gem 'mongoid-categorized_counter_cache'

class Author
  include Mongoid::Document

  has_many :books
end

class Book
  include Mongoid::Document
  
  belongs_to :author, counter_cache: true
end

author = Author.create
author.books.create

author.books_count
# => 1
# returns a cached count of author.books

class Author
  include Mongoid::Document

  has_many :books
end

class Book
  include Mongoid::Document
  include Mongoid::CategorizedCounterCache
  
  field :genre
  
  belongs_to :author, counter_cache: true

  categorized_counter_cache :author do |book|
    book.genre
  end
end

author = Author.create
author.books.create genre: 'fiction'
author.books.create genre: 'drama'

author.books_count
# => 2
# returns a cached count of all author.books

author.books_fiction_count
# => 1
# returns a cached count of all author.books which has a fiction genre

author.books_drama_count
# => 1
# returns a cached count of all author.books which has a drama genre