How to trigger `dependent: destroy` when updating collection_singular_ids - mysql

Question

Consider the following setup:
class Task
has_many :users, through: :task_users
end
class User
has_many :tasks, through: :tasks_users
end
class TaskUser
# Joins table
has_many :comments, dependent: :destroy
end
class Comment
belongs_to :task_user
end
Now if I perform a standard #destroy command, such as:
tu = TaskUser.first
tu.destroy
Then all comments associated to the task-user will also be destroyed.
However, suppose you want to update a user's tasks via #collection_singular_ids=, like so:
u = User.first
puts u.task_ids # => [1, 2, 3]
u.task_ids = [1, 2]
Doing this (without even calling #save explicitly!) will trigger SQL like:
(0.3ms) BEGIN
SQL (0.4ms) DELETE FROM `task_users` WHERE `task_users`.`task_id` = 3 AND `task_users`.`user_id` = 1
(2.0ms) COMMIT
...So the associated Comments get orphaned.
The same issue occurs if you use #attributes=:
u.attributes = { task_ids: [1, 2] }
Is there a clean way to ensure that the associated Comments will always be destroyed (i.e. never orphaned)?

Answer 1

Thanks to #engineersmnky for pointing me in the right direction.
This may not be the prettiest solution, but a viable option is to define a callback on the association, such as:
class User
has_many :tasks,
through: :tasks_users,
before_remove: ->(user, task) do
task_user = TaskUser.find_by!(user: user, task: task)
task_user.comments.each(&:destroy!)
end
# Or alternatively, this can be defined as a method:
has_many :tasks,
through: :tasks_users,
before_remove: :destroy_task_user_comments
private
def destroy_task_user_comments(task)
task_user = TaskUser.find_by!(user: self, task: task)
task_user.comments.each(&:destroy!)
end
end
Note that I have used bang (!) methods within the block, so that the entire transaction will rollback if the an exception is raised - as per the documentation:
if any of the before_remove callbacks throw an exception, the object will not be removed from the collection.