Controlling join table actions based on join table content

I have an active record association that when I delete the parent
object the ::dependent => :nullify option works correctly and sets
the FKs to nil in the join table. Is there a way to make this happen
when the record is updated too? When I remove an association from
the parent, the association is deleted instead of setting the FK to

To complicate things there are times when I would like the behavior to
be :dependent => :destroy_all (depending on what is in the join

I am using has_many :through and depending on what is in my join table
I may want to destroy instead of nullify.

Is this a job for an observer on the join table or maybe overriding
the destroy method :-\

Fabricated Example (i need a better example)

Lets say there is a company that manages shooting locations for
movies. The company offers 2 types of contracts (transferable and

class Movie < ActiveRecord::Base
has_many :contracts, :dependent => :nullify
has_many :locations, :through => :contracts
class Contract < ActiveRecord::Base
belongs_to :location # location_id is unique.
belongs_to :movie

There is a contract type. fixed or transferable

transferrable contracts can have a nil movie. A fixed one can not

class Location < ActiveRecord::Base
has_many :contracts
has_many :movies, :through => :contracts

A movie may purchase several locations (contracts). If the movie is
canceled, the locations are freed up (if using :dependent
=> :destroy_all), If any locations are changed the unreferenced
locations are deleted. This all works fine.

But what if I have a contract that allows me to transfer a location I
purchased to another movie. If my contract_type is transferable and I
delete the movie the contract should remain but not reference any
movie until I reassign it (dependent => :nullify). The contract
should also remain if I remove just 1 location from my movie and I
want to transfer it to another movie at a later point.

overriding the Movie destroy method works fine for killing ‘fixed’
contracts before ActiveRecord makes them null.

At 1st I was thinking of catching the nullify via a callback on the
Contract class (wasn’t sure what I was going to do here but it appears
the nullification doesn’t invoke the callbacks).

Next up. prevent location update on a movie object from deleting
transferable contracts. I guess I’ll need to make locations
attr_protected and manually make updates to the contracts.