set_primary_key
This is a method that you can use within a model to override the Active Record assumption that each table has a primary key labeled id. You may also provide a block rather than a pri- mary key label, and the result of the block will be the value Active Record attempts to use as the primary key.
In the following example, we override the Active Record assumptions for two models. In both models, Active Record would normally assume each table has a primary key in a field labeled id. However, in the first model, we say to use account_id as the primary key, and in the second model, we use a block to assign a primary key of comments_id:
# Using the set_table_name method
require 'rubygems'
require_gem 'activerecord'
ActiveRecord::Base.establish_connection(:adapter => "sqlserver",
:host => "localhost", :username => "sa", :password => "", :database => "testdb")
class Account < ActiveRecord::Base
  set_primary_key "account_id"
end
puts Account.find(1).inspect
class Comment < ActiveRecord::Base
  set_primary_key {
    d = ["comm","ents","_id"]
d.join }
end
puts Comment.find(1).inspect