How do I fetch just the ids via ActiveRecord

class PersonAddress < ActiveRecord::Base
end

I would like to fetch only the PersonAddress ids (primary keys) using a ActiveRecord query, how should I do that, something similar to PersonAddres开发者_运维问答s.find_all_by_person_id(person.id) which returns a set of address ids alone. (e.g. if the person has 3 addresses, then it should return 3 ids and not PersonAddress objects)

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What you could do is using :select parameter:

PersonAddress.find_all_by_person_id(person.id, :select => :id).map(&:id)
=> [2, 3, 5]  # Fake ids

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Depends on which Rails version you are using... but this might work:

PersonAddress.find_all_by_person_id(person.id).select("id")