= Composite Primary Keys for ActiveRecords

== Summary

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

== Installation

    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 verison to use)

== Versions

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 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

== 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]
  end

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]
  end

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

== 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"}>

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:

  Membership.find(1,1)

This usage is no longer supported.

== Databases

CPK supports the following databases:

 * PostgreSQL
 * MySQL
 * Oracle
 * DB2
 * SQLite
 * SQLServer

== Running Tests

See test/README_tests.rdoc

== Questions, Discussion and Contributions

For help please visit https://github.com/composite-primary-keys/composite_primary_keys.

== Author

First version was written by Dr Nic Williams.

Maintained by Charlie Savage

Contributions by many!