Introduce Inventory::Product write model to take care of Stock Level

This model will be used as a source of truth for the Stock Level. It'll
replace current Product Active Record stock level.

rails_application/app/models/inventory/product.rb

@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+module Inventory
+  class Product
+    include AggregateRoot
+
+    private attr_reader :id
+
+    def initialize(id)
+      @id = id
+      @stock_level = 0
+    end
+
+    def supply(quantity)
+      apply(StockLevelIncreased.new(data: { id:, quantity: }))
+    end
+
+    def withdraw(quantity)
+      raise "Not enough stock" if @stock_level < quantity
+      apply(StockLevelDecreased.new(data: { id:, quantity: }))
+    end
+
+    on StockLevelIncreased do |event|
+      @stock_level += event.data[:quantity]
+    end
+
+    on StockLevelDecreased do |event|
+      @stock_level -= event.data[:quantity]
+    end
+  end
+end

rails_application/app/models/inventory/stock_level_decreased.rb

@@ -0,0 +1,6 @@
+# frozen_string_literal: true
+
+module Inventory
+  StockLevelDecreased = Class.new(Infra::Event)
+end
+

rails_application/app/models/inventory/stock_level_increased.rb

@@ -0,0 +1,5 @@
+# frozen_string_literal: true
+
+module Inventory
+  StockLevelIncreased = Class.new(Infra::Event)
+end

rails_application/test/models/inventory/product_test.rb

@@ -0,0 +1,55 @@
+# frozen_string_literal: true
+
+require 'test_helper'
+
+module Inventory
+  class ProductTest < Infra::InMemoryTest
+    def test_supply
+      product_id = 1024
+
+      assert_events(stream_name(product_id), StockLevelIncreased.new(data: { id: product_id, quantity: 10 })) do
+        with_aggregate(product_id) do |product|
+          product.supply(10)
+        end
+      end
+    end
+
+    def test_withdraw
+      product_id = 1024
+
+      assert_events(stream_name(product_id),
+                    StockLevelIncreased.new(data: { id: product_id, quantity: 10 }),
+                    StockLevelDecreased.new(data: { id: product_id, quantity: 5 })
+      ) do
+        with_aggregate(product_id) do |product|
+          product.supply(10)
+          product.withdraw(5)
+        end
+      end
+    end
+
+    def test_withdraw_when_not_enough_stock_is_not_allowed
+      product_id = 1024
+
+      assert_nothing_published_within do
+        assert_raises("Not enough stock") do
+          with_aggregate(product_id) do |product|
+            product.withdraw(10)
+          end
+        end
+      end
+    end
+
+    private
+
+    def stream_name(product_id)
+      "Inventory::Product$#{product_id}"
+    end
+
+    def with_aggregate(product_id)
+      Infra::AggregateRootRepository.new(event_store).with_aggregate(Product, product_id) do |product|
+        yield product
+      end
+    end
+  end
+end