= 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 version 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 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]
  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 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:

  Membership.find(1,1)

This usage is no longer supported.

== Databases

CPK supports the following databases:

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

== Tests

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 install --without "mysql oracle sqlite sqlserver"

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="https://travis-ci.org/composite-primary-keys/composite_primary_keys.svg" alt="Build Status" />}[https://travis-ci.org/composite-primary-keys/composite_primary_keys]

=== DB2

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

Oracle is fully supported with all tests passing.

=== Postgresql

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

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

== 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!