Question

I need a validation to check whether an attribute is less or equal than another (virtual) attribute of the same record. How can I do that?

Sample code (not working – NoMethodError):

attr_reader :virt
attr_accessible :virt

validates :my_attr, :numericality => {:only_integer => true, :less_or_equal => self.virt}

(please be gentle and explicit, I’m a RoR newb :])

Answer 1

Since those validation lines are going to be executed when the class definition is first encountered, self.virt doesn’t exist.

You can usually pass in a lambda/proc instead that will be called at validation time from the scope of the object:

validates :my_attr, :numericality => { :less_or_equal => lambda { virt } }

This still isn’t that great, though. A better route would be to just define your own validation method:

validate :my_attr_is_within_range

def my_attr_is_within_range
  my_attr <= virtual_attribute
end

This is much cleaner and more explicit. Note that you don’t need to use self here since there is no ambiguity (if you were setting you would need the self).