Question

I have following model:

class Customer < ActiveRecord::Base
  has_one :cart
end

class Cart < ActiveRecord::Base
  belongs_to :customer

  validates :customer, :presence => true
end

In following case, Cart object is invalid and not saved to DB:

c = Cart.create
c.valid? # false

In following case, Cart object is valid and saved to DB:

c = Cart.create
cus = Customer.create
c.customer = cus
c.valid? # true

In following, third case, Cart object is also valid and saved to the DB:

cus = Customer.create
cus.cart = Cart.create

The question is – why the last case produces valid Cart record, that will be saved ?

When following stack-like code evaluation schema, Cart.create is eveluated before being passed to Customer.cart= method. Therefore, association of Cart with Customer is not established at this moment, and, theoretically, should produce invalid Cart that should not be saved to DB.

Why saving of Cart happens ?

I guess, cart object is marked as “to-be-saved”, when it was created with create method and is invalid. When passed to Customer.cart= method, cart is associated with customer and Cart.save will be called, when cart is marked as “to-be-saved”. So, we get cart model, saved to DB.

Am I right ?

Answer 1

…/activerecord-3.0.8/lib/active_record/base.rb:476 (create method)

# Creates an object (or multiple objects) and saves it to the database, if validations pass.
# The resulting object is returned whether the object was saved successfully to the database or not.

also …/activerecord-3.0.8/lib/active_record/associations.rb:1003 (has_one method)

# [association=(associate)]
#   Assigns the associate object, extracts the primary key, sets it as the foreign key,
#   and saves the associate object.