Example User model for Inboxes gem

user.rb

class User < ActiveRecord::Base
  devise :database_authenticatable, :registerable,
         :recoverable, :rememberable, :trackable, :validatable

  attr_accessible :email, :password, :password_confirmation, :remember_me, :name
  
  validates :name, :presence => true, :uniqueness => true
  
  inboxes
end

I think you should change inboxes method to be has_inboxes, since the documentation for inboxes refers to this gist.