btho.rb

##Class

class User
  has_one :profile # inverse_of :profile not necessary in 4.1+
end

class Profile
  belongs_to :user # inverse_of :user not necessary in 4.1+
end

##Migration

class AddUserRefToProfiles < ActiveRecord::Migration
  def change
    add_reference :profile, :user, index: true # adds user_id to profile table
  end
end


user.profile
profile.user