Dirty Associations with ActiveRecord

ActiveRecord automatically keeps track of when a record was created and updated, but what about when an association is created? Does that also count as an update?

If a blog has many posts and you add a new post to the blog, has the blog been updated? Maybe not technically, but conceptually I think you could argue it has. If you wanted to show when your blog was last updated, what you really probably is when the last post was written. You could look that up with a query, but it could make more sense to update the timestamp of the parent model itself.

When you update attributes on a model those changes are tracked in a “dirty” hash. When the model is saved it calls the changed? method which checks the hash to see if there’s anything in it. If there is then the updated_at timestamp is set to the current time. But associations are not tracked in this hash, so modifying them will not flag a model as being dirty. This is how you can add that behavior.

module DirtyAssociations

  attr_accessor :dirty_associations?

  def dirty_associations
    self.dirty_associations? = true
  end

  def changed?
    dirty_associations? || super
  end

end

class Blog
  include DirtyAssociations

  has_many :posts, {
    after_add: :dirty_associations,
    after_remove: :dirty_associations,
  }
end