Rails - Best-Practice: How to create dependent has_one relations

Best practice to create has_one relation is to use the ActiveRecord callback before_create rather than after_create. Or use an even earlier callback and deal with the issues (if any) of the child not passing its own validation step.

Because:

• with good coding, you have the opportunity for the child record's validations to be shown to the user if the validations fail
• it's cleaner and explicitly supported by ActiveRecord -- AR automagically fills in the foreign key in the child record after it saves the parent record (on create). AR then saves the child record as part of creating the parent record.

How to do it:

# in your User model...
has_one :profile
before_create :build_default_profile

private
def build_default_profile
  # build default profile instance. Will use default params.
  # The foreign key to the owning User model is set automatically
  build_profile
  true # Always return true in callbacks as the normal 'continue' state
       # Assumes that the default_profile can **always** be created.
       # or
       # Check the validation of the profile. If it is not valid, then
       # return false from the callback. Best to use a before_validation 
       # if doing this. View code should check the errors of the child.
       # Or add the child's errors to the User model's error array of the :base
       # error item
end

Your solution is definitely a decent way to do it (at least until you outgrow it), but you can simplify it:

# user.rb
class User < ActiveRecord::Base
  has_one      :profile
  after_create :create_profile
end

If this is a new association in an existing large database, I'll manage the transition like this:

class User < ActiveRecord::Base
  has_one :profile
  before_create :build_associations

  def profile
    super || build_profile(avatar: "anon.jpg")
  end

private
  def build_associations
    profile || true
  end
end

so that existing user records gain a profile when asked for it and new ones are created with it. This also places the default attributes in one place and works correctly with accepts_nested_attributes_for in Rails 4 onwards.

Probably not the cleanest solution, but we already had a database with half a million records, some of which already had the 'Profile' model created, and some of which didn't. We went with this approach, which guarantees a Profile model is present at any point, without needing to go through and retroactively generate all the Profile models.

alias_method :db_profile, :profile
def profile
  self.profile = Profile.create(:user => self) if self.db_profile.nil?
  self.db_profile
end