How to "touch" a parent model of a `belongs_to` association only if certain conditions are met?

Question

I am using Rails 3.1.0 and I would like to "touch" a parent model of a belongs_to association only if certain conditions are met.

For example, at this time I have:

belongs_to :article,
  :touch => true

I would "touch" the parent model only if it is "public". That is, the Article class has an attribute named access (@article.access => public or private) and I would like to check this value before "touching": if this value is not public, then "touch" it!

Is it possible to make that "directly" in the belongs_to association statement? If so, how?

Answer 1

You can try lambda as you said but I'm not sure if its going to work. Something like this:

belongs_to :article, :touch => Proc.new{|o| o.article && o.article.public }

According to the implementation maybe you can try to return nil instead of false in the proc, when it's not available

belongs_to :article, :touch => Proc.new{|o| o.article && o.article.public ? true : nil }

If this doesn't works use a before save callback like this:

class Model < ActiveRecord::Base
  belongs_to :article

  before_save :touch_public_parent

  def touch_public_parent
    article.touch if article && article.public?
  end
end

Let me know if you have any questions.

Update #1

The relevant part from add_touch_callbacks:

if touch_attribute == true
  association.touch unless association.nil?
else
  association.touch(touch_attribute) unless association.nil?
end

So if you pass true, then does a simple touch on updated_at attribute. If you pass a field name then updates that field unless you pass nil. If you pass nil doesn't updates nothing. That's why I said that maybe you can try the second version of belongs_to association.