Imagine you have the following models in a Rails app:

class Car
  has_many :wheels
end

class Wheel
  belongs_to :car
end

You could instantiate and save a new car with wheels like so:

car = Car.create
4.times { car.wheels.create }

What if we want to save the car and its wheels to the database as a single transaction? This might be important if we never want a car to be saved to the database without wheels. Makes sense, right? Here’s how we could do that:

car = Car.new
4.times { car.wheels.build }
car.save

The build method instantiates and associates new wheel objects, but does not persist them to the database. ActiveRecord is smart about deferring persistence in this way. When you call save on the parent object, car, ActiveRecord will also call save on the child objects. A handy pattern to use with closely coupled objects!