How can I disable a validation and callbacks in a rails STI derived model?

Question

Given a model

class BaseModel < ActiveRecord::Base
  validates_presence_of :parent_id
  before_save :frobnicate_widgets
end

and a derived model (the underlying database table has a type field - this is simple rails STI)

class DerivedModel < BaseModel
end

DerivedModel will in good OO fashion inherit all the behaviour from BaseModel, including the validates_presence_of :parent_id. I would like to turn the validation off for DerivedModel, and prevent the callback methods from firing, preferably without modifying or otherwise breaking BaseModel

What's the easiest and most robust way to do this?

Answer 1

I like to use the following pattern:

class Parent < ActiveRecord::Base
  validate_uniqueness_of :column_name, :if => :validate_uniqueness_of_column_name?
  def validate_uniqueness_of_column_name?
    true
  end
end

class Child < Parent
  def validate_uniqueness_of_column_name?
    false
  end
end

It would be nice if rails provided a skip_validation method to get around this, but this pattern works and handles complex interactions well.