT exchange_rates.exchange_ratio FROM exchange_rates WHERE exchange_rate_id = authors.exchange_rate_id)) AS computed_column")

我的目标是生成一个根据关联表属性计算的列。

这意味着，

book.exchange_rate.exchange_ratio + book.author.exchange_rate.exchange_ration AS computed_column

Answer 1

在computed_column

上添加attr_accessor作为Book

class Book
  attr_accessor :computed_column
end

然后，查询中的Book每个实例都会book.computed_column可用

供参考：Access SQL computed columns through ActiveRecord