ruby on rails - Howto use callbacks in a has_many through association?

Question

I have a Task model associated to a Project model via has_many through and need to manipulate the data before delete/insert via the association.

Since "Automatic deletion of join models is direct, no destroy callbacks are triggered." i can not use callbacks for this.

In Task i need all project_ids to calculate a value for Project after Task is saved. How can i disable delete or change delete to destroy on has_many through association? What is best practise for this problem?

class Task
  has_many :project_tasks
  has_many :projects, :through => :project_tasks

class ProjectTask
  belongs_to :project
  belongs_to :task

class Project
  has_many :project_tasks
  has_many :tasks, :through => :project_tasks

Answer 1

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