Question

I’ve got an abstract model that has a lot of implementing submodels. I’d like to be able to define some functions in the abstract model that rely on the submodel’s own definitions when called on the submodel. Is such a thing possible in Ruby on Rails?

Example, where the length attributes are defined in the submodels:

class Animal
  validates_inclusion_of :length, :in => MIN_LENGTH..MAX_LENGTH
end

class Elephant < Animal
  MIN_LENGTH = 5
  MAX_LENGTH = 20
end

Then when I go to Elephant.new.save! I’d like it to run the validation with Elephant’s constants.

The reason I want to do this is because the fact that there will be such validations is going to be universal to all of the submodels, the only thing that varies is the value of the constants.

When I tried to do it like this, or with lower-cased methods, in both cases I get a name error for the undefined values.

Is such a thing possible in Rails?

Answer 1

validates_inclusion_of :length, :in => MIN_LENGTH..MAX_LENGTH

gets executed when Ruby sees your Animal class. So, it’s naturally it uses the values of MIN/MAX_LENGTH as they appear at that time. To override this behavior you can do this:

validates_inclusion_of :length, :in => lambda {|animal| animal.class.age_range }

Then define in both your Animal and Elephant classes (or just in the last one if you don’t plan to “gave birth” to a “just Animal“):

def self.age_range
  # < some suitable range, like 5..20 >
end

This way your range for :in will be calculated when it is needed, providing :in with a suitable range for some species.

P.S. I guess you omitted the base class for your Animal just to save few keystrokes 😉