How to allow email blank in user table which is created using devise and cancan.

I have a user model in which username and email field, i am login with username when i am creating user i want it’s email may be blank, but when i am register user from another model it gives error User email can’t be blank.
I already make changes email field in confgi/devise.rb, but it doesn’t work.
Here is My User model
class User < ActiveRecord::Base
has_and_belongs_to_many :roles
devise :database_authenticatable, :registerable, :rememberable, :token_authenticatable, :trackable,
:validatable, :timeoutable ,:lockable, :recoverable , :authentication_keys => [:username]
attr_accessible :email, :username, :password, :password_confirmation, :remember_me, :role_ids, :roles
validates :email , :presence => false
end
Please, let me know where i am wrong.

Thanks,
Tushar Patil.

This thread may help you out http://stackoverflow.com/questions/11678508/email-cant-be-blank-devise-using-username-or-email