How to implement (Discussion N:M Tag) with ActiveRecord

discussion.rb

class Discussion < ActiveRecord::Base

  has_and_belongs_to_many :tags

  def self.my_scope
    # TODO: replace me with something meaningful
    where("discussions.id < 10")
  end

  def self.tags_count
    joins(:tags).
      group("tags.id").
      order("count_all desc").
      count
  end

end

main.rb

discussions = Discussion.my_scope
tags_with_count = TagsForDiscussions.perform(discussions: discussions)

migration_01_create_discussions.rb

class CreateDiscussions < ActiveRecord::Migration
  def change
    create_table :discussions do |t|
      t.string :name

      t.timestamps null: false
    end
  end
end

migration_02_create_tags.rb

class CreateTags < ActiveRecord::Migration
  def change
    create_table :tags do |t|
      t.string :name

      t.timestamps null: false
    end
  end
end

migration_03_create_join_table_discussions_tags.rb

class CreateJoinTableDiscussionsTags < ActiveRecord::Migration
  def change
    create_join_table :discussions, :tags do |t|
      t.index [:discussion_id, :tag_id]
      t.index [:tag_id, :discussion_id]
    end
  end
end

seeds.rb

20.times {|index| Discussion.create name: "discussion_#{index}"}
10.times {|index| Tag.create name: "tag_#{index}"}
Discussion.all.sample(10).each do |discussion|
  discussion.tag_ids = Tag.all.to_a.sample(5).map(&:id)
end

tag.rb

class Tag < ActiveRecord::Base
  has_and_belongs_to_many :discussions
end

tags_for_discussions.rb

module TagsForDiscussions

  def self.perform(discussions:)
    tags_count = discussions.tags_count
    tags_mapping = Tag.
      find(tags_count.keys).
      each_with_object({}) do |current, memo|
        memo[current.id] = current
      end
    tags_count.map do |tag_id, count|
      {
        tag: tags_mapping[tag_id],
        count: count
      }
    end
  end

end