With check constraints support in Rails 6.1, this functionality should no longer be needed.

If you’re on PostgreSQL and see the importance of data-layer constraints - this gem/plugin is for you. It integrates constraints into PostgreSQL adapter so you can add/remove them in your migrations. You get two simple methods for adding/removing constraints, as well as a pack of pre-made constraints.

As a gem

gem install maxim-sexy_pg_constraints --source http://gems.github.com

or as a plugin

script/plugin install git://github.com/maxim/sexy_pg_constraints.git

One more thing. Make sure that in your environment.rb file you have the following line uncommented.

config.active_record.schema_format = :sql

Otherwise your test database will not have these constraints replicated.

Say you have a table “books” and you want your Postgres DB to ensure that their title is not-blank, alphanumeric, and its length is between 3 and 50 chars. You also want to make sure that their isbn is unique. In addition you want to blacklist a few isbn numbers from ever being in your database. You can tell all that to your Postgres in no time. Generate a migration and write the following.

class AddConstraintsToBooks < ActiveRecord::Migration
  def self.up
    constrain :books do |t|
      t.title :not_blank => true, :alphanumeric => true, :length_within => 3..50
      t.isbn :unique => true, :blacklist => %w(badbook1 badbook2)
    end
  end

  def self.down
    deconstrain :books do |t|
      t.title :not_blank, :alphanumeric, :length_within
      t.isbn :unique, :blacklist
    end
  end
end

This will add all the necessary constraints to the database on the next migration, and remove them on rollback.

There’s also a syntax for when you don’t need to work with multiple columns at once.

constrain :books, :title, :not_blank => true, :length_within => 3..50

The above line works exactly the same as this block

constrain :books do |t|
  t.title :not_blank => true, :length_within => 3..50
end

Same applies to deconstrain.

Say you have the same table “books” only now you want to tell your Postgres to make sure that you should never have the same title + author_id combination. It means that you want to apply uniqueness to two columns, not just one. There is a special syntax for working with multicolumn constraints.

class AddConstraintsToBooks < ActiveRecord::Migration
  def self.up
    constrain :books do |t|
      t[:title, :author_id].all :unique => true # Notice how multiple columns are listed in brackets.
    end
  end

  def self.down
    deconstrain :books do |t|
      t[:title, :author_id].all :unique
    end
  end
end

It’s important to note that you shouldn’t mix multicolumn constraints with regular ones in one line. This may cause unexpected behavior.

In our table “books” we have column “author_id” which should reference the “id” column in the “authors” table. Here’s the very simple syntax for setting up foreign key constraint that will tell Postgres to enforce this relationship.

class AddConstraintsToBooks < ActiveRecord::Migration
  def self.up
    constrain :books do |t|
      t.author_id :reference => {:authors => :id, :on_delete => :cascade} # :on_delete is optional
    end
  end

  def self.down
    deconstrain :books do |t|
      t.author_id :reference
    end
  end
end

In this example we’re telling Postgres to enforce the connection of author_id to the column “id” in table “authors”. However, we’re also telling it to cascade on delete. This means that when an author is deleted - every book that referred to that author will be deleted as well.

Below is the list of constraints available and tested so far.

• whitelist

• blacklist

• not_blank

• within

• length_within

• email

• alphanumeric

• positive

• unique

• exact_length

• reference

• even

• odd

• format

• lowercase

• xor

All constraints are located in the lib/constraints.rb. Extending this module with more methods will automatically make constraints available in migrations. All methods in the Constraints module are under module_function directive. Each method is supposed to return a piece of SQL that is inserted “alter table foo add constraint bar #{RIGHT HERE};.”

• Add support for Rails schema.rb

• Create better API for adding constraints

• Empact (Big thanks for lots of work. Better flexibility, more tests, organizing code, bug fixes.)

• look (Extra constraints: lowercase and xor.)

Copyright © 2010 Maxim Chernyak

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The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

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