app/models/user.rb
class User < ActiveRecord::Base
# Include default devise modules. Others available are:
# :confirmable, :lockable, :timeoutable and :omniauthable
devise :database_authenticatable, :recoverable, :rememberable, :trackable, :validatable
has_many :addresses, dependent: :destroy
has_one :default_address, -> { where(default: true) }, class_name: Address
end