ruby.rb

def csv_stream
  sql = Request.where('created_at < ? AND client_id = ?', Time.now, 42).to_sql

  Enumerator.new do |y|
    conn = ActiveRecord::Base.connection.raw_connection
    conn.copy_data("COPY (#{sql}) TO STDOUT WITH (FORMAT CSV, HEADER TRUE);") do
      while (chunk = conn.get_copy_data)
        y << chunk
      end
    end
  end
end

stream = csv_stream