rails counter cache while destroy

Question

For example, I have three model user, question and answer, and the relationship between them are:

class User < ActiveRecord::Base
  has_many :answers
  has_many :questions
end

class Question < ActiveRecord::Base
  has_many :answers, :dependent => :destroy
  belongs_to :user, :counter_cache => true
end

class Answer < ActiveRecord::Base
  belongs_to :user, :counter_cache => true
  belongs_to :question, :counter_cache => true
end

Then when i want to destroy a question(with 1000 answers), these will happen: The answers will destroy one by one and will update the counter in the user model, even the counter in the question i want to destroy, and which will take a long time to do the counter update.

my question is how to make it faster?

Answer 1

I got my own solution like this:

step 1: remove the dependent destroy, which will call for counter update before destroy itself.

step 2: add my own before_destroy, like this

before_destroy :delete_dependents

and use delete_all function to delete without call any before_destroy, then call reset_counters function to reset the counter in User model.

full codes of class question:

class Question < ActiveRecord::Base
  has_many :answers
  has_many :answer_users, :through => :answers, :source => :user, :uniq => true
  belongs_to :user, :counter_cache => true

  before_destroy :delete_dependents

  private
  def delete_dependents
    answer_user_ids = self.answer_user_ids # eager loading for store the answer users id
    Answer.delete_all(:question_id => self.id)
    answer_user_ids.each do |u_id|
      User.reset_counters u_id, :answers
    end
  end
end

PS: If there are too many counters need to be reset, you might need a background job to work it out.