from typing import Annotated

from pydantic import BaseModel, PlainValidator
from pydantic_core import core_schema

class MyDict:
    @classmethod
    def __get_pydantic_core_schema__(cls, source, handler):
        return core_schema.dict_schema(
            keys_schema=handler.generate_schema(str),
            values_schema=handler.generate_schema(int),
            serialization=core_schema.filter_dict_schema(
                include={'a'},
            ),
        )


class Model(BaseModel):
    f: Annotated[MyDict, PlainValidator(val)]

# If core schema validation is disabled:
Model(f={'a': 1, 'b': 1}).model_dump()
#> {'f': {'a': 1, 'b': 1}}
# Should be: {'f': {'a': 1}}