Using named_scope with counts of child models

Question

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.

Answer 1

I would use the counter cache for this. Therefore you need the following migration:

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

Than you need too change you Bar model like this:

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

Now the count of bars is cached in the foo model, that will speed up your queries for the count of bars.

Your named_scopes then have too look like this:

#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") }

That way you can save time counting bars for each foo every time you make such a request.

I got this idea watching the railscast about counter cache: http://railscasts.com/episodes/23-counter-cache-column

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

Answer 2

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

Called like so:

Foo.with_n_bars(2)