Using named_scope with counts of child models
-
18-04-2021 - |
문제
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.
Is this possible?
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
I'm hoping to do something like Foo.with_one_bar
I could write methods on the parent class for something like this, but I'd rather have the power of the named scope
해결책
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)
다른 팁
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