Question

我有一个有很多孩子的简单父对象。我正在试图弄清楚如何使用命名范围来带回具有特定数量子女的父母。

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Foo.with_one_bar

之类的事情

我可以在父类上编写类似这样的方法，但我宁愿拥有命名范围的强大功能

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

这样称呼：

Foo.with_n_bars(2)

Answer 2

我会使用counter cache来做这件事。因此，您需要进行以下迁移：

class AddBarCount < ActiveRecord::Migration
  def self.up  
    add_column :foos, :bars_count, :integer, :default => 0  

    Foo.reset_column_information  
    Foo.all.each do |p|  
      p.update_attribute :bars_count, p.bars.length  
    end  
  end  

  def self.down  
    remove_column :foos, :bars_count  
  end
end

你需要改变你的Bar模型：

class Bar < ActiveRecord::Base
  belongs_to :foo, :counter_cache => true
end

现在，bars模型中的foo被缓存，这将加快您对bars计数的查询速度。

你的named_scopes看起来也是这样的：

#rails 2
named_scope :with_no_bars, :conditions => { :bars_count => 0 }
named_scope :with_one_bar, :conditions => { :bars_count => 1 }
named_scope :with_more_than_one_bar, :conditions => ["bars_count > 1"]

#rails 3 & ruby 1.9+
scope :with_no_bars, where(bars_count: 0)
scope :with_one_bar, where(bars_count: 1)
scope :with_more_than_on_bar, where("bars_count > 1")

#rails 4* & ruby 1.9+
scope :with_no_bars, -> { where(bars_count: 0) }
scope :with_one_bar, -> { where(bars_count: 1) }
scope :with_more_than_one_bar, -> { where("bars_count > 1") }

这样，每次发出此类请求时，您都可以为每个bars节省foo的时间。

我有这个想法观看关于计数器缓存的railscast：http://railscasts.com/episodes/23-counter-cache-column

* What's new in Active Record [Rails 4 Countdown to 2013]

使用带有子模型计数的named_scope

2 个答案: