ruby-on-rails - 将 named_scope 与子模型的计数一起使用

Question

我有一个简单的父对象，有很多 child 。我试图弄清楚如何使用命名范围来带回具有特定数量 child 的 parent 。

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

我希望做类似 Foo.with_one_bar 的事情

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

Answer 1

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

像这样调用:

Foo.with_n_bars(2)