test: update test to reflect nil assignment prevention

test/membership_fees/changes/validate_same_interval_test.exs

@@ -93,7 +93,7 @@ defmodule Mv.MembershipFees.Changes.ValidateSameIntervalTest do
       assert changeset.valid?
     end
 
-    test "allows removal of membership fee type" do
+    test "prevents removal of membership fee type" do
       yearly_type = create_fee_type(%{interval: :yearly})
       member = create_member(%{membership_fee_type_id: yearly_type.id})
 
@@ -104,7 +104,13 @@ defmodule Mv.MembershipFees.Changes.ValidateSameIntervalTest do
         })
         |> ValidateSameInterval.change(%{}, %{})
 
-      assert changeset.valid?
+      refute changeset.valid?
+      assert %{errors: errors} = changeset
+
+      assert Enum.any?(errors, fn error ->
+               error.field == :membership_fee_type_id and
+                 error.message =~ "Cannot remove membership fee type"
+             end)
     end
 
     test "does nothing when membership_fee_type_id is not changed" do