application.rb

module DataAPI

  class Application < ActiveRecord::Base
    # Validations ...
    
    attr_accessible :name, :domain, :description

    has_many :rooms, dependent: :destroy
  end

end

application_test.rb

require_relative '../helper'

class ApplicationTest < Test

  def test_it_has_rooms
    app  = create_app
    room = create_room application: app

    assert_kind_of Array, app.rooms
    assert_equal   room,  app.rooms.first
  end

  def test_it_has_dependent_rooms
    app = create_app
    room = create_room application: app

    app.destroy

    assert_predicate app,  :destroyed?
    assert_predicate room, :destroyed?
  end

end

gistfile1.txt

$ bundle exec ruby test/models/application_test.rb 
Run options: --seed 30518

# Running tests:

F.

Finished tests in 0.176238s, 11.3483 tests/s, 34.0448 assertions/s.

  1) Failure:
test_it_has_dependent_rooms(ApplicationTest) [test/models/application_test.rb:25]:
Expected #<DataAPI::Room id: 1, application_id: 1, name: "App Room", created_at: "2012-03-21 14:25:09", updated_at: "2012-03-21 14:25:09"> to be destroyed?.

2 tests, 6 assertions, 1 failures, 0 errors, 0 skips

room.rb

module DataAPI

  class Room < ActiveRecord::Base
    attr_accessible :name, :application

    belongs_to :application
  end

end

test_helper.rb

def create_app(fields={})
  fields.update({
    name: "App",
    domain: "app.cmix.com",
    description: "An app"
  }) { |_, old, new|
    return new unless old
    old == :null ? nil : old
  }
  Application.create(fields)
end

def create_room(fields={})
  fields.update({
    name: "App Room",
  }) { |_, old, new|
    return new unless old
    old == :null ? nil : old
  }
  Room.create(fields)
end