Create two depending models at the same time - ruby-on-rails

Question

I'm trying to model a system where each user may have many emails (at least one).
Following good normalization rules I created two migrations (some fields removed for brevity):
create_table :users do |t|
end
create_table :user_emails do |t|
t.integer :user_id, null: false
t.string :email, null: false
end
add_index :user_emails, :email, :unique => true
add_foreign_key :user_emails, :users, dependent: :delete
and the following rails models:
class User < ActiveRecord::Base
has_many :emails, class_name: 'UserEmail', dependent: :destroy
def self.find_by_email(email)
UserEmail.find_by(email: email).try(:user)
end
validate do
if emails.count < 1
errors.add(:emails, "is empty")
end
end
end
class UserEmail < ActiveRecord::Base
belongs_to :user
validates_presence_of :user_id, :email
validates_uniqueness_of :email
end
Now I am not able to create any of those. User cannot be created since it requires at least an UserEmail. At the same time, UserEmail cannot be created beforehand since it requires an user_id.
I believe I have tried any combination of #user.emails.build and #user.emails << e that I can think of.
How can I solve this really simple problem without renouncing to data consistency (one is saved and the other is not)?
P.s.: I tought that maybe relaxing the validations and using transactions may solve the problem consistently. However I've never used transactions in rails, so any help is really appreciated.
Thanks

Answer 1

validates_presence_of :user_id only check for any valid integer which might not be a valid user.
But if you use validates_presence_of :user and validates_presence_of :emails, they will check the user and emails association are valid and not blank.
Also, when you are trying to create the user and the associated email, you are able to do the following code on application level.
user = User.new(name: 'user name')
user.emails.build(email_address: 'email address') #build(email: '') for your case.
Then, you can save to database with
user.save!
Below is the code I tested with.
class User < ActiveRecord::Base
has_many :emails, class_name: 'UserEmail', dependent: :destroy
validates_presence_of :emails, :message => 'User should have at least one email address.'
end
class UserEmail < ActiveRecord::Base
belongs_to :user
validates_presence_of :user
validates_presence_of :email_address
end
[40] pry(main)> u = User.create!(name: 'doh')
(0.1ms) begin transaction
(0.0ms) rollback transaction
ActiveRecord::RecordInvalid: Validation failed: Emails User should have at least one email address.
[1] pry(main)> u = User.new(name: 'nice')
=> #<User id: nil, name: "nice", created_at: nil, updated_at: nil>
[2] pry(main)> u.emails.build(email_address: 'hello#world.blah')
=> #<UserEmail id: nil, user_id: nil, email_address: "hello#world.blah", created_at: nil, updated_at: nil>
[3] pry(main)> u.save!
(0.1ms) begin transaction
SQL (0.5ms) INSERT INTO "users" ("created_at", "name", "updated_at") VALUES (?, ?, ?) [["created_at", "2014-07-26 10:14:52.184245"], ["name", "nice"], ["updated_at", "2014-07-26 10:14:52.184245"]]
SQL (0.2ms) INSERT INTO "user_emails" ("created_at", "email_address", "updated_at", "user_id") VALUES (?, ?, ?, ?) [["created_at", "2014-07-26 10:14:52.189757"], ["email_address", "hello#world.blah"], ["updated_at", "2014-07-26 10:14:52.189757"], ["user_id", 5]]
(0.7ms) commit transaction
=> true
[4] pry(main)> u.emails.count
(0.2ms) SELECT COUNT(*) FROM "user_emails" WHERE "user_emails"."user_id" = ? [["user_id", 5]]
=> 1
[5] pry(main)> u.id
=> 5
[6] pry(main)> u.name
=> "nice"
[7] pry(main)> u.emails
=> [#<UserEmail id: 4, user_id: 5, email_address: "hello#world.blah", created_at: "2014-07-26 10:14:52", updated_at: "2014-07-26 10:14:52">]
[8] pry(main)> u
=> #<User id: 5, name: "nice", created_at: "2014-07-26 10:14:52", updated_at: "2014-07-26 10:14:52">
[19] pry(main)> UserEmail.create!(email_address: 'test#test.org')
(0.1ms) begin transaction
(0.1ms) rollback transaction
ActiveRecord::RecordInvalid: Validation failed: User can't be blank
Hope this helps.

Answer 2

I just realized that I adding a NOT NULL constraint on the storage layer is sufficient. I can remove the :user_id presence validation and have the same consistency behavior, with the exception that it now works.
However this looks quite hacky from the rails POW. Any better solution is still really appreciated...