Made DetermineVatRate process more readable

Used the Infra::ProcessManager module - consistent with our new direction for processes.
Enabled mutant for this class.
Added stream_name test which I'm not happy about (testing internals).

Data.define still feels a bit redundant with all the params.
In processes.rb I couldn't make it to work with .subscribed_events, it failed on acceptance tests.

Author: andrzejkrzywda

ecommerce/processes/.mutant.yml

@@ -15,5 +15,4 @@ matcher:
     - Processes::OrderItemInvoicingProcess*
     - Processes::SyncShipmentFromPricing*
     - Processes::SyncInventoryFromOrdering*
-    - Processes::NotifyPaymentsAboutOrderValue*
-    - Processes::DetermineVatRatesOnOrderPlaced*
+    - Processes::NotifyPaymentsAboutOrderValue*

ecommerce/processes/lib/processes/determine_vat_rates_on_order_placed.rb

@@ -2,17 +2,38 @@ module Processes
   class DetermineVatRatesOnOrderPlaced
     include Infra::Retry
 
-    def initialize(event_store, command_bus)
-      @event_store = event_store
-      @command_bus = command_bus
+    ProcessState = Data.define(:offer_accepted, :order_placed, :order_id, :order_lines) do
+      def initialize(offer_accepted: false, order_placed: false, order_id: nil, order_lines: [])
+        super(offer_accepted:, order_placed:, order_id:, order_lines: order_lines)
+      end
+
+      def placed? = offer_accepted && order_placed
+    end
+
+    include Infra::ProcessManager.with_state(ProcessState)
+    subscribes_to(
+      Pricing::OfferAccepted,
+      Fulfillment::OrderRegistered
+    )
+
+    def apply(event)
+      case event
+      when Pricing::OfferAccepted
+        state.with(
+          offer_accepted: true,
+          order_lines: event.data.fetch(:order_lines),
+          order_id: event.data.fetch(:order_id)
+        )
+      when Fulfillment::OrderRegistered
+        state.with(order_placed: true)
+      end
     end
 
-    def call(event)
-      state = build_state(event)
-      determine_vat_rates(state) if state.placed?
+    def act
+      determine_vat_rates if state.placed?
     end
 
-    def determine_vat_rates(state)
+    def determine_vat_rates
       state.order_lines.each do |line|
         product_id = line.fetch(:product_id)
         command = Taxes::DetermineVatRate.new(order_id: state.order_id, product_id: product_id)
@@ -22,41 +43,9 @@ def determine_vat_rates(state)
 
     private
 
-    attr_reader :event_store, :command_bus
-
-    def build_state(event)
-      with_retry do
-        stream_name = "DetermineVatRatesOnOrderPlacedProcess#{event.data.fetch(:order_id)}"
-        past_events = event_store.read.stream(stream_name).to_a
-        last_stored = past_events.size - 1
-        event_store.link(event.event_id, stream_name: stream_name, expected_version: last_stored)
-        ProcessState.new.tap do |state|
-          past_events.each { |ev| state.call(ev) }
-          state.call(event)
-        end
-      end
+    def fetch_id(event)
+      event.data.fetch(:order_id)
     end
 
-    class ProcessState
-      def initialize
-        @offer_accepted = false
-        @order_placed = false
-      end
-
-      attr_reader :order_id, :order_lines
-
-      def call(event)
-        case event
-        when Pricing::OfferAccepted
-          @offer_accepted = true
-          @order_lines = event.data.fetch(:order_lines)
-          @order_id = event.data.fetch(:order_id)
-        when Fulfillment::OrderRegistered
-          @order_placed = true
-        end
-      end
-
-      def placed? = @offer_accepted && @order_placed
-    end
   end
-end
+end

ecommerce/processes/test/determine_vat_rate_test.rb

@@ -29,6 +29,14 @@ def test_placed_but_not_accepted
       assert_no_command
     end
 
+    def test_stream_name
+      process = DetermineVatRatesOnOrderPlaced.new(event_store, command_bus)
+      given([order_placed]).each do |event|
+        process.call(event)
+      end
+      assert_equal "Processes::DetermineVatRatesOnOrderPlaced$#{order_id}", process.send(:stream_name)
+    end
+
     private
 
     def offer_accepted(order_id, product_id)