Defaulter

Defaulter allows you to mark and maintain default objects in ActiveRecord association collections with minimal code and minimal fuss.

For example, a User model can have many Email models using ActiveRecords has_many functionality. But which email address do you send mail to? Certainly not all, that's just irritating. Instead, marking a particular record as default and sending mail there is a much better idea. The defaulter gem allows you to achieve that rather simply.

Besides Rails 4 beta, defaulter now supports Rails 3.2. It may work on lower rails versions, but your mileage may vary.

Put this in your project's Gemfile:

gem 'defaulter'

Don't forget to run the bundle command to fetch the gem.

Next, you'll need to add one column to whichever table where you want to mark default records. Since both default and primary are reserved words in SQL, we go with the name prime. If you're making a migration afresh, tuck the following line in your migration's create_table call:

t.boolean :prime, null: false, default: false

If already have a migration, you will need to create a new one and add the column like so:

add_column :table_name, :prime, null: false, default: false

Now, run the migrations.

NOTE: Adding proper indexes will speed-up your finds.

There's no Step 3. We're ready to roll.

The defaulter gem gives you a has_default call that sets up the model and the has_many association automatically for you. Going with our User and Email example, a user can have many emails, one of which may be a default record. The User class can look like so:

class User < ActiveRecord::Base
  has_default :email
end

NOTE: There is no need to add has_many :emails, has_default :email does that for you automatically. Also all of the has_many options, including polymorphism are supported.

The Email class can go unchanged:

class Email < ActiveRecord::Base
  belongs_to :user
end

Usage is surprisingly simple. Defaulter extends ActiveRecord for getting and setting a default record like so.

If you've followed along carefully, user.emails should get you all the email addresses; and, user.emails.default should get you the default email address.

To set a new default email, user.emails.default = user.emails.last will set a new default email address. The older default record will now longer be the default.

NOTE: The new default record will have to exist in the collection before you can set it as a new default.

Defaulter overides ActiveRecord's << so that a new record or collections can be added. If the new record is marked as default by setting prime to true, the existing default record will cease to be one. If a collection has more than one item marked as default, the last item will take precendence. In short, defaulter will do eveything it can to ensure that there is just one default record on the database for an association collection at any given time.

I threw this gem together in about half an hour, there is scope for improvement. I will appreciate contributions in the following areas:

1. Tests, Tests, Tests
2. Ability the configure the name of the default column, by passing it with the has_default call like so: has_default :email, default_column: :primordial
3. Dynamically generated utitlity instance methods like default_email and default_address
4. Optionally, prevent default records from being destroyed
5. Backport to Rails 3.2 (v0.0.9)

MIT License. Copyright © 2013 Amol Hatwar.