Rails `find_in_batches` solves a vary particular problem, by checking your model's primary keys. If your primary key is never repeated you can use it with ease. If, on the other hand, your primary key repeats, you've got a problem on your hands... You can go from the other side of the relationship or you can brute force it...

batches.rb

module ActiveRecord
  module Batches
    def brute_batch(options={})
      options.reverse_merge!(batch_size: 1000, offset: 0)
      offset = options[:offset].to_i

      while true do
        results = offset(offset).limit(options[:batch_size])
        offset += results.count
        yield results
        break unless results.any?
      end

      offset
    end
  end
end