use belongs_to instead of field for references

lib/cannery/ammo/ammo_group.ex

@@ -1,6 +1,7 @@
 defmodule Cannery.Ammo.AmmoGroup do
   use Ecto.Schema
   import Ecto.Changeset
+  alias Cannery.{Accounts, Ammo, Containers, Tags}
 
   @primary_key {:id, :binary_id, autogenerate: true}
   @foreign_key_type :binary_id
@@ -8,10 +9,11 @@ defmodule Cannery.Ammo.AmmoGroup do
     field :count, :integer
     field :notes, :string
     field :price_paid, :float
-    field :tag_id, :binary_id
-    field :ammo_type_id, :binary_id
-    field :container_id, :binary_id
-    field :user_id, :binary_id
+
+    belongs_to :tag, Tags.Tag
+    belongs_to :ammo_type, Ammo.AmmoType
+    belongs_to :container, Containers.Container
+    belongs_to :user, Accounts.User
 
     timestamps()
   end
@@ -19,7 +21,15 @@ defmodule Cannery.Ammo.AmmoGroup do
   @doc false
   def changeset(ammo_group, attrs) do
     ammo_group
-    |> cast(attrs, [:count, :price_paid, :notes])
-    |> validate_required([:count, :price_paid, :notes])
+    |> cast(attrs, [:count, :price_paid, :notes, :tag_id, :ammo_type_id, :container_id, :user_id])
+    |> validate_required([
+      :count,
+      :price_paid,
+      :notes,
+      :tag_id,
+      :ammo_type_id,
+      :container_id,
+      :user_id
+    ])
   end
 end