show average cost per round for ammo types

lib/cannery/ammo.ex

@@ -37,6 +37,30 @@ defmodule Cannery.Ammo do
   @spec get_ammo_type!(AmmoType.id()) :: AmmoType.t()
   def get_ammo_type!(id), do: Repo.get!(AmmoType, id)
 
+  @doc """
+  Gets the average cost of a single ammo type
+
+  Raises `Ecto.NoResultsError` if the Ammo type does not exist.
+
+  ## Examples
+
+      iex> get_ammo_type!(123)
+      %AmmoType{}
+
+      iex> get_ammo_type!(456)
+      ** (Ecto.NoResultsError)
+
+  """
+  @spec get_average_cost_for_ammo_type!(AmmoType.t()) :: float()
+  def get_average_cost_for_ammo_type!(%{id: ammo_type_id}) do
+    Repo.one!(
+      from ag in AmmoGroup,
+        where: ag.ammo_type_id == ^ammo_type_id,
+        where: not (ag.price_paid |> is_nil()),
+        select: sum(ag.price_paid) / sum(ag.count)
+    )
+  end
+
   @doc """
   Creates a ammo_type.
 

lib/cannery_web/live/ammo_type_live/show.ex

@@ -14,11 +14,14 @@ defmodule CanneryWeb.AmmoTypeLive.Show do
 
   @impl true
   def handle_params(%{"id" => id}, _, socket) do
+    ammo_type = Ammo.get_ammo_type!(id) |> Repo.preload(:ammo_groups)
+
     socket =
       socket
       |> assign(
         page_title: page_title(socket.assigns.live_action),
-        ammo_type: Ammo.get_ammo_type!(id) |> Repo.preload(:ammo_groups)
+        ammo_type: ammo_type,
+        avg_cost_per_round: ammo_type |> Ammo.get_average_cost_for_ammo_type!()
       )
 
     {:noreply, socket}

lib/cannery_web/live/ammo_type_live/show.html.heex

@@ -76,6 +76,16 @@
         </span>
       <% end %>
     <% end %>
+
+    <%= if @avg_cost_per_round do %>
+      <h3 class="mb-2 sm:mr-4 title text-lg">
+        Average Price paid:
+      </h3>
+
+      <span class="mb-4 sm:mb-2 text-primary-600">
+        <%= @avg_cost_per_round |> :erlang.float_to_binary(decimals: 2) %>
+      </span>
+    <% end %>
   </div>
 
   <hr class="hr">