Implementing HABTM counter_cache - between 2 models (Tag, Question)

console.rb

## I imagine this could be done in the migration, but it was giving me problems so I moved it to the model 
## and just did it on the command line
 
> Tag.reset_questions_count

migration-sometimestamp.rb

class AddTagCounterToQuestion < ActiveRecord::Migration
  def up
    add_column :tags, :questions_count, :integer, default: 0, null: false
  end
  
  def down
    remove_column :tags, :questions_count
  end
    
end

question.rb

class Question < ActiveRecord::Base
  ## the validates_tag method is just to prevent duplicate tags being added to the db - not fully necessary for this exercise, 
  ## but I figured I would leave it just in case
  has_and_belongs_to_many :tags, before_add: :validates_tag
  
  after_create :increment_tags_counter
  after_destroy :decrement_tags_counter
  
  def validates_tag(tag)
    self.tags.include? tag
  end
 
  def increment_tags_counter
    self.tags.each do |t|
      Tag.find_by_name(t.name).questions_count += 1
    end
  end
 
  ## I haven't tested the callback on this, but it should work. What I am not 100% sure of is if the record is saved even
  ## a destroy, such that we can call `self.tags` on that same record and it works. If it doesn't work, then this can just
  ## be moved to a 'before_destroy'.
  
  def decrement_tags_counter
    self.tags.each do |t|
      Tag.find_by_name(t.name).questions_count -= 1
    end
  end
 
end

tag.rb

class Tag < ActiveRecord::Base
  attr_accessible :name, :questions_count
  
  has_and_belongs_to_many :questions, uniq: true
 
  def self.reset_questions_count
    Tag.all.each do |t|
      question_count = t.questions.count
      t.update_attributes(:questions_count => question_count)
    end
  end  
 
end

Hi, thank you for your code.
Does it work if I remove the association without deleting a Tag?

i would suggest adding the reset_questions_count function to the migration itself, as that code needs to be used only need to be used once.

You can do:

def increment_tags_counter
  self.tags.each do |t|
    t.increment!(:questions_count)
  end
end

you already have tags so no need to find them, and finding by name is slow because it doesn't use index. If you need to find records, use id or other indexed column.

also :after_destroy doesn't work, using :before_destroy worked for me.

This isn't a good code. Rails should do it automatically