Fix custom field serializer inside a flattened field

serde/se.py

@@ -307,11 +307,7 @@ def wrap(cls: type[T]) -> type[T]:
             add_func(scope, union_key, render_union_func(cls, union_args, tagging), g)
             scope.union_se_args[union_key] = union_args
 
-        for f in sefields(cls, serialize_class_var):
-            if f.skip_if:
-                g[f.skip_if.name] = f.skip_if
-            if f.serializer:
-                g[f.serializer.name] = f.serializer
+        add_field_serializers_to_scope(sefields(cls, serialize_class_var), g, serialize_class_var)
 
         add_func(
             scope,
@@ -603,6 +599,29 @@ def sefields(cls: type[Any], serialize_class_var: bool = False) -> Iterator[SeFi
         yield f
 
 
+def add_field_serializers_to_scope(
+    sfields: Iterator[SeField[Any]], g: dict[str, Any], serialize_class_var: bool = False
+) -> None:
+    """
+    Register custom field serializers and skip_if predicates in the generated globals.
+
+    A flattened dataclass field is rendered inline in the enclosing class's serialize
+    function, so the custom serializers and skip_if predicates of its inner fields must
+    be available in this scope too, not only those of the top-level fields (#453).
+    """
+    for f in sfields:
+        if f.skip_if:
+            g[f.skip_if.name] = f.skip_if
+        if f.serializer:
+            g[f.serializer.name] = f.serializer
+        if f.flatten:
+            inner = f[0] if is_opt(f.type) else f
+            if is_dataclass(inner.type):
+                add_field_serializers_to_scope(
+                    sefields(inner.type, serialize_class_var), g, serialize_class_var
+                )
+
+
 jinja2_env = jinja2.Environment(
     loader=jinja2.DictLoader(
         {

tests/test_flatten.py

@@ -30,6 +30,27 @@ class Foo:
     assert from_json(Foo, s) == f
 
 
+def test_flatten_custom_field_serializer() -> None:
+    # A custom field serializer on a flattened dataclass's field must be in
+    # scope of the enclosing class's generated serializer (#453).
+    @serde
+    class Child:
+        value: int = field(serializer=str)
+
+    @serde
+    class Parent:
+        child: Child = field(flatten=True)
+
+    assert to_json(Parent(child=Child(value=3))) == '{"value":"3"}'
+
+    # also works through a nested flatten
+    @serde
+    class GrandParent:
+        parent: Parent = field(flatten=True)
+
+    assert to_json(GrandParent(parent=Parent(child=Child(value=7)))) == '{"value":"7"}'
+
+
 @pytest.mark.parametrize("se,de", all_formats)
 def test_flatten(se: Any, de: Any) -> None:
     @serde