fix: decode cached JSON as the annotated return type

changelog.d/249.bugfix.md

@@ -0,0 +1 @@
+Decode cached JSON into the annotated return type on Pydantic v2.

fastapi_cache/coder.py

@@ -15,15 +15,12 @@
 
 import pendulum
 from fastapi.encoders import jsonable_encoder
+from pydantic import TypeAdapter
 from starlette.responses import JSONResponse
 from starlette.templating import (
     _TemplateResponse as TemplateResponse,  # pyright: ignore[reportPrivateUsage]
 )
 
-
-class ModelField:
-    pass
-
 _T = TypeVar("_T", bound=type)
 
 
@@ -67,12 +64,9 @@ def encode(cls, value: Any) -> bytes:
     def decode(cls, value: bytes) -> Any:
         raise NotImplementedError
 
-    # (Shared) cache for endpoint return types to Pydantic model fields.
-    # Note that subclasses share this cache! If a subclass overrides the
-    # decode_as_type method and then stores a different kind of field for a
-    # given type, do make sure that the subclass provides its own class
-    # attribute for this cache.
-    _type_field_cache: ClassVar[Dict[Any, ModelField]] = {}
+    # Subclasses share this. Override the attribute if decode_as_type stores
+    # something other than a TypeAdapter for a given type.
+    _type_adapter_cache: ClassVar[Dict[Any, TypeAdapter[Any]]] = {}
 
     @overload
     @classmethod
@@ -92,7 +86,17 @@ def decode_as_type(cls, value: bytes, *, type_: Optional[_T]) -> Union[_T, Any]:
 
         """
         result = cls.decode(value)
-        return result
+        if type_ is None:
+            return result
+
+        adapter: TypeAdapter[Any]
+        try:
+            adapter = cls._type_adapter_cache[type_]
+        except KeyError:
+            adapter = TypeAdapter(type_)
+            cls._type_adapter_cache[type_] = adapter
+        validated: Any = adapter.validate_python(result)
+        return validated
 
 
 class JsonCoder(Coder):

tests/test_codecs.py

@@ -1,5 +1,5 @@
 from dataclasses import dataclass
-from typing import Any, Optional, Tuple, Type
+from typing import Any, List, Optional, Tuple, Type
 
 import pytest
 from pydantic import BaseModel, ValidationError
@@ -51,6 +51,10 @@ def test_pickle_coder(value: Any) -> None:
         ({"some_key": 1, "other_key": 2}, None),
         (DCItem(name="foo", price=42.0, description="some dataclass item", tax=0.2), DCItem),
         (PDItem(name="foo", price=42.0, description="some pydantic item", tax=0.2), PDItem),
+        (
+            [PDItem(name="foo", price=42.0, description="some pydantic item", tax=0.2)],
+            List[PDItem],
+        ),
     ],
 )
 def test_json_coder(value: Any, return_type: Type[Any]) -> None: