Fix union validation logic when extra='allow'

src/validators/model.rs

@@ -204,7 +204,6 @@ impl Validator for ModelValidator {
             for field_name in validated_fields_set {
                 fields_set.add(field_name)?;
             }
-            state.add_fields_set(fields_set.len());
         }
 
         force_setattr(py, model, intern!(py, DUNDER_DICT), validated_dict.to_object(py))?;
@@ -244,11 +243,9 @@ impl ModelValidator {
             };
             force_setattr(py, self_instance, intern!(py, DUNDER_FIELDS_SET_KEY), &fields_set)?;
             force_setattr(py, self_instance, intern!(py, ROOT_FIELD), &output)?;
-            state.add_fields_set(fields_set.len());
         } else {
             let (model_dict, model_extra, fields_set): (Bound<PyAny>, Bound<PyAny>, Bound<PyAny>) =
                 output.extract(py)?;
-            state.add_fields_set(fields_set.len().unwrap_or(0));
             set_model_attrs(self_instance, &model_dict, &model_extra, &fields_set)?;
         }
         self.call_post_init(py, self_instance.clone(), input, state.extra())
@@ -287,11 +284,10 @@ impl ModelValidator {
             };
             force_setattr(py, &instance, intern!(py, DUNDER_FIELDS_SET_KEY), &fields_set)?;
             force_setattr(py, &instance, intern!(py, ROOT_FIELD), output)?;
-            state.add_fields_set(fields_set.len());
         } else {
-            let (model_dict, model_extra, val_fields_set) = output.extract(py)?;
+            let (model_dict, model_extra, val_fields_set): (Bound<PyAny>, Bound<PyAny>, Bound<PyAny>) =
+                output.extract(py)?;
             let fields_set = existing_fields_set.unwrap_or(&val_fields_set);
-            state.add_fields_set(fields_set.len().unwrap_or(0));
             set_model_attrs(&instance, &model_dict, &model_extra, fields_set)?;
         }
         self.call_post_init(py, instance, input, state.extra())

src/validators/model_fields.rs

@@ -150,6 +150,7 @@ impl Validator for ModelFieldsValidator {
         let mut model_extra_dict_op: Option<Bound<PyDict>> = None;
         let mut errors: Vec<ValLineError> = Vec::with_capacity(self.fields.len());
         let mut fields_set_vec: Vec<Py<PyString>> = Vec::with_capacity(self.fields.len());
+        let mut fields_set_count: usize = 0;
 
         // we only care about which keys have been used if we're iterating over the object for extra after
         // the first pass
@@ -184,6 +185,7 @@ impl Validator for ModelFieldsValidator {
                         Ok(value) => {
                             model_dict.set_item(&field.name_py, value)?;
                             fields_set_vec.push(field.name_py.clone_ref(py));
+                            fields_set_count += 1;
                         }
                         Err(ValError::Omit) => continue,
                         Err(ValError::LineErrors(line_errors)) => {
@@ -327,6 +329,7 @@ impl Validator for ModelFieldsValidator {
             Err(ValError::LineErrors(errors))
         } else {
             let fields_set = PySet::new_bound(py, &fields_set_vec)?;
+            state.add_fields_set(fields_set_count);
 
             // if we have extra=allow, but we didn't create a dict because we were validating
             // from attributes, set it now so __pydantic_extra__ is always a dict if extra=allow

src/validators/validation_state.rs

@@ -18,6 +18,10 @@ pub enum Exactness {
 pub struct ValidationState<'a, 'py> {
     pub recursion_guard: &'a mut RecursionState,
     pub exactness: Option<Exactness>,
+    // This is used as a tie-breaking mechanism for union validation.
+    // Note: the count of the fields set is not always equivalent to the length of the
+    // `model_fields_set` attached to a model. `model_fields_set` includes extra fields
+    // when extra='allow', whereas this tally does not.
     pub fields_set_count: Option<usize>,
     // deliberately make Extra readonly
     extra: Extra<'a, 'py>,

tests/validators/test_union.py

@@ -1280,3 +1280,56 @@ class ModelB:
             )
             assert isinstance(result, ModelB)
             assert isinstance(result.b, SubModelW)
+
+
+@pytest.mark.parametrize('extra_behavior', ['forbid', 'ignore', 'allow'])
+def test_smart_union_extra_behavior(extra_behavior) -> None:
+    class Foo:
+        foo: str = 'foo'
+
+    class Bar:
+        bar: str = 'bar'
+
+    class Model:
+        x: Union[Foo, Bar]
+
+    validator = SchemaValidator(
+        core_schema.model_schema(
+            Model,
+            core_schema.model_fields_schema(
+                fields={
+                    'x': core_schema.model_field(
+                        core_schema.union_schema(
+                            [
+                                core_schema.model_schema(
+                                    Foo,
+                                    core_schema.model_fields_schema(
+                                        fields={
+                                            'foo': core_schema.model_field(
+                                                core_schema.with_default_schema(core_schema.str_schema(), default='foo')
+                                            )
+                                        }
+                                    ),
+                                    extra_behavior=extra_behavior,
+                                ),
+                                core_schema.model_schema(
+                                    Bar,
+                                    core_schema.model_fields_schema(
+                                        fields={
+                                            'bar': core_schema.model_field(
+                                                core_schema.with_default_schema(core_schema.str_schema(), default='bar')
+                                            )
+                                        }
+                                    ),
+                                    extra_behavior=extra_behavior,
+                                ),
+                            ]
+                        )
+                    )
+                }
+            ),
+        )
+    )
+
+    assert isinstance(validator.validate_python({'x': {'foo': 'foo'}}).x, Foo)
+    assert isinstance(validator.validate_python({'x': {'bar': 'bar'}}).x, Bar)