fix: change_attribute failing on a union attribute (#778)

lib/ash/type/union.ex

@@ -530,7 +530,7 @@ defmodule Ash.Type.Union do
     |> Map.new()
     |> case do
       %{tag: field, tag_value: tag} ->
-        if new_value[field] == tag || new_value[to_string(field)] == tag,
+        if get_tag(new_value, field) == tag,
           do: do_prepare_change(type_name, old_value, new_value, constraints),
           else: {:ok, new_value}
 

test/type/union_test.exs

@@ -58,6 +58,22 @@ defmodule Ash.Test.Type.UnionTest do
                       ]
                     ]
       end
+
+      attribute :thing, :union,
+        constraints: [
+          types: [
+            foo: [
+              type: Foo,
+              tag: :type,
+              tag_value: "foo"
+            ],
+            bar: [
+              type: Bar,
+              tag: :type,
+              tag_value: "bar"
+            ]
+          ]
+        ]
     end
   end
 
@@ -234,6 +250,16 @@ defmodule Ash.Test.Type.UnionTest do
     end
   end
 
+  test "it handles changing union attribute on a resource" do
+    Example
+    |> Ash.Changeset.for_create(:create, %{thing: %Foo{type: "foo", foo: "foo"}})
+    |> Ash.Test.AnyApi.create!()
+    |> Ash.Changeset.new()
+    |> Ash.Changeset.change_attribute(:thing, %Bar{type: "bar", bar: "bar"})
+    |> Ash.Changeset.for_update(:update)
+    |> Ash.Test.AnyApi.update!()
+  end
+
   test "it handles paths on a resource" do
     Example
     |> Ash.Changeset.for_create(:create, %{things: [%{type: :foo, foo: "bar"}]})