Rails find record with zero has_many records associated

This seems fairly simple but I can't get it to turn up on Google.

If I have:

class City < ActiveRecord::Base

  has_many :photos

end



class Photo < ActiveRecord::Base

  belongs_to :city

end

I want to find all cities that have no photos. I'd love to be able to call something like...

City.where( photos.empty? )

...but that doesn't exist. So, how do you do this kind of query?

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Update:
Having now found an answer to the original question, I'm curious, how do you construct the inverse?

IE: if I wanted to create these as scopes:

scope :without_photos, includes(:photos).where( :photos => {:city_id=>nil} )

scope :with_photos, ???