hot.rb

##Class

class User
  has_one :profile
  has_one :avatar, through: :profile
end

class Profile
  belongs_to :user
  has_one :avatar
end

class Avatar
  belongs_to :profile
end

##Migration

class AddAvatarRefToUsers < ActiveRecord::Migration
  def change
    add_reference :avatar, :profile, index: true
    add_reference :profile, :user, index: true
  end
end


user.avatar