Reset parent association when updating from nil

lib/ecto/repo/schema.ex

@@ -1142,11 +1142,21 @@ defmodule Ecto.Repo.Schema do
   defp reset_parent?(changes, data, assoc) do
     %{field: field, owner_key: owner_key, related_key: related_key} = assoc
 
-    with %{^owner_key => owner_value} <- changes,
-         %{^field => %{^related_key => related_value}} when owner_value != related_value <- data do
-      true
-    else
-      _ -> false
+    case changes do
+      %{^owner_key => owner_value} ->
+        case data do
+          %{^field => %{^related_key => related_value}} when owner_value != related_value ->
+            true
+
+          %{^field => nil} when owner_value != nil ->
+            true
+
+          _ ->
+            false
+        end
+
+      _ ->
+        false
     end
   end
 

test/ecto/repo/belongs_to_test.exs

@@ -496,4 +496,18 @@ defmodule Ecto.Repo.BelongsToTest do
     assert updated_schema.assoc_id == 2
     assert %Ecto.Association.NotLoaded{} = updated_schema.assoc
   end
+
+  test "reset assoc when foreign key update results in a mismatch and parent is nil" do
+    schema = %MySchema{} |> TestRepo.insert!() |> TestRepo.preload(:assoc)
+    assert schema.assoc_id == nil
+    assert schema.assoc == nil
+
+    updated_schema =
+      schema
+      |> Ecto.Changeset.change(%{assoc_id: 2})
+      |> TestRepo.update!()
+
+    assert updated_schema.assoc_id == 2
+    assert %Ecto.Association.NotLoaded{} = updated_schema.assoc
+  end
 end