We recently upgraded one of our services to use Rails 3.2. The downside I discovered pretty quickly is that rails broke support for the 3.0 way of streaming data.

The 3.0 way, simplified a bit:

self.response_body =  proc{ |response, output|
  @records.each do |record|
     output.write << CSV.generate_line(record.csv_values)
}

In >= 3.1, the proc method has been deprecated. What are we to do then? Well it's actually pretty simple if we go digging in the Rack source code.

      if body.respond_to? :to_str
        write body.to_str
      elsif body.respond_to?(:each)
        body.each { |part|
          write part.to_s
        }
      else
        raise TypeError, "stringable or iterable required"
      end

Basically, if we set response_body to something that responds to 'each' then rack will automatically send those chunks back to the browser without having to do anything special on our end. In my case I wrote a little streamer class that would take care of it.

class RecordCsvStreamer
   attr_reader :records

   def initialize(record_scope)
      @records = record_scope
   end

   def each
     records.each do |record|
        yield csv_row(record)
     end
   end

   def csv_row(record)
   # however you want to format it
   end
end

Then in my controller I do

class RecordController
  def index
    @records = Record.all

    self.response_body = new RecordCsvStreamer(@records)
  end
end

Happy streaming!