Composite Primary Keys

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Ruby on Rails does not support composite primary keys. This free software is an extension
to the database layer of Rails – ActiveRecords – to support composite primary keys as transparently as possible.

Any Ruby script using ActiveRecords can use Composite Primary Keys with this library.


sudo gem install composite_primary_keys

Rails: Add the following to the bottom of your environment.rb file

require 'composite_primary_keys'

Ruby scripts: Add the following to the top of your script

require 'rubygems'
require 'composite_primary_keys'

The basics

A model with composite primary keys would look like…

class Membership < ActiveRecord::Base
  # set_primary_keys *keys - turns on composite key functionality
  set_primary_keys :user_id, :group_id
  belongs_to :user
  belongs_to :group
  has_many :statuses, :class_name => 'MembershipStatus', :foreign_key => [:user_id, :group_id]

A model associated with a composite key model would be defined like…

class MembershipStatus < ActiveRecord::Base
  belongs_to :membership, :foreign_key => [:user_id, :group_id]

That is, associations can include composite keys too. Nice.

Demonstration of usage

Once you’ve created your models to specify composite primary keys (such as the Membership class) and associations (such as MembershipStatus#membership), you can uses them like any normal model with associations.

But first, lets check out our primary keys.

MembershipStatus.primary_key # => "id"    # normal single key
Membership.primary_key  # => [:user_id, :group_id] # composite keys
Membership.primary_key.to_s # => "user_id,group_id"

Now we want to be able to find instances using the same syntax we always use for ActiveRecords…

MembershipStatus.find(1)    # single id returns single instance
=> <MembershipStatus:0x392a8c8 @attributes={"id"=>"1", "status"=>"Active"}>
Membership.find(1,1)  # composite ids returns single instance
=> <Membership:0x39218b0 @attributes={"user_id"=>"1", "group_id"=>"1"}>

Using Ruby on Rails? You’ll want to your url_for helpers
to convert composite keys into strings and back again…

Membership.find(:first).to_param # => "1,1"

And then use the string id within your controller to find the object again

params[:id] # => '1,1'
=> <Membership:0x3904288 @attributes={"user_id"=>"1", "group_id"=>"1"}>

That is, an ActiveRecord supporting composite keys behaves transparently
throughout your application. Just like a normal ActiveRecord.

Other tricks

Pass a list of composite ids to the #find method

Membership.find [1,1], [2,1]
=> [
  <Membership:0x394ade8 @attributes={"user_id"=>"1", "group_id"=>"1"}>, 
  <Membership:0x394ada0 @attributes={"user_id"=>"2", "group_id"=>"1"}>

Perform #count operations

MembershipStatus.find(:first).memberships.count # => 1

Routes with Rails

From Pete Sumskas:

I ran into one problem that I didn’t see mentioned on this list
and I didn’t see any information about what I should do to address it in the
documentation (might have missed it).

The problem was that the urls being generated for a ‘show’ action (for
example) had a syntax like:


for a two-field composite PK. The default routing would not match that,
so after working out how to do the routing I added:

map.connect ':controller/:action/:id', :id => /\w+(,\w+)*/

to my route.rb file.

Which databases?

A suite of unit tests have been run on the following databases supported by ActiveRecord:

Database Test Success User feedback
mysql YES YES (Yes! or No…)
sqlite3 YES YES (Yes! or No…)
postgresql YES YES (Yes! or No…)
oracle YES YES (Yes! or No…)
sqlserver ??? (I can help) ??? (Yes! or No…)
db2 ??? (I can help) ??? (Yes! or No…)
firebird ??? (I can help) ??? (Yes! or No…)
sybase ??? (I can help) ??? (Yes! or No…)
openbase ??? (I can help) ??? (Yes! or No…)
frontbase ??? (I can help) ??? (Yes! or No…)

Dr Nic’s Blog – for future announcements and
other stories and things.


How to submit patches

Read the 8 steps for fixing other people’s code and for section 8b: Submit patch to Google Groups, use the Google Group above.

The source for this project is available via git. You can browse and/or fork the source, or to clone the project locally:

git clone git://


This code is free to use under the terms of the MIT licence.


Comments are welcome. Send an email to Dr Nic Williams.

Dr Nic, 25th October 2008
Theme extended from Paul Battley