conditional validations in Rails 4+

conditiona_validations.rb

module ConditionalValidations
  extend ActiveSupport::Concern
  
  attr_accessor :conditional_validations

  def conditional_validations
    @conditional_validations || []
  end
  
  def conditionally_validate(attribute, options=nil)
    unless options.nil? 
      # Passing options for built-in validators
      # http://guides.rubyonrails.org/active_record_validations_callbacks.html#validation-helpers
      unless options[:if].nil?
        options[:if] = "#{options[:if]} && conditional_validations?(:#{attribute})"
        validates attribute, options
      else
        validates attribute, options.merge(if: "conditional_validations?(:#{attribute})")
      end
    else
      # Block validation
      # will initiate a validator on attribute using a method called validate_#{attribute}
      validate :"validate_#{attribute}", if: "conditional_validations?(:#{attribute})"
    end
  end
end

demo.rb

class Demo < ActiveRecord::Base
  include ConditionalValidations
  
  conditionally_validate :my_field, presence: true
end

usage.rb

d = Demo.new
d.valid?

#=> true

d.conditional_validations = [:my_field]
d.valid?

#=> false

d.my_field = "expert level"
d.valid?

#=> true