Rails /活动记录，按关联排序/ has_many计数

Question

I have a collection of books and they can have many topics associated through another table.

class Book < ActiveRecord::Base
  has_many :book_topics, dependent: :destroy
  has_many :topics, through: :book_topics
end

Then I have a helper to list out all the topics, but I want to list topics to be ordered by the number of books that have that topic.

I've tried variations of:

def topic_list
  Topic.joins(:book_topics)
       .group("book_topics.topic_id")
       .order("count(book_topics.book_id) desc")
end

With no luck.

Answer 1

Think that proper way to do this would be to implement counter cache

in book_topic.rb you would have

class BookTopic < ActiveRecord::Base
  belongs_to :topic, counter_cache: true
end

topic.rb should stay the same

class Topic < ActiveRecord::Base
  has_many :book_topics, dependent: :destroy
end

however, you would have to add the counter column to the topics table

class AddBookTopicsCountToTopics < ActiveRecord::Migration
  def change
    add_column :topics, :book_topics_count, :integer, default: 0
  end
end

migrate it, update counter val if you have existing records, and from there it should be easy to sort topics by the number of books they have.

i realize you want a query to solve this, but in my experience this should be much faster.

sry if there are mistakes. counter name is not the best, but there's an example in the docs how to change it. if it's not enough there's also a railscast vid.