lazy_object.rb

ActiveRecord::Schema.define do
  create_table :posts do |table|
    table.string :title
    table.string :content
  end
end

class Post < ActiveRecord::Base
end

class LazyObject

  class ObjectNotFoundError < StandardError; end

  def initialize(klass, id_or_object)
    @klass = klass

    case id_or_object
    when ActiveRecord::Base
      @object = id_or_object
    when Integer, String
      @id = if @klass.columns_hash['id'].type == :integer
              id_or_object.to_i
            else
              id_or_object.to_s
            end
    else
      raise ArgumentError.new("The second argument should be object of ActiveRecord::Base or id of that #{klass}")
    end
  end

  def object
    @object ||= @klass.find_by(id: @id)
    @object or raise ObjectNotFoundError
  end

  def id
    @id ||= object.id
  end

  def method_missing(method, *args, &block)
    object.send(method, *args, &block)
  end

  def respond_to_missing?(method, include_private = true)
    super || object.respond_to?(method, include_private)
  end
end

# Suppose we've got 1,000 posts in the database.
# id: 10, title: 'post 10', content: 'hello world * 10'
# id: 11, title: 'post 11', content: 'hello world * 11'

lo = LazyObject.new(Post, 10)
puts lo.id
puts lo.title