Stuck on an issue?

Describe the bug

The documentation states: https://github.com/collerek/ormar/blob/master/docs/queries/filter-and-sort.md#python-style-filters

isnull: not null can be written as Track.album.name.is_null(False)

But this does not actually seem to work.

To Reproduce

$ python --version
Python 3.9.1

$  pip freeze
aiosqlite==0.17.0
click==7.1.2
databases==0.4.1
Faker==8.1.0
fastapi==0.63.0
fastapi-pagination==0.7.0
h11==0.12.0
ormar==0.10.4
pydantic==1.8
python-dateutil==2.8.1
six==1.15.0
SQLAlchemy==1.3.23
starlette==0.13.6
text-unidecode==1.3
typing-extensions==3.7.4.3
uvicorn==0.13.4

Try to run this script with uvicorn main:app:

from typing import List, Optional

import databases
import sqlalchemy
from fastapi import FastAPI

import ormar

app = FastAPI()
metadata = sqlalchemy.MetaData()
database = databases.Database("sqlite:///test.db")
app.state.database = database


class Item(ormar.Model):
    class Meta:
        tablename = "items"
        metadata = metadata
        database = database

    id: int = ormar.Integer(primary_key=True)
    name: Optional[str] = ormar.String(nullable=True, max_length=100)


@app.on_event("startup")
async def startup() -> None:
    engine = sqlalchemy.create_engine(str(database.url))
    metadata.drop_all(engine)
    metadata.create_all(engine)

    await database.connect()

    for _ in range(100):
        await Item.objects.create(name="Name")


@app.on_event("shutdown")
async def shutdown() -> None:
    database_ = app.state.database
    if database_.is_connected:
        await database_.disconnect()


@app.get("/items/", response_model=List[Item])
async def get_items():
    items = await Item.objects.filter(Item.name.is_null(False)).all()
    return items

Error when run:

$ uvicorn main3:app --reload
INFO:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)
INFO:     Started reloader process [63484] using statreload
INFO:     Started server process [63487]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     127.0.0.1:60078 - "GET /docs HTTP/1.1" 200 OK
INFO:     127.0.0.1:60078 - "GET /openapi.json HTTP/1.1" 200 OK
INFO:     127.0.0.1:60081 - "GET /items/ HTTP/1.1" 500 Internal Server Error
ERROR:    Exception in ASGI application
Traceback (most recent call last):
  File "./venv/lib/python3.9/site-packages/uvicorn/protocols/http/h11_impl.py", line 396, in run_asgi
    result = await app(self.scope, self.receive, self.send)
  File "./venv/lib/python3.9/site-packages/uvicorn/middleware/proxy_headers.py", line 45, in __call__
    return await self.app(scope, receive, send)
  File "./venv/lib/python3.9/site-packages/fastapi/applications.py", line 199, in __call__
    await super().__call__(scope, receive, send)
  File "./venv/lib/python3.9/site-packages/starlette/applications.py", line 111, in __call__
    await self.middleware_stack(scope, receive, send)
  File "./venv/lib/python3.9/site-packages/starlette/middleware/errors.py", line 181, in __call__
    raise exc from None
  File "./venv/lib/python3.9/site-packages/starlette/middleware/errors.py", line 159, in __call__
    await self.app(scope, receive, _send)
  File "./venv/lib/python3.9/site-packages/starlette/exceptions.py", line 82, in __call__
    raise exc from None
  File "./venv/lib/python3.9/site-packages/starlette/exceptions.py", line 71, in __call__
    await self.app(scope, receive, sender)
  File "./venv/lib/python3.9/site-packages/starlette/routing.py", line 566, in __call__
    await route.handle(scope, receive, send)
  File "./venv/lib/python3.9/site-packages/starlette/routing.py", line 227, in handle
    await self.app(scope, receive, send)
  File "./venv/lib/python3.9/site-packages/starlette/routing.py", line 41, in app
    response = await func(request)
  File "./venv/lib/python3.9/site-packages/fastapi/routing.py", line 201, in app
    raw_response = await run_endpoint_function(
  File "./venv/lib/python3.9/site-packages/fastapi/routing.py", line 148, in run_endpoint_function
    return await dependant.call(**values)
  File "./main3.py", line 46, in get_items
    items = await Item.objects.filter(Item.name .is_null(False)).all()
  File "./venv/lib/python3.9/site-packages/ormar/queryset/field_accessor.py", line 46, in __getattr__
    return object.__getattribute__(self, item)  # pragma: no cover
AttributeError: 'FieldAccessor' object has no attribute 'is_null'

Expected behavior

Query to complete successfully and filter out Items with null name.