ruby 41 lines · 8 steps

Streaming a CSV export in Rails

How ActionController::Live streams a large CSV to the client row-by-row without buffering it all in memory.

Explained by highlit

Rails Advanced streaming csv-export batching http-headers resource-cleanup

1class Admin::OrdersExportController < Admin::BaseController
include ActionController::Live
3 
def show
  response.headers["Content-Type"] = "text/csv"
  response.headers["Content-Disposition"] = %(attachment; filename="orders-#{Date.current.iso8601}.csv")
  response.headers["Last-Modified"] = Time.current.httpdate
  response.headers["X-Accel-Buffering"] = "no"
9 
  writer = CSV.new(response.stream)
  writer << %w[id placed_at customer_email status subtotal tax total currency]
12 
  scope
    .includes(:customer)
    .find_each(batch_size: 500) do |order|
      writer << [
        order.id,
        order.placed_at&.iso8601,
        order.customer.email,
        order.status,
        order.subtotal_cents / 100.0,
        order.tax_cents / 100.0,
        order.total_cents / 100.0,
        order.currency,
      ]
    end
rescue ActionController::Live::ClientDisconnected
  logger.info("Orders export aborted: client disconnected")
ensure
  response.stream.close
end
32 
private
34 
def scope
  orders = current_account.orders.order(:id)
  orders = orders.where(status: params[:status]) if params[:status].present?
  orders = orders.where(placed_at: params[:from]..params[:to]) if params[:from].present?
  orders
end
41end

01 / 01

STEP 01

Walkthrough

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Three takeaways

1Streaming responses with ActionController::Live let you send data as it's produced instead of building the whole payload in memory.
2Pairing find_each with a live stream keeps both database and response memory bounded no matter how many rows you export.
3A live stream must always be closed in ensure, and client disconnects should be caught rather than crash the action.

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