Rails 3 ActiveRecord: Order by count on association

Solution 1:

Using named scopes:

class Song
  has_many :listens
  scope :top5,
    select("songs.id, OTHER_ATTRS_YOU_NEED, count(listens.id) AS listens_count").
    joins(:listens).
    group("songs.id").
    order("listens_count DESC").
    limit(5)

Song.top5 # top 5 most listened songs

Solution 2:

Even better, use counter_cache which will be faster because you'll only because using one table in your query

Here is your song class:

class Song < ActiveRecord::Base
  has_many :listens

  def self.top
    order('listens_count DESC').limit(5)
  end
end

Then, your listen class:

class Listen < ActiveRecord::Base
  belongs_to :song, counter_cache: true
end

Make sure you add a migration:

add_column :comments, :likes_count, :integer, default: 0

Bonus points, add test:

describe '.top' do
  it 'shows most listened songs first' do
    song_one = create(:song)
    song_three = create(:song, listens_count: 3)
    song_two = create(:song, listens_count: 2)

    popular_songs = Song.top

    expect(popular_songs).to eq [song_three, song_two, song_one]
  end
end

Or, if you want to go with the above method, here it is a bit more simply, and using a class method rather than scope

def self.top
    select('comments.*, COUNT(listens.id) AS listens_count').
      joins(:listens).                                                   
      group('comments.id').
      order('listens_count DESC').
      limit(5)
end