Question

class Car << ActiveRecord::Base
end

Car.all.each do |car|
  # do stuff
end

This loads all the objects of type Car into memory (I think) and iterates through them. What I want instead is to iterate through all the ids and load them one at a time like this:

Car.all_ids.each do |id|
  c = Car.find id
  # do stuff
end

But all_ids doesn’t exist, is there an equivalent?

Answer 1

For Rails 2.3 and up (including Rails 3) the easiest solution is find_each:

Car.find_each do |car|
  # do stuff
end

This executes the query in batches for you automatically. The default batch size is 1000 but you can set your own. It also works alongside named scopes and ActiveRecord::Relations:

Car.hotrods.where(:color => 'red').find_each(:batch_size => 10) { do |car| ... }

See http://guides.rubyonrails.org/active_record_querying.html#retrieving-multiple-objects-in-batches