Problem

The FilterSchema definition is a simple but effective approach to centralize logic around filtering QuerySets. A similar approach could be followed for ordering.
Current for ordering and filtering in the same handler we would the following:

@api.get("/books")
def list_books(request, filters: BookFilterSchema = Query(...), order_by: list[str] | None = None):
    books = Book.objects.order_by(**order_by)
    books = filters.filter(books)
    return books

But what if we want to limit the API to allow ordering by just some of the fields. Or if we want to customize ordering based on custom fields and logic. What if we want to have a similar approach for other data sources, for example ElasticSearch.

Proposal

This PR propose to include a helper schema class, similar to FilteringSchema, but for ordering. It';s a simple schema class, with only one field: order_by, that accepts a list of string.
The allowed fields can be specified through the Config inner class, and a Pydantic validator will check that the provided query values are part of the allowed fields.
The schema then will provide a .sort() method (similar to .filter()) that we can use to pass the query set, and expect it ordered as a returned value.
The values can be provided using django standard behavior for descending order.

Example

Using it with out-of-the-box definition, allowing all fields from the model.

from ninja import OrderingSchema

@api.get("/books")
def list_books(request, filters: BookFilterSchema = Query(...), ordering: OrderingSchema = Query(...)):
    books = Book.objects.all()
    books = filters.filter(books)
    books = ordering.sort(books)
    return books

Using it with custom definition of allowed fields

from ninja import OrderingSchema

class BookOrderingSchema(OrderingSchema):
    class Config(OrderingSchema.Config):
        allowed_fields = ['name', 'created_at']  # Leaving out `author` field

@api.get("/books")
def list_books(request, filters: BookFilterSchema = Query(...), ordering: BookOrderingSchema = Query(...)):
    books = Book.objects.all()
    books = filters.filter(books)
    books = ordering.sort(books)
    return books

Other ideas not followed

I also considered to have a default value field in the config, but decided to go with field default definition on custom schema level
Another consideration was to create a class method factory in the OrderingSchema, so it can be define inline, but I wasn't sure if it would be used:

@api.get("/books")
def list_books(
    request,
    filters: BookFilterSchema = Query(...),
    ordering: OrderingSchema.with_allowed_fields('name', 'created_at') = Query(...)
):
    ...

Notes

I didn't add field validation with Model definition, to keep the practices followed in the FilterSchema definition.
Also, I think is a good idea to keep this helpers class as simple as possible, and give room to personalization for more complex scenarios. However, let me know if validating allowed_fields with Model fields is something we would like to have, and I can update the PR.

This was really useful in a personal project, were we needed to provide different ordering behaviors for a QuerySet and for an OpenSearch query. We had to do similar personalizations for pagination and filtering.