feat: PaginatedResponse[T] as discriminated union annotation (#142)

docs/module/schemas.md

@@ -102,7 +102,9 @@ async def list_events(
 
 #### [`PaginatedResponse[T]`](../reference/schemas.md#fastapi_toolsets.schemas.PaginatedResponse)
 
-Base class and return type for endpoints that support **both** pagination strategies via a `pagination_type` query parameter (using [`paginate()`](crud.md#unified-paginate--both-strategies-on-one-endpoint))
+Return type for endpoints that support **both** pagination strategies via a `pagination_type` query parameter (using [`paginate()`](crud.md#unified-paginate--both-strategies-on-one-endpoint)).
+
+When used as a return annotation, `PaginatedResponse[T]` automatically expands to `Annotated[Union[CursorPaginatedResponse[T], OffsetPaginatedResponse[T]], Field(discriminator="pagination_type")]`, so FastAPI emits a proper `oneOf` + discriminator in the OpenAPI schema with no extra boilerplate:
 
 ```python
 from fastapi_toolsets.crud import PaginationType

src/fastapi_toolsets/schemas.py

@@ -1,9 +1,9 @@
 """Base Pydantic schemas for API responses."""
 
 from enum import Enum
-from typing import Any, ClassVar, Generic, Literal
+from typing import Annotated, Any, ClassVar, Generic, Literal, TypeVar, Union
 
-from pydantic import BaseModel, ConfigDict
+from pydantic import BaseModel, ConfigDict, Field
 
 from .types import DataT
 
@@ -138,9 +138,11 @@ class PaginatedResponse(BaseResponse, Generic[DataT]):
 
     Base class and return type for endpoints that support both pagination
     strategies.  Use :class:`OffsetPaginatedResponse` or
-    :class:`CursorPaginatedResponse` when the strategy is fixed;  use
-    ``PaginatedResponse`` as the return annotation for unified endpoints that
-    dispatch via :meth:`~fastapi_toolsets.crud.factory.AsyncCrud.paginate`.
+    :class:`CursorPaginatedResponse` when the strategy is fixed.
+
+    When used as ``PaginatedResponse[T]`` in a return annotation, subscripting
+    returns ``Annotated[Union[CursorPaginatedResponse[T], OffsetPaginatedResponse[T]], Field(discriminator="pagination_type")]``
+    so FastAPI emits a proper ``oneOf`` + discriminator in the OpenAPI schema.
     """
 
     data: list[DataT]
@@ -148,6 +150,16 @@ class PaginatedResponse(BaseResponse, Generic[DataT]):
     pagination_type: PaginationType | None = None
     filter_attributes: dict[str, list[Any]] | None = None
 
+    def __class_getitem__(  # type: ignore[invalid-method-override]
+        cls, item: type[Any] | tuple[type[Any], ...]
+    ) -> type[Any]:
+        if cls is PaginatedResponse and not isinstance(item, TypeVar):
+            return Annotated[  # type: ignore[invalid-return-type]
+                Union[CursorPaginatedResponse[item], OffsetPaginatedResponse[item]],  # type: ignore[invalid-type-form]
+                Field(discriminator="pagination_type"),
+            ]
+        return super().__class_getitem__(item)
+
 
 class OffsetPaginatedResponse(PaginatedResponse[DataT]):
     """Paginated response with typed offset-based pagination metadata.

tests/test_schemas.py

@@ -304,7 +304,7 @@ class TestPaginatedResponse:
             page=1,
             has_more=False,
         )
-        response = PaginatedResponse[dict](
+        response = PaginatedResponse(
             data=[],
             pagination=pagination,
         )
@@ -313,6 +313,26 @@ class TestPaginatedResponse:
         assert response.data == []
         assert response.pagination.total_count == 0
 
+    def test_class_getitem_with_concrete_type_returns_discriminated_union(self):
+        """PaginatedResponse[T] with a concrete type returns a discriminated Annotated union."""
+        import typing
+
+        alias = PaginatedResponse[dict]
+        args = typing.get_args(alias)
+        # args[0] is the Union, args[1] is the FieldInfo discriminator
+        union_args = typing.get_args(args[0])
+        assert CursorPaginatedResponse[dict] in union_args
+        assert OffsetPaginatedResponse[dict] in union_args
+
+    def test_class_getitem_with_typevar_returns_generic(self):
+        """PaginatedResponse[TypeVar] falls through to Pydantic generic parametrisation."""
+        from typing import TypeVar
+
+        T = TypeVar("T")
+        alias = PaginatedResponse[T]
+        # Should be a generic alias, not an Annotated union
+        assert not hasattr(alias, "__metadata__")
+
     def test_generic_type_hint(self):
         """PaginatedResponse supports generic type hints."""
         pagination = OffsetPagination(