business.rb

class Business < ActiveRecord::Base
  include SubuserExtender
  
  # ...
end

lender.rb

class Lender < ActiveRecord::Base
  include SubuserExtender
  
  # ...
end

mti_accessors.rb

module UserAccessorMethods
  def define_user_accessors
    all_attributes = User.content_columns.map(&:name) + [:skip_confirmation]
    # Prevent delegating duplicate fields to user, also check if the database hasn't been migrated yet and if the columsn don't exist
    class_attributes = self.table_exists? ? self.content_columns.map(&:name) : []
    ignored_attributes = class_attributes + ["created_at", "updated_at"]
    attributes_to_delegate = all_attributes - ignored_attributes

    attributes_to_delegate.each do |attr|
      class_eval <<-RUBY
        def #{attr}
          user.#{attr}
        end

        def #{attr}=(value)
          self.user.#{attr} = value
        end

        def #{attr}?
          self.user.#{attr}?
        end
      RUBY
    end
  end
end

subuser_extender.rb

require 'mti_accessors.rb'  

module SubuserExtender
  include UserAccessorMethods

  def self.included(base)
    # Sub-User Associations
    base.belongs_to :user
    base.validate :user_must_be_valid
    base.alias_method_chain :user, :autobuild

    # Delegate User Methods / Properties
    base.extend UserAccessorMethods
    base.define_user_accessors

    # Callbacks / Special Options
    base.before_create :generate_uuid
    base.before_save :save_user

    base.devise :registerable
  end

  def save_user
    # On a recursive save, carry over the user errors
    if !user.save
      add_user_errors  
    end
  end

  def user_with_autobuild
    user_without_autobuild || build_user
  end

  def method_missing(meth, *args, &blk) 
    # Check if the model has the current method
    super
  rescue NoMethodError
    p '!subuser_extender: nomethod Error'
    # If the model doesn't have the method, delegate if it
    # isn't arleady delegated from another MTI call
    if !errors[:user_mti_method_missing].any?
      errors.add(:user_mti_method_missing, 'no method')
      user.send(meth, *args, &blk)
    else
      raise NoMethodError
    end
  end

  protected

  def add_user_errors
    user.errors.each do |attr, message|
      errors.add(attr, message)
    end
  end

  # Enforce Validation
  def user_must_be_valid
    unless user.valid?
      add_user_errors
    end
  end
end

user.rb

class User < ActiveRecord::Base
  has_many :businesses, :autosave => true, dependent: :destroy
  has_one :lender, :autosave => true, dependent: :destroy
  include UserSubclassMethods
  
  # ...
end

user_with_subclass.rb

module UserSubclassMethods
  def method_missing(meth, *args, &blk)
    super
  rescue NoMethodError
    if !errors[:user_mti_method_missing].any?
      errors.add(:user_mti_method_missing, 'no method')
      self.send(self.main_type.downcase).send(meth)
    else 
      raise NoMethodError
    end
  end
end