Fix usage of AliasGenerator with computed_field decorator (#8806)

Co-authored-by: Alex Hall <alex.mojaki@gmail.com>

pydantic/_internal/_generate_schema.py

@@ -973,7 +973,7 @@ def _apply_alias_generator_to_field_info(
         # Apply an alias_generator if
         # 1. An alias is not specified
         # 2. An alias is specified, but the priority is <= 1
-        if alias_generator and (
+        if (
             field_info.alias_priority is None
             or field_info.alias_priority <= 1
             or field_info.alias is None
@@ -1009,6 +1009,49 @@ def _apply_alias_generator_to_field_info(
             if field_info.validation_alias is None:
                 field_info.validation_alias = validation_alias or alias
 
+    @staticmethod
+    def _apply_alias_generator_to_computed_field_info(
+        alias_generator: Callable[[str], str] | AliasGenerator,
+        computed_field_info: ComputedFieldInfo,
+        computed_field_name: str,
+    ):
+        """Apply an alias_generator to alias on a ComputedFieldInfo instance if appropriate.
+
+        Args:
+            alias_generator: A callable that takes a string and returns a string, or an AliasGenerator instance.
+            computed_field_info: The ComputedFieldInfo instance to which the alias_generator is (maybe) applied.
+            computed_field_name: The name of the computed field from which to generate the alias.
+        """
+        # Apply an alias_generator if
+        # 1. An alias is not specified
+        # 2. An alias is specified, but the priority is <= 1
+
+        if (
+            computed_field_info.alias_priority is None
+            or computed_field_info.alias_priority <= 1
+            or computed_field_info.alias is None
+        ):
+            alias, validation_alias, serialization_alias = None, None, None
+
+            if isinstance(alias_generator, AliasGenerator):
+                alias, validation_alias, serialization_alias = alias_generator.generate_aliases(computed_field_name)
+            elif isinstance(alias_generator, Callable):
+                alias = alias_generator(computed_field_name)
+                if not isinstance(alias, str):
+                    raise TypeError(f'alias_generator {alias_generator} must return str, not {alias.__class__}')
+
+            # if priority is not set, we set to 1
+            # which supports the case where the alias_generator from a child class is used
+            # to generate an alias for a field in a parent class
+            if computed_field_info.alias_priority is None or computed_field_info.alias_priority <= 1:
+                computed_field_info.alias_priority = 1
+
+            # if the priority is 1, then we set the aliases to the generated alias
+            # note that we use the serialization_alias with priority over alias, as computed_field
+            # aliases are used for serialization only (not validation)
+            if computed_field_info.alias_priority == 1:
+                computed_field_info.alias = serialization_alias or alias
+
     def _common_field_schema(  # C901
         self, name: str, field_info: FieldInfo, decorators: DecoratorInfos
     ) -> _CommonField:
@@ -1659,20 +1702,12 @@ def _computed_field_schema(
             filter_field_decorator_info_by_field(field_serializers.values(), d.cls_var_name),
             computed_field=True,
         )
-        # Handle alias_generator using similar logic to that from
-        # pydantic._internal._generate_schema.GenerateSchema._common_field_schema,
-        # with field_info -> d.info and name -> d.cls_var_name
+
         alias_generator = self._config_wrapper.alias_generator
-        if alias_generator and (d.info.alias_priority is None or d.info.alias_priority <= 1):
-            alias = None
-            if isinstance(alias_generator, AliasGenerator) and alias_generator.alias is not None:
-                alias = alias_generator.alias(d.cls_var_name)
-            elif isinstance(alias_generator, Callable):
-                alias = alias_generator(d.cls_var_name)
-            if not isinstance(alias, str):
-                raise TypeError(f'alias_generator {alias_generator} must return str, not {alias.__class__}')
-            d.info.alias = alias
-            d.info.alias_priority = 1
+        if alias_generator is not None:
+            self._apply_alias_generator_to_computed_field_info(
+                alias_generator=alias_generator, computed_field_info=d.info, computed_field_name=d.cls_var_name
+            )
 
         def set_computed_field_metadata(schema: CoreSchemaOrField, handler: GetJsonSchemaHandler) -> JsonSchemaValue:
             json_schema = handler(schema)

tests/test_aliases.py

@@ -702,3 +702,26 @@ class Foo(BaseModel):
     assert f.a == 'a'
     assert f.model_dump(by_alias=True) == {'a_ser_alias': 'a'}
     assert f.model_dump(by_alias=False) == {'a': 'a'}
+
+
+def test_alias_generator_with_computed_field_for_serialization() -> None:
+    """Tests that the alias generator is used for computed fields, with serialization_alias taking precedence over alias."""
+
+    class Rectangle(BaseModel):
+        model_config = ConfigDict(
+            alias_generator=AliasGenerator(
+                validation_alias=lambda field_name: f'{field_name}_val_alias',
+                alias=lambda field_name: f'{field_name}_alias',
+                serialization_alias=lambda field_name: f'{field_name}_ser_alias',
+            )
+        )
+
+        width: int
+        height: int
+
+        @computed_field
+        def area(self) -> int:
+            return self.width * self.height
+
+    r = Rectangle(width_val_alias=10, height_val_alias=20)
+    assert r.model_dump(by_alias=True) == {'width_ser_alias': 10, 'height_ser_alias': 20, 'area_ser_alias': 200}