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Describe the bug

Fields with nullable=True aren’t considered nullable when also using choices=...

To Reproduce

Create a model with the following:

from enum import Enum
from ormar import Model, Integer, String
import sqlalchemy
import databases

URL = "postgresql://..."  # url to postgres db

metadata = sqlalchemy.MetaData()
database = database.Database(URL)

class MyChoices(Enum):
    A = "A"
    B = "B"

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

    id = Integer(primary_key=True)
    my_field = String(max_length=1, nullable=True, choices=list(MyChoices))

app = FastAPI()
app.state.database = database

@app.on_event("startup")
async def startup() -> None:
    database_ = app.state.database
    if not database_.is_connected:
        await database_.connect()


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

@app.post("/upload/")
async def upload(item: Item):
    await Item.save()

Upon trying a POST request to that endpoint, if my_field is set to None, it will fail with a ValueError:

{"detail": [{"loc": ["body", 0, "__root__"], "msg": "my_field: 'None' not in allowed choices set: ['A', 'B']", "type": "value_error"}]}

Expected behavior

The model accepts "A", "B", or None for my_field, because it has a set of valid choices and is nullable. It can be worked around by adding None to the enum, but that does not seem natural to me:

  class MyChoices(Enum):
      A = "A"
      B = "B"
+     NONE = None

Versions

• Database backend used: postgres
• Python version: 3.9.7
• ormar version: 0.10.19
• pydantic version: 1.8.2
• fastapi version: 0.68.1