Question

试图在铁路3号公路上做一个范围。

:book has_many :chapters

我想有以下几章:一长,将书本交回,有10章。

如何最好地构建这一范围(不使用反海滩)?

感谢!

Answer 1

你们应当:

class Book
  scope :long, joins(:chapters).
                 select( books.id, count(chapters.id) as n_chapters ).
                 group( books.id ).
                 having( n_chapters > 10 )
end

这是否有助于?

Answer 2

Ah - 在以上评论中回答我自己的问题,我不得不在《消除家庭暴力法》中考虑:

class Book
  scope :long, joins(:chapters).
    select( books.id ).
    group( books.id ).
    having( count(chapters.id) > 10 )
end

Answer 3

In rails 4.0 this version works. You have to count() in the having clause. It seems that having clause doesn t see as n_chapters .

Answer 4

另一种办法是进行排序。虽然合并更为正确(而且可能也会导致更好的业绩),但如果把试图进行分组的多种范围结合起来,你最终可能会产生奇怪的结果。等级比侵扰性低得多。例如,情况如下:

class Book
  scope :with_chapters_count, -> {
    select( books.* ).
    select( (select count(chapters.id) from chapters where chapters.book_id = books.id) as chapters_count )
  }
  scope :long, -> {
    with_chapters_count.where("chapters_count > 10")
  }
end

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