From 1fad133608c1e5ff5782fa3a9ba37861d8733339 Mon Sep 17 00:00:00 2001
From: Lukasz Samson <lukaszsamson@gmail.com>
Date: Sat, 18 Jul 2026 09:05:32 +0200
Subject: [PATCH 1/2] Fix stale belongs_to key in apply_changes

---
 lib/ecto/changeset.ex        | 9 ++++++---
 test/ecto/changeset_test.exs | 4 +++-
 2 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/lib/ecto/changeset.ex b/lib/ecto/changeset.ex
index f79066955f..7a8acf140a 100644
--- a/lib/ecto/changeset.ex
+++ b/lib/ecto/changeset.ex
@@ -4323,9 +4323,12 @@ defmodule Ecto.Changeset do
 
     acc = Map.put(acc, key, relation_changed)
 
-    with %Ecto.Association.BelongsTo{related_key: related_key} <- relation,
-         %{^related_key => id} <- relation_changed do
-      Map.put(acc, relation.owner_key, id)
+    with %Ecto.Association.BelongsTo{related_key: related_key, owner_key: owner_key} <- relation do
+      case relation_changed do
+        %{^related_key => id} -> Map.put(acc, owner_key, id)
+        nil -> Map.put(acc, owner_key, nil)
+        _ -> acc
+      end
     else
       _ -> acc
     end
diff --git a/test/ecto/changeset_test.exs b/test/ecto/changeset_test.exs
index 3f1b7910b6..d4b2054ead 100644
--- a/test/ecto/changeset_test.exs
+++ b/test/ecto/changeset_test.exs
@@ -1083,8 +1083,10 @@ defmodule Ecto.ChangesetTest do
     assert changed_post.title == "foo"
     assert changed_post.category_id == category.category_id
 
+    post_with_category = %Post{category_id: 5}
+
     changeset =
-      post
+      post_with_category
       |> changeset(%{"title" => "foo"})
       |> put_assoc(:category, nil)
 

From 5f1a69294eea73f151abd4c29b4dcf97fe17dd21 Mon Sep 17 00:00:00 2001
From: Lukasz Samson <lukaszsamson@gmail.com>
Date: Sat, 18 Jul 2026 09:24:49 +0200
Subject: [PATCH 2/2] Enforce belongs_to relation invariant in apply_changes

---
 lib/ecto/changeset.ex | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/lib/ecto/changeset.ex b/lib/ecto/changeset.ex
index 7a8acf140a..09dd81ebb4 100644
--- a/lib/ecto/changeset.ex
+++ b/lib/ecto/changeset.ex
@@ -4323,14 +4323,13 @@ defmodule Ecto.Changeset do
 
     acc = Map.put(acc, key, relation_changed)
 
-    with %Ecto.Association.BelongsTo{related_key: related_key, owner_key: owner_key} <- relation do
-      case relation_changed do
-        %{^related_key => id} -> Map.put(acc, owner_key, id)
-        nil -> Map.put(acc, owner_key, nil)
-        _ -> acc
-      end
-    else
-      _ -> acc
+    case relation do
+      %Ecto.Association.BelongsTo{related_key: related_key, owner_key: owner_key} ->
+        id = relation_changed && Map.fetch!(relation_changed, related_key)
+        Map.put(acc, owner_key, id)
+
+      _ ->
+        acc
     end
   end