CreatedAt mixin (base class) in python SQLModel

Question

I'm using SQLModel and would like to define base class with created_at field to inherit from it in others models.

class CreatedAtMixin(SQLModel):
    created_at: datetime = Field(default=datetime.now(timezone.utc), nullable=False)

But when i'm trying to use it in other model like this and making migrations via alembic:

class NetworkBase(SQLModel, CreatedAtMixin):
    name: str = Field(max_length=64, description='Name of network')
    short_name: str = Field(max_length=32, description='Short name of the network')

Im facing a problem:

Traceback (most recent call last):
  File "/Users/nikitasamaev/PycharmProjects/new-checkout-backend/venv/bin/alembic", line 8, in <module>
    sys.exit(main())
  File "/Users/nikitasamaev/PycharmProjects/new-checkout-backend/venv/lib/python3.9/site-packages/alembic/config.py", line 588, in main
    CommandLine(prog=prog).main(argv=argv)
  File "/Users/nikitasamaev/PycharmProjects/new-checkout-backend/venv/lib/python3.9/site-packages/alembic/config.py", line 582, in main
    self.run_cmd(cfg, options)
  File "/Users/nikitasamaev/PycharmProjects/new-checkout-backend/venv/lib/python3.9/site-packages/alembic/config.py", line 559, in run_cmd
    fn(
  File "/Users/nikitasamaev/PycharmProjects/new-checkout-backend/venv/lib/python3.9/site-packages/alembic/command.py", line 227, in revision
    script_directory.run_env()
  File "/Users/nikitasamaev/PycharmProjects/new-checkout-backend/venv/lib/python3.9/site-packages/alembic/script/base.py", line 563, in run_env
    util.load_python_file(self.dir, "env.py")
  File "/Users/nikitasamaev/PycharmProjects/new-checkout-backend/venv/lib/python3.9/site-packages/alembic/util/pyfiles.py", line 92, in load_python_file
    module = load_module_py(module_id, path)
  File "/Users/nikitasamaev/PycharmProjects/new-checkout-backend/venv/lib/python3.9/site-packages/alembic/util/pyfiles.py", line 108, in load_module_py
    spec.loader.exec_module(module)  # type: ignore
  File "<frozen importlib._bootstrap_external>", line 850, in exec_module
  File "<frozen importlib._bootstrap>", line 228, in _call_with_frames_removed
  File "app/db/migrations/env.py", line 12, in <module>
    from app.models import *
  File "/Users/nikitasamaev/PycharmProjects/new-checkout-backend/app/models/__init__.py", line 1, in <module>
    from app.models.blockchain import Network
  File "/Users/nikitasamaev/PycharmProjects/new-checkout-backend/app/models/blockchain.py", line 9, in <module>
    class NetworkBase(SQLModel, CreatedAtMixin):
  File "/Users/nikitasamaev/PycharmProjects/new-checkout-backend/venv/lib/python3.9/site-packages/sqlmodel/main.py", line 277, in __new__
    new_cls = super().__new__(cls, name, bases, dict_used, **config_kwargs)
  File "pydantic/main.py", line 149, in pydantic.main.ModelMetaclass.__new__
  File "pydantic/config.py", line 118, in pydantic.config.inherit_config
TypeError: Cannot create a consistent method resolution
order (MRO) for bases Config, Config

Of course I can set field created_at in my NetworkBase and others models manually, it would work without any problems, but i'd like to inherit from CreatedAtMixin.

What are the ways to solve the problem?

The problem exists because both the model and the MixIn reference `SQLModel`. Easiest is to change `CreatedAtMixin` to `CreatedAtBase` and let that be the only BaseClass of `NetworkBase`. — rfkortekaas, Apr 03 '22 at 17:45
See also [this](https://stackoverflow.com/questions/29214888/typeerror-cannot-create-a-consistent-method-resolution-order-mro) question. — Paul P, Apr 04 '22 at 07:24

score 0 · Answer 1 · answered Jun 20 '22 at 16:39

You need to take out the SQLModel inheritance, as it is already an SQLModel class in the first class:

class CreatedAtMixin(SQLModel):
    created_at: datetime = Field(default=datetime.now(timezone.utc), nullable=False)

class NetworkBase(CreatedAtMixin):
    name: str = Field(max_length=64, description='Name of network')
    short_name: str = Field(max_length=32, description='Short name of the network')

CreatedAt mixin (base class) in python SQLModel

1 Answers1