Composite Primary Keys for ActiveRecords


ActiveRecord infamously doesn’t support composite primary keys. This gem, composite_primary_keys, or CPK for short, extends ActiveRecord to support composite keys.


gem install composite_primary_keys

If you are using Rails add the following to your Gemfile:

gem 'composite_primary_keys', '=x.x.x' (see next section about what version to use)


Every major version of ActiveRecord has included numerous internal changes. As a result, CPK has to be rewritten for each version of ActiveRecord. To help keep things straight, here is the mapping:

Version 14.x is designed to work with ActiveRecord 7.0.x
Version 13.x is designed to work with ActiveRecord 6.1.x
Version 12.x is designed to work with ActiveRecord 6.0.x
Version 11.x is designed to work with ActiveRecord 5.2.x
Version 10.x is designed to work with ActiveRecord 5.1.x
Version  9.x is designed to work with ActiveRecord 5.0.x
Version  8.x is designed to work with ActiveRecord 4.2.x
Version  7.x is designed to work with ActiveRecord 4.1.x
Version  6.x is designed to work with ActiveRecord 4.0.x
Version  5.x is designed to work with ActiveRecord 3.2.x
Version  4.x is designed to work with ActiveRecord 3.1.x

Run the following command to list available versions:

gem list composite_primary_keys -ra

The basics

A model with composite primary keys is defined like this:

class Membership < ActiveRecord::Base
  self.primary_keys = :user_id, :group_id
  belongs_to :user
  belongs_to :group
  has_many :statuses, :class_name => 'MembershipStatus', :foreign_key => [:user_id, :group_id]

Note the addition of the line:

self.primary_keys = :user_id, :group_id

A model associated with a composite key model is defined like this:

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

That is, associations can include composite keys too. All Rails association types are supported. Nice.


Once you’ve created your models to specify composite primary keys (such as the Membership class) and associations (such as MembershipStatus#membership), you can use 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"}>

Notice the use of an array to specify the composite key values.

NOTE - API CHANGE. CPK Version 6.x and earlier used to allow composite keys to be listed out like this:


This usage is no longer supported.


CPK supports the following databases:

* PostgreSQL
* MariaDB
* Oracle
* DB2
* SQLite
* SQLServer


To run tests you first need to install the appropriate gems for the database you want to test. Database gems are divided into the following bundler groups:

* mysql
* oracle
* postgresql
* sqlite
* sqlserver

Since it is likely you do not have all the above databases installed on your computer, you want to install just the gems for your database. For example, to test postgresql you would install the appropriate gems like this:

bundler config set --local without "mysql oracle sqlite sqlserver"
bundler install

Once you have installed the appropriate gems, the next step is to create the test database. There is a rake command for each database. Using our example:

rake postgresql:build_database

You can also rebuild the database if it already exists using this command:

rake postgresql:rebuild_database

To get a list of commands for your database use:

Rake -T

Finally, to run tests:

rake postgresql:test

Travis build status: <img src=“” alt=“Build Status” />


DB2 is no longer supported due to difficulties in getting the ibm_db2 gem to build. Thus tests have not been run against db2.

MariaDb (mysql)

MariaDb is fully supported with all tests passing.


Oracle is fully supported with all tests passing.


Postgresql is fully supported with all tests passing.

Sqlite 3

The sqlite database is created at the path composite_primary_keys/db. Note you must first create the database using the built-in rake task before running tests:

rake sqlite:build_database

For sqlite3 to work correctly, you must manually require ‘composite_primary_keys/connection_adapters/sqlite3_adapter’ after loading the CPK gem.


SqlServer is partially supported. There are a number of failing tests - patches welcomed.

Questions, Discussion and Contributions

For help please visit


First version was written by Dr Nic Williams.

Maintained by Charlie Savage

Contributions by many!