Create instance-based validation

gistfile1.rb

require 'rubygems'
gem 'activemodel', '3.0.10'
require 'active_model'

class ChildValidator < ActiveModel::EachValidator
  def validate_each(object, attribute, child)
    klass = "#{object.process}_child_validator".camelize.constantize rescue nil
    child.validates_with klass unless klass.nil?
    unless child.errors.empty?
      object.errors[attribute] <<  child.errors.full_messages
    end
  end
end

class ClassicChildValidator < ActiveModel::Validator
  def validate(child)
    child.validates_presence_of :city
  end
end

class FvkChildValidator < ActiveModel::Validator
  def validate(child)
    child.validates_presence_of :name
  end
end

class A
  include ActiveModel::Validations
  
  attr_accessor :child, :process
  
  def initialize
    @process = 'classic'
  end
  
  validates :child, :child=>true
  validates_inclusion_of :process, :in=>%w{classic fvk fdb}
  
end

class ChildOfA
  include ActiveModel::Validations
  
  attr_accessor :name, :city
end

a = A.new.tap{ |a| a.process = 'classic' }
a.child = ChildOfA.new.tap { |c| c.name = "Hubi" }
puts a.valid? ? "Valid" : a.errors.full_messages.join(', ')

b = A.new.tap{ |a| a.process = 'fvk' }
b.child = ChildOfA.new.tap { |c| c.city = "Aachen" }
puts b.valid? ? "Valid" : b.errors.full_messages.join(', ')

c = A.new.tap{ |a| a.process = 'invalid_process' }
c.child = ChildOfA.new
puts c.valid? ? "Valid" : c.errors.full_messages.join(', ')