How can I delete an entry from a HABTM join table in rails?

Question

Through many iterations of testing, I just noticed that my join table that represents a HABTM relationship between two models isn't removing entries when instances of these models get deleted. Do I need to do something special when removing an instance of a model that has HABTM relationships?

Answer 1

EDIT: Upon closer inspection HABTM relationships should be removing join table entries. However neither HABTM relationships or the relationship I described in the original version of this solution will remove those join table entries when you eliminate the record with the delete method. ActiveRecord::Base#delete does not trigger any call backs, such as the ones a HABTM relationship establishes to remove orphaned entries from the join table. Instead you should be using ActiveRecord::Base#destroy.

You will have to use raw SQL to remove the unneeded entries. If you decide to create a join model, you can iterate through entries in the join model, deleting those without an association.

Example:

class Foo < ActiveRecord::Base
  has_many :foo_bars, :dependent => :destroy
  has_many :bars, :through => :foo_bars
end

class FooBar < ActiveRecord::Base
  belongs_to :foo
  belongs_to :bar
end

class Bar < ActiveRecord::Base
  has_many :foo_bars, :dependent => :destroy
  has_many :foos, :through => :foo_bars
end

FooBar.all.each{|fb| fb.destroy? if fb.foo.nil? || fb.bar.nil? }

Answer 2

The entries in the join table should be getting deleted without you doing anything special. You can add the :delete_sql option to your code to change the behavior if you have a weird situation. Deleting the object on the other side of the join is not a default behavior.