All about Rails

Counter caches in RoR eliminate the need to issue an sql query to determine the size of an associated collection. Using counter cache is quite simple. Here’s an example:

class Author < ActiveRecord::Base
  has_many :books
end

class Book < ActiveRecord::Base
  belongs_to :author, :counter_cache => true
end

add_column :authors, :books_count, :integer

Author.first.books.count  # gets the value from the books_count column, 
                              # rather than doing an sql count

There’s a problem if you use this alongside acts_as_paranoid. AAP is a plugin that provides ‘soft delete’ functionality. In order to achieve this, AAP overrides AR finder methods to check the ‘deleted_at’ column to determine if a record has been ‘deleted’. Due to a condition placed on the count (counting only undeleted objects), the counter cache is not used and instead an sql statement is issued.

class Author < ActiveRecord::Base
  has_many :books
end

class Book < ActiveRecord::Base
  acts_as_paranoid
  belongs_to :author, :counter_cache => true
end

Author.first.books.count # SELECT count(*) AS count_all FROM `books` 
                             # WHERE (`books`.author_id = 11524) AND 
                             # (`books`.deleted_at IS NULL OR 
                             # `books`.deleted_at > '2008-07-27 13:27:22') 

Deleting an object from a ‘paranoid’ collection will still do the right thing (decrement the counter cache), making this easy to fix. You just have to add your own count method for the collection.

class Author < ActiveRecord::Base
  has_many :books do
    def count
       books_count
    end
  end
end

Author.first.books.count  # calls the count method, returning
                       # the books counter 

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