Question

I have a Rails model that has a belongs_to relationship to another model.

class Foo < ActiveRecord::Base
  belongs_to :bar
end

Every time a bar is set / updated on the foo I want “intercept” the modification of that change and update something on the foo.

My guess is that I can just override the setter for bar and do what I want. But, since I’m relatively new to Ruby / Rails, my concern is that there are “magical” ways that the bar can get set on the foo that don’t go through my setter (perhaps with mass-assignment, update/update_all calls, etc).

What is the safest way to ensure that any change to bar runs a block of code that reacts to that change?

Answer 1

Depending on your needs, you can use callbacks on your bar to execute specific code on your foo.

All the documentation you need is here http://api.rubyonrails.org/classes/ActiveRecord/Callbacks.html

For example :

class Bar < ActiverRecord::Base
    has_many :foos
    before_save :do_something

    private
        def do_something
            self.foos.each {|f| f.update_attribute('bar_has_changed', true) }
        end
end