将 named_scope 与子模型的计数一起使用 [英] Using named_scope with counts of child models

问题描述

我有一个简单的父对象，有很多子对象.我正在尝试弄清楚如何使用命名范围来仅带回具有特定数量孩子的父母.

I have a simple parent object having many children. I'm trying to figure out how to use a named scope for bringing back just parents with specific numbers of children.

这可能吗?

class Foo < ActiveRecord::Base
    has_many :bars
    named_scope :with_no_bars, ... # count of bars == 0
    named_scope :with_one_bar, ... # count of bars == 1
    named_scope :with_more_than_one_bar, ... # count of bars > 1
end

class Bar < ActiveRecord::Base
    belongs_to :foo
end

我希望做一些像 Foo.with_one_bar

我可以在父类上为这样的事情编写方法，但我宁愿拥有命名范围的力量

I could write methods on the parent class for something like this, but I'd rather have the power of the named scope

推荐答案

class Foo < ActiveRecord::Base
  has_many :bars

  # I don't like having the number be part of the name, but you asked for it.
  named_scope :with_one_bar, :joins => :bars, :group => "bars.foo_id", :having => "count(bars.foo_id) = 1"

  # More generically...
  named_scope :with_n_bars, lambda {|n| {:joins => :bars, :group => "bars.foo_id", :having => ["count(bars.foo_id) = ?", n]}}
  named_scope :with_gt_n_bars, lambda {|n| {:joins => :bars, :group => "bars.foo_id", :having => ["count(bars.foo_id) > ?", n]}}

end

这样称呼:

Foo.with_n_bars(2)

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