mixin_models.rb

module Food
  def self.included(model)
    model.property :id,       Serial
    model.property :price,    Integer, :min => 0 # builtin validation works just fine
    model.property :calories, Integer, :min => 0

    model.before(:valid?, :custom_validation)
  end
    
  def custom_validation
    # does some stuff
    if delicious?
      return true
    else
      return [false, "error message! Not delicious enough!!!!!!!"]
    end
  end
end

class Taco
  include DataMapper::Resource
  include Food
end

class Ingredient
  include DataMapper::Resource
  include Food
end