The "ActiveMapper" concept. Some terms could use refactoring.

activemapper_gist.rb

require 'active_record'
require 'yaml'
require 'pry'

dbconfig = YAML::load_file('config/database.yml')
ActiveRecord::Base.establish_connection(dbconfig)

class String
  def camelize
      self.split('_').map {|w| w.capitalize}.join
  end
end

class ActiveMapper
  def self.set_relations(relation_hash_string)
    eval <<-qot
      def relations
        @relations ||= #{relation_hash_string}
      end
    qot
  end
  def self.set_relation(klass_name, attrs)
    attrs.each do |attr|
      eval <<-qot
        def #{attr}
          relations[:#{klass_name}].#{attr}
        end
        def #{attr}=(value)
          relations[:#{klass_name}].#{attr} = value
        end
      qot
    end
  end
  def initialize(attrs = {})
    keys = relations.keys
    if attrs[:id]
      keys.each do |key|
        relations[key] = eval(key.to_s.camelize).find(attrs[:id])
      end
    else
      keys.each do |key|
        relations[key] = eval(key.to_s.camelize).new
      end
      keys = attrs.keys
      keys.each do |key|
        self.send(key.to_s, [attrs[key]])
      end
    end
  end
end

class Order < ActiveMapper
  set_relations "{orders_relation: nil, order_details_relation: nil}"
  set_relation :orders_relation, [:client_id, :status, :ship_date]
  set_relation :order_details_relation, [:customer_number]
end

class OrdersRelation < ActiveRecord::Base
  self.table_name = :orders
  self.primary_key = :order_id
end

class OrderDetailsRelation < ActiveRecord::Base
  self.table_name = :order_details
  self.primary_key = :order_id
end

binding.pry