gistfile1.rb

$ rails g model switchable_plan origin:references replacement:references 'switch_plan_fee:decimal{15,2}'

# app/models/switchable_plan.rb
class SwitchablePlan < ActiveRecord::Base
  belongs_to :origin
  belongs_to :replacement
end

# db/migrate/20131128082637_create_switchable_plans.rb
class CreateSwitchablePlans < ActiveRecord::Migration
  def change
    create_table :switchable_plans do |t|
      t.references :origin, index: true
      t.references :replacement, index: true
      t.decimal :switch_plan_fee, precision: 15, scale: 2

      t.timestamps
    end
  end
end

# spec/factories/switchable_plans.rb
# Read about factories at https://github.com/thoughtbot/factory_girl

FactoryGirl.define do
  factory :switchable_plan do
    origin nil
    replacement nil
    switch_plan_fee "9.99"
  end
end

# spec/models/switchable_plan_spec.rb
require 'spec_helper'

describe SwitchablePlan do
  pending "add some examples to (or delete) #{__FILE__}"
end