Specify a default instance that should be returned when Model#default is called from an ActiveRecord model
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README.markdown

HasDefault

Specify a "default" instance that is to be returned for an ActiveRecord model when Model.default is called.

Installation

Install the has_default gem

gem install has_default --source http://gemcutter.org

Then configure in rails

config.gem 'has_default',
  :lib => 'has_default',
  :source => 'http://gemcutter.org'

For whatever model you need to set a default for, run the a similar migration

class AddDefaultsToModels < ActiveRecord::Migration
  def self.up
    change_table :plans do |t|
      t.boolean :default
    end
    add_index :plans, :default, :unique => true
  end

  def self.down
    remove_index :plans, :default
    change_table :plans do |t|
      t.remove :default
    end
  end
end

Example

Let's pretend our paid web application includes the models Plan and Account. When somebody creates an account, we want to make sure that at the very least it has a "Free Plan" associated with it.

class Plan
  has_many :accounts
  has_default
end

class Account
  belongs_to :plan

  before_validate :set_default_plan, :unless => Proc.new{|a| a.plan.present? }

protected
  def set_default_plan
    self.plan = Plan.default
  end
end

Maybe in our seed.rb file in Rails or when we startup our application for the first time, we specify the plans that we're going to sell.

Plan.create(:name => 'Free', :default => true)
Plan.create(:name => 'Medium')
Plan.create(:name => 'High')

Finally, somewhere in our application (maybe in AccountsController#new), we call Account#new and the set_default_plan method sets the plan.

Account.new.plan.name # => 'Free'

Down the road when we want to change the default plan that's assigned to an account, we just set default to true and has_default takes care of the rest.

Plan.find_by_name('High').update_attribute(:default, true)
Account.new.plan.name # => 'High'

License

Copyright (c) 2009 Brad Gessler, released under the MIT license

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

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

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.