Fix

lib/chore_tracker/chores.ex

@@ -61,11 +61,15 @@ defmodule ChoreTracker.Chores do
   end
 
   def change_chore(%Chore{} = chore, attrs \\ %{}) do
-    assignees = Accounts.list_users_by_id(attrs["assignee_ids"])
-
     chore
     |> Chore.changeset(attrs)
-    |> Changeset.put_assoc(:assignees, assignees)
+    |> then(fn changeset ->
+      if ids = attrs["assignee_ids"] do
+        Changeset.put_assoc(changeset, :assignees, Accounts.list_users_by_id(ids))
+      else
+        changeset
+      end
+    end)
   end
 
   def change_chore_assignee(%Chore{} = chore, user) do

lib/chore_tracker_web/live/chore_live/form_component.ex

@@ -75,6 +75,8 @@ defmodule ChoreTrackerWeb.ChoreLive.FormComponent do
   end
 
   defp save_chore(socket, :edit, chore_params) do
+    chore_params = Map.merge(%{"assignee_ids" => []}, chore_params)
+
     case Chores.update_chore(socket.assigns.chore, chore_params) do
       {:ok, chore} ->
         notify_parent({:saved, chore})
@@ -109,8 +111,8 @@ defmodule ChoreTrackerWeb.ChoreLive.FormComponent do
   defp user_options(form) do
     existing_ids =
       form.source
-      |> Ecto.Changeset.get_change(:assignees, [])
-      |> Enum.map(& &1.data.id)
+      |> Ecto.Changeset.get_field(:assignees)
+      |> Enum.map(& &1.id)
 
     Accounts.list_users()
     |> Enum.map(&[key: Accounts.display_user(&1), value: &1.id, selected: &1.id in existing_ids])