Rails 3, ActiveRecord, Detecting changes in has_many association

I have the following associations

class User < ActiveRecord::base 
  has_many :memberships
end

What I would like to do is to detect in the users_contro开发者_StackOverflowller update action that some memberships have been added or removed from the user.memberships.

Any idea or suggestion on how to implement this?

Thanks for any help.

EDIT: My apologies if the question was not clear.

Users are linked together by a parent-child relationship. This relationship is implemented through memberships to a Family circle.

What I want is that when a child user joins a Classroom circle automagically the parent of the child also joins the same Classroom circle. In other words adding/removing memberships to a child user needs to be propagated to his parents users.

My apologies if the context was not very clear. So first some context clarification:

Users are linked together by a parent-child relationship. This relationship is implemented through memberships to a Family circle.

What I want is that when a child user joins a Classroom circle automagically the parent of the child also joins the same Classroom circle. In other words adding/removing memberships to a child user needs to be propagated to his parents users.

Now the solution:
The way I have solved this is by adding the after_save and before_destroy methods to the Membership model to check is the user is a child and in that case to create/destroy membership for the parents. There are several tricky edge-cases I had to solve but the code seems to be working fine.

A final note: my first idea was to somehow detect in the user controller that memberships have been added or removed and then check if the user was a child user....

I know I'm late to the party here, but for future travelers ActiveRecord exposes before_add, after_add, before_remove, and after_remove hooks for exactly these scenarios.

Added how? Since the last visit?

If so, then make a "read" boolean column that updates when you visit.

Otherwise, elaborate. :)