Unable to use .save() method on a nullable ForeignKeyField

About the bug Returns wrong value when trying to save FK field, which may be null

To Reproduce

import asyncio
import logging

from tortoise import Tortoise, fields, models

logging.basicConfig(level=logging.DEBUG)

POSTGRES_URL = "postgres://postgres:postgres@localhost/postgres"


class ModelA(models.Model):
    string = fields.TextField(default="")


class ModelB(models.Model):
    a_model = fields.ForeignKeyField(
        "models.ModelA", related_name="b_models", null=True
    )


async def main():
    await Tortoise.init(db_url=POSTGRES_URL, modules={"models": ["__main__"]})
    await Tortoise.generate_schemas()

    a, _ = await ModelA.get_or_create()
    b, _ = await ModelB.get_or_create()

    b.a_model = a
    await b.save()

    await Tortoise.close_connections()


asyncio.run(main())

This code uses None as a_model_id on update query. But if there is .create or .get_or_create method with passed a_model arg, then the code uses the right values.

Expected behavior .save method should use the right values when updating nullable foreign key.

Additional context Python version: CPython 3.7.4 Installed packages: aiosqlite==0.10.0 asyncpg==0.18.3 atlastk==0.10.7 ciso8601==2.1.1 PyPika==0.35.4 tortoise-orm==0.13.3

[nsidnev]

By the way, your bug reports are really good I’m a big fan!

Thanks! 😄 I’m glad that this project exists and I am happy to help it this way, at least until I haven’t enough knowledge to open the PR with fix.

[nsidnev]

FYI, the partial/property change is only in the models file here: tortoise/tortoise-orm/pull/185/files#diff-2b3e042db0b773b93d8a98880434ee35

Thank you, I’ll watch it.

Just checked on my project. It worked as it should, thanks for the quick fix!