How to Order ActiveRecord Results by Association Count in Rails 3?

Ordering ActiveRecord Results by Association Count

In Rails 3, you can encounter scenarios where you need to retrieve data from a model and sort the results based on the number of associated records in another model. This can be achieved using ActiveRecord's named scopes.

Consider the example of a model Song that has many associated Listen records. To find the five songs that have been listened to the most, you can utilize the named scope approach:

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)

# Retrieve the top 5 songs based on listen count
Song.top5

In this named scope, we use the select method to include the columns of interest along with a custom column listens_count that represents the listen count. The joins method establishes the association between Song and Listen models. The group method groups the results by Song.id to aggregate the listen count for each song. Finally, the order and limit methods sort the results in descending order of listen count and limit the output to the top five songs.

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