15 years of Rails with Domain Driven Design - lessons learnt

How to use AI to transform your Rails app towards a DDD application. The resulting architecture is AI-native and allows non-technical people to create some of the new features - especially read models.

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1. AI Business Developers Code push slop use more AI push slop more complexity more tokens pay more
3. How can we solve it?
4. AI Business Developers Code writes new features proud of code architect infra gets cleaner generates tests transforms the code makes NO MISTAKES
5. Andrzej Krzywda, Vienna 2026 15 years with Rails and DDD CQRS Events
7. AI harness event bus event store
8. Extracted in 2015, perfect for existing Rails apps
9. The guardrails for AI, Rails and DDD are here
10. Inventory Pricing CRM Catalog Shipments Taxes Invoicing x14 Domains Orders Customers Products ClientOrders ClientInbox PublicO ff er Read models x16 Reservation Shipment InvoiceGeneration Onboarding Process Managers x20 Modern DDD building blocks commands + events
12. Which parts of your codebase could allow AI code?
13. Step 1: Separate reads
14. 80% of Rails app tra ff i c is reads typical Rails schema write schema as core reads separated
18. Understand the conventions but let the agents write
19. create_table "customers", id: :uuid, force: :cascade do |t| t.string "name" t.boolean "vip", t.decimal "paid_orders_summary", t.uuid "account_id" t.uuid "store_id" end
20. module Customers class Customer < ApplicationRecord self.table_name = "customers" end private_constant :Customer def self.customers_for_store(store_id) Customer.where(store_id: store_id) end def self.find_customer_in_store(customer_id, store_id) Customer.where(store_id: store_id).find(customer_id) end class Configuration def call(event_store) event_store.subscribe(RegisterCustomer.new, to: [Crm::CustomerRegistered]) event_store.subscribe(AssignStoreToCustomer.new, to: [Stores::CustomerRegistered]) event_store.subscribe(PromoteToVip.new, to: [Crm::CustomerPromotedToVip]) event_store.subscribe(UpdatePaidOrdersSummary.new, to: [Fulfillment::OrderConfirmed]) event_store.subscribe(ConnectAccount.new, to: [Authentication::AccountConnectedToClient]) end end end 80 LOC Read model
21. module Customers class Customer < ApplicationRecord self.table_name = "customers" end private_constant :Customer end
22. module Customers class Configuration def call(event_store) event_store.subscribe(RegisterCustomer.new, to: [Crm::CustomerRegistered]) event_store.subscribe(AssignStoreToCustomer.new, to: [Stores::CustomerRegistered]) event_store.subscribe(PromoteToVip.new, to: [Crm::CustomerPromotedToVip]) event_store.subscribe(UpdatePaidOrdersSummary.new, to: [Fulfillment::OrderConfirmed]) end end end Read model
23. module Customers class PromoteToVip def call(event) promote_to_vip(event) end private def promote_to_vip(event) find(event.data.fetch(:customer_id)).update(vip: true) end def find(customer_id) Customer.where(id: customer_id).first end end end Read model
24. Where do we get events from?
28. Too many read models, long transaction problem?
29. https://railseventstore.org/docs/advanced-topics/outbox Outbox pattern RailsEventStore gem
30. Step 2: Separate domains
31. Inventory Pricing CRM Catalog Shipments Taxes Invoicing x14 Domains Orders Customers Products ClientOrders ClientInbox PublicO ff er Read models x16 Reservation Shipment InvoiceGeneration Onboarding Process Managers x20
32. Domain module Inventory class Supply < Infra::Command attribute :product_id, Infra::UUID attribute :quantity, Infra::Integer end end 220 LOC
33. Domain module Inventory class StockLevelChanged < Infra::Event end end def supply(quantity) apply StockLevelChanged.new( data: { product_id: @product_id, quantity: quantity, stock_level: (@in_stock || 0) + quantity } ) apply_availability_changed end
34. module Inventory class InventoryEntry include AggregateRoot InventoryNotAvailable = Class.new(StandardError) InventoryNotEvenReserved = Class.new(StandardError) def initialize(product_id) @product_id = product_id @reserved = 0 end def supply(quantity) apply StockLevelChanged.new( data: { product_id: @product_id, quantity: quantity, stock_level: (@in_stock || 0) + quantity } ) apply_availability_changed end Domain def apply_availability_changed apply AvailabilityChanged.new( data: { product_id: @product_id, available: availability } ) if stock_level_defined? end on StockLevelChanged do |event| @in_stock = event.data.fetch(:stock_level) end
35. Step 3: Separate processes
36. Inventory Pricing CRM Catalog Shipments Taxes Invoicing x14 Domains Orders Customers Products ClientOrders ClientInbox PublicO ff er Read models x16 Reservation Shipment InvoiceGeneration Onboarding Process Managers x20
37. module Processes class ShipmentProcess include Infra::ProcessManager.with_state { ProcessState } subscribes_to( Shipping::ShippingAddressAddedToShipment, Fulfillment::OrderRegistered, Fulfillment::OrderConfirmed, Stores::OfferRegistered ) private def act case state in { shipment: :address_set, order: :placed } register_shipment submit_shipment in { shipment: :address_set, order: :confirmed } register_shipment submit_shipment authorize_shipment else end end 67 LOC def apply(event) case event when Shipping::ShippingAddressAddedToShipment state.with(shipment: :address_set) when Fulfillment::OrderRegistered state.with(order: :placed) when Fulfillment::OrderConfirmed state.with(order: :confirmed) when Stores::OfferRegistered state.with(store_id: event.data.fetch(:store_id)) end end def register_shipment return unless state.store_id command_bus.call( Stores::RegisterShipment.new( shipment_id: id, store_id: state.store_id ) ) end def submit_shipment command_bus.call(Shipping::SubmitShipment.new(order_id: id)) end def authorize_shipment command_bus.call(Shipping::AuthorizeShipment.new(order_id: id)) end def fetch_id(event) event.data.fetch(:order_id) end ProcessState = Data.define(:order, :shipment, :store_id) do def initialize(order: nil, shipment: nil, store_id: nil) = super end end end Process Managers
38. module Processes class ShipmentProcess include Infra::ProcessManager.with_state { ProcessState } subscribes_to( Shipping::ShippingAddressAddedToShipment, Fulfillment::OrderRegistered, Fulfillment::OrderConfirmed, Stores::OfferRegistered )
39. module Processes class ShipmentProcess include Infra::ProcessManager.with_state { ProcessState } subscribes_to( Shipping::ShippingAddressAddedToShipment, Fulfillment::OrderRegistered, Fulfillment::OrderConfirmed, Stores::OfferRegistered )
40. module Processes class ShipmentProcess include Infra::ProcessManager.with_state { ProcessState } def act case state in { shipment: :address_set, order: :placed } register_shipment submit_shipment in { shipment: :address_set, order: :confirmed } register_shipment submit_shipment authorize_shipment else end end
41. def submit_shipment command_bus.call(Shipping::SubmitShipment.new(order_id: id)) end def authorize_shipment command_bus.call(Shipping::AuthorizeShipment.new(order_id: id)) end
42. Inventory Pricing CRM Catalog Shipments Taxes Invoicing x14 Domains Orders Customers Products ClientOrders ClientInbox PublicO ff er Read models x16 Reservation Shipment InvoiceGeneration Onboarding Process Managers x20 Controller
43. Inventory Pricing CRM Catalog Shipments Taxes Invoicing x14 Domains Orders Customers Products ClientOrders ClientInbox PublicO ff er Read models x16 Reservation Shipment InvoiceGeneration Onboarding Process Managers x20 class CustomersController < ApplicationController def index @customers = Customers.customers_for_store(current_store_id) end def create ActiveRecord::Base.transaction do create_customer(params[:customer_id], params[:name]) end rescue Crm::Customer::AlreadyRegistered flash[:notice] = "Customer was already registered" render "new" else redirect_to customers_path end def create_customer(customer_id, name) command_bus.(create_customer_cmd(customer_id, name)) command_bus.(register_customer_in_store_cmd(customer_id)) end def create_customer_cmd(customer_id, name) Crm::RegisterCustomer.new( customer_id: customer_id, name: name) end
44. CRM Domains Customers Read models Onboarding Process Managers class CustomersController < ApplicationController def index @customers = Customers.customers_for_store end def create ActiveRecord::Base.transaction do create_customer(params[:customer_id], params[:name]) end rescue Crm::Customer::AlreadyRegistered flash[:notice] = "Customer was already registered" render "new" else redirect_to customers_path end 1 2 3 4 5 cmd event Communication cmd Inbox
45. But tokens are expensive!
46. Read models, domains, processes are tiny enough to allow local LLMs
47. How to keep agents from deleting tests?
48. Mutant keeps agents from misbehaving mutant > simplecov Where is your 100% coverage?
49. Can AI extract the read models for you?
51. Packwerk output as a technical debt documentation
53. AI harness
54. Prepare the architecture for what's already happening
55. Domain experts writing read models with AI
56. Thanks! andrzej@arkency.com @andrzejkrzywda