How to make a database appear partitioned by some columns with Active::Record

Question

Suppose a column client_id is ubiquitous through out our database, and for a given session or request, we will be 'in the context' of a client the whole time.

Is there a way to simulate having each client's data stored in a separate database, while keeping them in the same table for simpler database-management ? What I want is similar to a default scope, but since it will change for every request, it cant' be fixed at load-time.

  # invoices table data
  # -------------------
  # id       client_id     amount
  # 1        1             100.00
  # 2        2             100.00
  with_client( Client.find(1) ) do
     Invoices.all      # finds only invoice 1
     Invoices.find(2)  # finds nothing or raises
  end 

How can I do this with ActiveRecord, or at what points could I surgically alter AR to affect this behavior ?

Extra points: Id like to prevent the updating of this client_id column - it should be fixed at create-time

Answer 1

class Client < ActiveRecord::Base
  has_one :invoice, :dependent => :destroy
end

class Invoice < ActiveRecord::Base
  belongs_to :client
end

In controller

   @client= Client.find(1)
   @client.invoice #finds  id  =1     client_id  =1   amount=100.0

Answer 2

There is a presentation here that discusses multi-tenanted applications. It has some interesting ideas to utilise database schemas, particularly for postgres.