Rails Association Id Cleanup On Destroy

rails_association_id_cleanup.rb

# I found a bit of odd rails behavior recently.  Consider the following classes.

class Contact < ActiveRecord::Base
    belongs_to :gallery
end

class Gallery < ActiveRecord::Base
    has_many :contacts
end


# All is well and good.  But let's say I have a Gallery and a Contact associated with it.

g = Gallery.first   # id = 1
c = g.contacts.first
c.gallery_id # => 1

# So now we destroyed the gallery, but we still want the contact to stick around, which 
# it does.  We would assume the relationship is also gone from the contact to the gallery 
# which it is.

g.destroy
c.gallery # => nil

# However, the gallery_id value on the contact remains 1 even though we no longer have 
# a gallery with id = 1.  

c.gallery_id # => 1

# This is fine by rails as it's smart enough to handle this, but what about if we want 
# to do checks using gallery_id.

# For example if you have the contact in memory and only want to check the existence 
# of gallery association you can do that by checking gallery_id and thus you don't have 
# to make another call to the DB.  This rails functionality of keeping the gallery_id 
# even when the gallery doesn't exist however makes this check faulty.

# The Fix
class Gallery < ActiveRecord::Base
    has_many :contacts
    before_destroy { |gallery| gallery.contacts.clear }
end


# Adding this before_destroy tells rails to clear all the gallery contacts associations 
# (and associated id's) before we remove the gallery.  Now you have nice clean association 
# ids that actually match your association existence.