http://blog.guiltry.com/everyone-manage-their-object-creation-process-with-factory-pattern

factory_user.rb

class Factory::User
  def self.make_user(_user_params)

    if user_params[:type] == "Knight"
      @user = Knight.new(_user_params)
      @user.increase_strength(10)
      @user.decrease_agility(4)

      # You can add specific logic that come from "unexpected feature" here.
      # Without editing all logic-contained files.
      
    elsif user_params[:type] == "Archer"
      @user = Archer.new(_user_params)
      @user.decrease_strength(6)
      @user.increase_agility(8)
    end
  
    # ...

    return @user
  end

end

member_controller.rb

class User < ActiveRecord::Base
end
 
class Knight < User
end
 
class Archer < User
end
 
class UserController < ApplicationController
  def create
    @user = Factory::User.make_user(user_params)
  
    # ...
  end
end
 
class Admin::UserController < ApplicationController
  def create
    @user = Factory::User.make_user(user_params)
  
    # ...
  end
end