Console test

rails console
user = User.new
user.orders << Order.create!
user.save

#will create a User with a single new Order

Generation

rails generate model User
rails generate model Order user_id:integer
rake db:migrate

model classes

#app/models/user.rb
class User < ActiveRecord::Base
  has_many :orders
end

#app/models/order.rb
class Order < ActiveRecord::Base
  belongs_to :user
end