Named scope and has_many

Rails
1

Hi
I have the models User and Payment. The relation between them

user has_many payments
payment belongs_to user

 payments table has a field paid_date. Now for finding out the last

paid_date for a particular user I did like

class payment < ActiveRecord::Base
named_scope :last_payment_date, :select => ‘MAX(date_paid) as
date_paid’
end

Then @user = User.find(id)
@user.payments.last_payment_date

  I get the correct answer. But actually is this correct? Because

sql generated is

SELECT MAX(date_paid) as date_paid FROM payments WHERE
(payments.user_id = 8) AND (payments.user_id = 8)

 I would like to know why (`payments`.user_id = 8)  appears twice

Thanks