Support subclasses in lists in Union of List types

src/serializers/type_serializers/list.rs

@@ -116,4 +116,8 @@ impl TypeSerializer for ListSerializer {
     fn get_name(&self) -> &str {
         &self.name
     }
+
+    fn retry_with_lax_check(&self) -> bool {
+        self.item_serializer.retry_with_lax_check()
+    }
 }

src/serializers/type_serializers/union.rs

@@ -75,9 +75,10 @@ impl TypeSerializer for UnionSerializer {
         exclude: Option<&PyAny>,
         extra: &Extra,
     ) -> PyResult<PyObject> {
-        // try the serializers in with error_on fallback=true
+        // try the serializers in left to right order with error_on fallback=true
         let mut new_extra = extra.clone();
         new_extra.check = SerCheck::Strict;
+
         for comb_serializer in &self.choices {
             match comb_serializer.to_python(value, include, exclude, &new_extra) {
                 Ok(v) => return Ok(v),

tests/serializers/test_union.py

@@ -454,3 +454,59 @@ class Item(BaseModel):
     )
 
     assert s.to_python(DBUser(name='John', password='secret')) == {'name': 'John'}
+
+
+def test_union_serializes_list_of_model_subclass_from_definition() -> None:
+    class BaseModel:
+        __slots__ = '__dict__', '__pydantic_fields_set__', '__pydantic_extra__', '__pydantic_private__'
+
+        def __init__(self, **kwargs: Any):
+            for key, value in kwargs.items():
+                setattr(self, key, value)
+
+    class User(BaseModel):
+        name: str
+
+    class DBUser(User):
+        password: str
+        __pydantic_serializer__: ClassVar[SchemaSerializer]
+
+    DBUser.__pydantic_serializer__ = SchemaSerializer(
+        core_schema.model_schema(
+            DBUser,
+            core_schema.model_fields_schema(
+                {
+                    'name': core_schema.model_field(core_schema.str_schema()),
+                    'password': core_schema.model_field(core_schema.str_schema()),
+                }
+            ),
+        )
+    )
+
+    class Item(BaseModel):
+        price: float
+
+    s = SchemaSerializer(
+        core_schema.definitions_schema(
+            core_schema.union_schema(
+                [
+                    core_schema.list_schema(core_schema.definition_reference_schema('User'), strict=False),
+                    core_schema.list_schema(core_schema.definition_reference_schema('Item'), strict=False),
+                ]
+            ),
+            [
+                core_schema.model_schema(
+                    User,
+                    core_schema.model_fields_schema({'name': core_schema.model_field(core_schema.str_schema())}),
+                    ref='User',
+                ),
+                core_schema.model_schema(
+                    Item,
+                    core_schema.model_fields_schema({'price': core_schema.model_field(core_schema.float_schema())}),
+                    ref='Item',
+                ),
+            ],
+        )
+    )
+
+    assert s.to_python([DBUser(name='John', password='secret')]) == [{'name': 'John'}]