# Rein

Database constraints made easy for ActiveRecord. Rein currently supports PostgreSQL and MySQL (foreign keys only) for both Rails 2 & 3.


## Introduction

[Database integrity](http://en.wikipedia.org/wiki/Database_integrity) is a "good thing". Constraining the allowed values in your database at the database-level (rather than solely at the application-level) is a much more robust way of ensuring your data stays sane.

Unfortunately, ActiveRecord doesn't encourage (or even allow) you to use database integrity without resorting to hand-crafted SQL. Rein adds a handful of methods to your ActiveRecord migrations so that you can easily tame your database.


## Quick Start

Install the gem:

    gem install rein

Add a foreign key constraint:

    add_foreign_key_constraint :books, :authors

Add a numericality constraint (PostgreSQL only):

    add_numericality_constraint :books, :publication_month, :greater_than_or_equal_to => 1, :less_than_or_equal_to => 12

Add an inclusion constraint (PostgreSQL only):

    add_inclusion_constraint :books, :state, :in => %w(available on_loan)


## Example

Let's look at constraining values for this simple library application.

Here we have a table of authors:

    create_table :authors do |t|
      t.string :name, :null => false

      t.timestamps
    end

And here we have a table of books:

    create_table :books do |t|
      t.belongs_to :author,          :null => false
      t.string     :name,            :null => false
      t.string     :state,           :null => false
      t.integer    :published_year,  :null => false
      t.integer    :published_month, :null => false

      t.timestamps
    end

    # A book should always belong to an author.
    # The database should prevent us from deleteing an author who has books.
    add_foreign_key_constraint :books, :authors, :on_delete => :restrict

    # Our library doesn't deal in classics.
    add_numericality_constraint :books, :published_year, :greater_than => 1980

    # Month is always between 1 and 12.
    add_numericality_constraint :books, :published_month, :greater_than_or_equal_to => 1, :less_than_or_equal_to => 12

    # State is always either "available" or "on_loan".
    add_inclusion_constraint :books, :state, :in => %w(available on_loan)