Code for step 6

lib/book_store/books/book_process.ex

@@ -0,0 +1,76 @@
+defmodule BookStore.Books.BookProcess do
+  use GenServer, restart: :transient
+
+  require Logger
+
+  alias BookStore.Repo
+  alias BookStore.Books.Book
+  alias Ecto.Changeset
+
+  def start_link(%Book{} = book) do
+    GenServer.start_link(__MODULE__, book,
+      name: {:via, Registry, {BookStore.BookRegistry, book.id}}
+    )
+  end
+
+  @impl true
+  def init(%Book{} = state) do
+    {:ok, state}
+  end
+
+  @impl true
+  def handle_call(:read, _from, %Book{} = state) do
+    {:reply, state, state}
+  end
+
+  @impl true
+  def handle_call({:update, attrs}, _from, %Book{} = state) do
+    state
+    |> update_book(attrs)
+    |> case do
+      {:ok, %Book{} = updated_book} ->
+        {:reply, updated_book, updated_book, {:continue, :persist_book_changes}}
+
+      error ->
+        {:reply, error, state}
+    end
+  end
+
+  @impl true
+  def handle_call(:order_copy, _from, %Book{quantity: 0} = state) do
+    {:reply, :no_copies_available, state}
+  end
+
+  @impl true
+  def handle_call(:order_copy, _from, %Book{quantity: quantity} = state) do
+    state
+    |> update_book(%{quantity: quantity - 1})
+    |> case do
+      {%Book{} = updated_book, changeset} ->
+        {:reply, :ok, updated_book, {:continue, {:persist_book_changes, changeset}}}
+
+      error ->
+        {:reply, error, state}
+    end
+  end
+
+  @impl true
+  def handle_continue({:persist_book_changes, changeset}, state) do
+    Repo.update(changeset)
+
+    {:noreply, state}
+  end
+
+  defp update_book(book, attrs) do
+    book
+    |> Book.changeset(attrs)
+    |> case do
+      %Changeset{valid?: true} = changeset ->
+        updated_book = Changeset.apply_changes(changeset)
+        {updated_book, changeset}
+
+      error_changeset ->
+        {:error, error_changeset}
+    end
+  end
+end