Declarative AM validations depending on arbitrary attributes

validations_for.rb

# class Page
#   include ValidationsFor
#
#   validations_for page_type: 'offer' do
#     validates :amount, presence: true
#   end
#
#   validations_for page_type: 'page' do
#     validates :title, presence: true
#   end
# end

module ValidationsFor
  extend ActiveSupport::Concern

  class ValidatorWrapper < SimpleDelegator
    include ActiveModel::Validations
    def self.model_name
      ActiveModel::Name.new(ValidatorWrapper)
    end
  end

  included do
    mattr_accessor :_validation_spec
    validate do |record|
      spec = self._validation_spec || []
      spec.each do |(attrs, wrapper)|
        if attrs.all?{ |k, v| record[k] == v }
          validator = wrapper.new(record)
          if !validator.valid?
            validator.errors.each do |k, msg|
              record.errors.add(k, msg)
            end
          end
        end
      end
    end
  end

  class_methods do
    def validations_for(attrs, &block)
      wrapper = Class.new(ValidatorWrapper, &block)
      self._validation_spec ||= []
      self._validation_spec << [attrs, wrapper]
    end
  end
end