Adding custom filtering for relationships ModelConverterBase._prepare_select_options

[maxim-f1]

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Is your feature related to a problem? Please describe.

I want to filter relationships in a query, but I can't because the ModelConverterBase._prepare_select_options function generates the query in a hardcode.

Describe the solution you would like.

class CustomModelConverter(ModelConverter):
    select_option_stmts = {
        'field': select(Model).where(Model.level > 1)
    }


class ModelBaseConverter:
    select_option_stmts: Dict[str, Select]
    
    async def _prepare_select_options(
        self,
        prop: RelationshipProperty,
        session_maker: sessionmaker,
    ) -> list[tuple[str, Any]]:
        target_model = prop.mapper.class_
        if prop.key in self.select_option_stmts:
            stmt = self.select_option_stmts[prop.key]
        else:
            stmt = select(target_model)

        if is_async_session_maker(session_maker):
            async with session_maker() as session:
                objects = await session.execute(stmt)
                return [
                    (str(self._get_identifier_value(obj)), str(obj))
                    for obj in objects.scalars().unique().all()
                ]
        else:
            with session_maker() as session:
                objects = await anyio.to_thread.run_sync(session.execute, stmt)
                return [
                    (str(self._get_identifier_value(obj)), str(obj))
                    for obj in objects.scalars().unique().all()
                ]


class View(ModelView, model=Model):
    form_converter = CustomModelConverter

Describe alternatives you considered

There is an option to inherit from ModelConverter and override the _prepare_select_options method in CustomModelConverter. But it looks like a huge crutch.

class CustomModelConverter(ModelConverter):
    async def _prepare_select_options(
       self,
       prop: RelationshipProperty,
       session_maker: sessionmaker,
   ) -> list[tuple[str, Any]]:
       target_model = prop.mapper.class_
       stmt = <custom query>

       if is_async_session_maker(session_maker):
           async with session_maker() as session:
               objects = await session.execute(stmt)
               return [
                   (str(self._get_identifier_value(obj)), str(obj))
                   for obj in objects.scalars().unique().all()
               ]
       else:
           with session_maker() as session:
               objects = await anyio.to_thread.run_sync(session.execute, stmt)
               return [
                   (str(self._get_identifier_value(obj)), str(obj))
                   for obj in objects.scalars().unique().all()
               ]
               
               
class View(ModelView, model=Model):
   form_converter = CustomModelConverter

Additional context

No response