15 years with Rails and DDD (AI Edition)

This talk presents what is the current problem with Rails apps from AI agents perspective. The lack of modularity in Rails apps make agents confused - the context is too big for them. The solution is making a big number of small modules with well defined interfaces.

Domain Driven Design (DDD) is a technique which, together with event-driven and CQRS results in smaller modules.

The app I am presenting has 50 small modules, composed via commands and events.

I am also presenting an algorithm which helps reducing the size of ActiveRecord models - by gradually moving their responsibilities to read models and aggregates.

Look at this repo to see the modularity mentioned - https://github.com/RailsEventStore/ecommerce

Transcript

1. Andrzej Krzywda, RubyConf Thailand, Bangkok 15 years with Rails and DDD
2. AI
3. agent coding since March 2025
4. 6 apps vibe coded in December
5. Fallout-themed rpg roguelike game • dragonruby (mruby) • voice-driven dev • perks, challenges, SPECIAL, levels • built with my 11 yrs son
6. • iOS app • Swift • chess
7. • Chrome extension • chess-related • originally JS • Claude rewrote to opalrb
8. • OS X menu app • Swift • never looked at the code
9. • The Rails Way • Business-heavy • Dashboard
11. AI and Rails
12. CRUD architecture
13. God objects >50 columns
14. Layers (mvc), but no modules
15. often Callbacks Hell
16. Usually Rails teams hit the wall after 2-5 years of CRUDs now it can be 2-5 days of vibe coding at ~100 ActiveRecords complexity
17. Andrzej Krzywda, Arkency CEO we help with legacy Rails apps since 2007
18. https://railseventstore.org/
19. Structure is missing
20. one big module is too much for the limited context of agents
21. modularity separation + composability
22. for AI and for humans: we need multiple small modules that compose together easily
23. DDD might be the missing part
24. You as a Rails Architect
25. gh clone RailsEventStore/ecommerce (CLAUDE.md included)
26. Speed and con fi dence
27. CLAUDE.md
28. 1. Perfect DDD 2. From legacy to DDD
29. 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
30. 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
31. Read models
33. 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
34. 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::OrderConfirm event_store.subscribe(ConnectAccount.new, to: [Authentication::AccountConnectedTo end end end 80 LOC Read model
35. module Customers class Customer < ApplicationRecord self.table_name = "customers" end private_constant :Customer end
36. 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
37. 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
38. 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
39. 220 LOC Domain module Inventory class Supply < Infra::Command attribute :product_id, Infra::Types::UUID attribute :quantity, Infra::Types::Coercible::Integer.constrained(gteq: 1) end end 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
40. 220 LOC Domain
41. Domain module Inventory class Supply < Infra::Command attribute :product_id, Infra::UUID attribute :quantity, Infra::Integer end end
42. 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
43. 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 dispatch(quantity) end def reserve(quantity) end def release(quantity) end private 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
44. 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
45. 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
46. module Processes class ShipmentProcess include Infra::ProcessManager.with_state { ProcessState } subscribes_to( Shipping::ShippingAddressAddedToShipment, Fulfillment::OrderRegistered, Fulfillment::OrderConfirmed, Stores::OfferRegistered )
47. module Processes class ShipmentProcess include Infra::ProcessManager.with_state { ProcessState } subscribes_to( Shipping::ShippingAddressAddedToShipment, Fulfillment::OrderRegistered, Fulfillment::OrderConfirmed, Stores::OfferRegistered )
48. 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
49. 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
50. 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
51. 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
52. 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
53. 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 def create_customer_cmd(customer_id, name) Crm::RegisterCustomer.new( customer_id: customer_id, name: name) end
54. 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 def create_customer_cmd(customer_id, name) Crm::RegisterCustomer.new( customer_id: customer_id, name: name) end
55. 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 module Crm class CustomerRegistered attribute :customer_id attribute :name, end end
56. Inventory Pricing CRM Catalog Shipments Taxes Communication x14 Domains Orders Customers Products ClientOrders ClientInbox PublicO ff er Read models x16 Reservation Shipment InvoiceGeneration Onboarding Process Managers x20 SendWelcomeMessage module Crm class CustomerRegistered attribute :customer_id attribute :name, end end
57. 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 def index @customers = Customers.customers_for_store end
58. 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 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
59. From Legacy to DDD
60. Step 1 app/domains app/read_models app/process_managers
61. Step 2 gem "rails_event_store"
62. Step 3 app/read_models/monthly_report.rb
63. Step 4 denormalized schema migration
64. Step 5 for each column needed: identify event(s)
65. Step 6 app/domains/sales/order_con fi rmed.rb
66. Step 7 event_store.publish(OrderCon fi rmed.new(data: {}) in the service object
67. Step 8 Keep comparing data in production and log di ff erences
68. Step 9 Get rid of original view code
69. Step 10 Remove ActiveRecord view-related code associations usually
70. Step 11 Replace callbacks with process managers
71. Step 12 Replace ActiveRecord with event-sourced aggregates
72. Bonus: mutation testing
73. Mutant keeps agents from misbehaving I wouldn't work with agents without mutant
75. Rails + DDD + AI + mutant
76. Thanks! andrzej@arkency.com @andrzejkrzywda gh clone RailsEventStore/ecommerce (CLAUDE.md included)