Howto use callbacks in a has_many through association?

Seems like i have to use associations callbacks before_add, after_add, before_remove or after_remove

class Task
  has_many :project_tasks
  has_many :projects, :through => :project_tasks, 
                      :before_remove => :my_before_remove, 
                      :after_remove => :my_after_remove
  protected

  def my_before_remove(obj)
    ...
  end

  def my_after_remove(obj)
    ...
  end
end   

Updating joins model associations, Rails add and remove records on the collection. To remove the records, Rails use the delete method and this one will not call any destroy callback.

You can force Rails to call destroy instead delete when is removing records. To do that, install the gem replace_with_destroy and pass the option replace_with_destroy: true to the has_many association.

class Task
  has_many :project_tasks
  has_many :projects, :through => :project_tasks,
            replace_with_destroy: true
  ...
end

class ProjectTask
  belongs_to :project
  belongs_to :task

  # any destroy callback in this model will be executed
  #...

end

class Project
  ...
end

With this, you ensure Rails invoke all the destroy callbacks. This could be very useful if you are using paranoia.

This is what I did

in the model:

class Body < ActiveRecord::Base
  has_many :hands, dependent: destroy
  has_many :fingers, through: :hands, after_remove: :touch_self
end

in my Lib folder:

module ActiveRecord
  class Base
  private
    def touch_self(obj)
      obj.touch && self.touch
    end
  end
end