How can I run validations on derived values for new objects? - ruby-on-rails

Question

I have a model that has several attributes that are provided at creation. The model also has some additional attributes that are derived from the provided attributes, which I also want to calculate at creation. More problematically, I want to be able to run validations on these derived values (since there are inputs that are valid on their own that lead to invalid derived values).
The problem is that when I do this:
class MyClass < ActiveRecord::Base
attr_accessible :given1, :given2, :derived
before_validation :derivation
validates_uniqueness_of :derived
def derivation
self.derived = self.given1 + self.given2
end
end
MyClass.new(:given1 => aNumber, :given2 => otherNumber)
I always get errors saying I can't add nil to nil. Apparently self.attribute is nil until farther into the validation & creation process.
Obviously I could set my derived values in a later stage, and add a custom validation that works on the given attributes, but that would entail doing the derivation twice, which wouldn't be very DRY.
Is there some other way to get at assigned but not yet validated attributes in the before_validates stage?
Edit: To clarify, I want to call MyClass.new(:given1 => aNumber, :given2 => otherNumber) and have the derived value calculated before the validations check, so that the validations check as if I had called MyClass.new(:given1 => aNumber, :given2 => otherNumber, :derived => aNumber + otherNumber). The problem is that I can't seem to access the passed-in values for :given1 and :given2 in a before_validations method.

Answer 1

I wrote my own snippet of code that looks like this:
class User < ActiveRecord::Base
attr_accessible :email, :first_name, :last_name
validates :email, uniqueness: true
before_validation :derivation
def derivation
self.email = self.first_name + self.last_name
end
end
Running the following yielded no errors:
» u = User.new first_name: "leo", last_name: "correa"
=> #<User:0x007ff62dd8ace0> {
:id => nil,
:first_name => "leo",
:last_name => "correa",
:email => nil,
:created_at => nil,
:updated_at => nil,
}
» u.valid?
User Exists (0.9ms) SELECT 1 AS one FROM "users" WHERE "users"."email" = 'leocorrea' LIMIT 1
=> true
Running u.save saved the record successfully and upon repeating the User.new and saving that new record it returned with ROLLBACK because email was already used.
In any case, make sure you are assigning whatever variables you are using to the given1, given2 and whatever the result is make sure is not giving you false either because it will cancel the before_validate callback and the record won't save.