[Rails] validates_uniqueness_of before ActiveRecord::RecordNotUnique

I have the following class in a Rails 3.1.1 app:

class User < ActiveRecord::Base

attr_accessible :email, :password, :password_confirmation, :remember_me, :username, :admin, :moderator, :bio
validates_presence_of :username
validates_uniqueness_of :username, :case_sensitive => false
validates_uniqueness_of :email, :case_sensitive => false
and in my migrations:

add_index :users, :email, :unique => true
add_index :users, :username, :unique => true

However, when I try to create two users with duplicate emails or
usernames I get a DB level exception rather than a failing validation:

ActiveRecord::RecordNotUnique in RegistrationsController#create
PGError: ERROR: duplicate key value violates unique constraint
"index_users_on_username"

There are other validations that work perfectly, but why aren't the
uniqueness validations being performed before the DB create?

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