Uniqueness of users with devise and acts_as_tenant in rails 3

The only way to do this is by removing the validatable module from devise and run your own validations like so:

class User < ActiveRecord::Base
  acts_as_tenant :account
  attr_accessible :email, :password, :remember_me

  #remove :validatable
  devise :database_authenticatable, :registerable,
    :recoverable, :rememberable, :trackable

  #run own validations
  #I've omitted any emailformatting checks for clarity's sake.
  validates :email, 
    presence: true,
    uniqueness: { scope: :account_id, case_sensitive: false }
  validates :password,
    presence: true,
    length: { :in => 6..20 },
    :if => :password_required?

protected
  # copied from validatable module
  def password_required?
    !persisted? || !password.nil? || !password_confirmation.nil?
  end

end