remove record when counter cache reached 0

Question

I have the following model with the following relations:

+------+ 1    n +------------+ n    1 +-----+
| Post |--------| TagMapping |--------| Tag |
+------+        +------------+        +-----+

Now, in my application the Post count of a Tag is read very frequently and only changed when a new post is added, which compared to reading happens very rarely. Because of this I decided to add posts_count attribute to the Tag Model.

Here are my ActiveRecord models:

Post.rb:

class Post < ActiveRecord::Base

  # other stuff ... 

  # relations
  has_many :tag_mappings, dependent: :destroy
  has_many :tags, through: :tag_mappings

  # assign a new set of tags
  def tags=(new_tags)
    # generate a list of tag objects out of a listing
    if new_tags && !new_tags.instance_of?(Array)
      new_tags = new_tags.split(/\s+/).map do |tag|
        tag = Tag.find_or_initialize_by_name tag
        tag.save ? tag : false
      end.select {|v| v }
    end

    # remove the spare tags which aren't used by any posts
    ((tags || []) - new_tags).each do |tag|
      tag.destroy if tag.posts.count <= 1
    end

    # replace the old tags with the new ones
    tags.delete_all
    new_tags.each do |tag|
      # prevent tagging the post twice with the same tag
      tags << tag unless TagMapping.exists? post_id: self[:id], tag_id: tag.id
    end
  end
end

TagMapping.rb:

class TagMapping < ActiveRecord::Base

  # other stuff ...

  # create a cache for the post count in the Tag model
  belongs_to :tag, counter_cache: :posts_count
  belongs_to :post
end

Tag.rb:

class Tag < ActiveRecord::Base

  # other stuff ...

  has_many :tag_mappings, dependent: :destroy
  has_many :posts, through: :tag_mappings
end

When I destroy all the posts of a tag the posts_count reduces to correctly to 0. But the Tag record still exists. How can I delete the record if posts_count reached 0?

Answer 1

Instead of using the counter cache I simply used the after_destroy and the after_create hook to keep track of the post count:

class TagMapping < ActiveRecord::Base

  # other stuff

  after_create :increment_post_count
  after_destroy :decrement_post_count

  def increment_post_count
    tag.posts_count += 1
    tag.save
  end

  def decrement_post_count
    tag.posts_count -= 1
    tag.save
    tag.destroy if tag.posts_count <= 0
  end
end