自动生成Alembic修订版本时出现SQLModel错误

我有一个repository.models.models文件，代码如下：

from datetime import date
from typing import Type, Optional, List
from sqlmodel import SQLModel, Field, Relationship


class Compra(SQLModel, table=True):
    com_id: Optional[int] = Field(default=None, primary_key=True)
    data: Type[date]
    item: str
    valor: float
    forn_id: Optional[int] = Field(default=None, foreign_key="fornecedor.forn_id")

    fornecedor: Optional["Fornecedor"] = Relationship(back_populates="compras")


class Fornecedor(SQLModel, table=True):
    forn_id: Optional[int] = Field(default=None, primary_key=True)
    nome: str
    rating: int = 5

    compras: List[Compra] = Relationship(back_populates="fornecedores")

执行Alembic自动生成修订命令：

alembic revision --autogenerate -m "Create models"

触发如下错误回溯：

Traceback (most recent call last):
  File "/home/andre/.cache/pypoetry/virtualenvs/rest-api-for-bizagi-9TxUSah3-py3.10/bin/alembic", line 8, in <module>
    sys.exit(main())
  File "/home/andre/.cache/pypoetry/virtualenvs/rest-api-for-bizagi-9TxUSah3-py3.10/lib/python3.10/site-packages/alembic/config.py", line 590, in main
    CommandLine(prog=prog).main(argv=argv)
  File "/home/andre/.cache/pypoetry/virtualenvs/rest-api-for-bizagi-9TxUSah3-py3.10/lib/python3.10/site-packages/alembic/config.py", line 584, in main
    self.run_cmd(cfg, options)
  File "/home/andre/.cache/pypoetry/virtualenvs/rest-api-for-bizagi-9TxUSah3-py3.10/lib/python3.10/site-packages/alembic/config.py", line 561, in run_cmd
    fn(
  File "/home/andre/.cache/pypoetry/virtualenvs/rest-api-for-bizagi-9TxUSah3-py3.10/lib/python3.10/site-packages/alembic/command.py", line 229, in revision
    script_directory.run_env()
  File "/home/andre/.cache/pypoetry/virtualenvs/rest-api-for-bizagi-9TxUSah3-py3.10/lib/python3.10/site-packages/alembic/script/base.py", line 569, in run_env
    util.load_python_file(self.dir, "env.py")
  File "/home/andre/.cache/pypoetry/virtualenvs/rest-api-for-bizagi-9TxUSah3-py3.10/lib/python3.10/site-packages/alembic/util/pyfiles.py", line 94, in load_python_file
    module = load_module_py(module_id, path)
  File "/home/andre/.cache/pypoetry/virtualenvs/rest-api-for-bizagi-9TxUSah3-py3.10/lib/python3.10/site-packages/alembic/util/pyfiles.py", line 110, in load_module_py
    spec.loader.exec_module(module)  # type: ignore
  File "<frozen importlib._bootstrap_external>", line 883, in exec_module
  File "<frozen importlib._bootstrap>", line 241, in _call_with_frames_removed
  File "/home/andre/Projects/rest_api_for_bizagi/migrations/env.py", line 9, in <module>
    from repository.models import Fornecedor, Compra  # noqa
  File "/home/andre/Projects/rest_api_for_bizagi/./repository/models/__init__.py", line 1, in <module>
    from .models import Fornecedor  # noqa
  File "/home/andre/Projects/rest_api_for_bizagi/./repository/models/models.py", line 6, in <module>
    class Compra(SQLModel, table=True):
  File "/home/andre/.cache/pypoetry/virtualenvs/rest-api-for-bizagi-9TxUSah3-py3.10/lib/python3.10/site-packages/sqlmodel/main.py", line 293, in __new__
    col = get_column_from_field(v)
  File "/home/andre/.cache/pypoetry/virtualenvs/rest-api-for-bizagi-9TxUSah3-py3.10/lib/python3.10/site-packages/sqlmodel/main.py", line 421, in get_column_from_field
    sa_type = get_sqlachemy_type(field)
  File "/home/andre/.cache/pypoetry/virtualenvs/rest-api-for-bizagi-9TxUSah3-py3.10/lib/python3.10/site-packages/sqlmodel/main.py", line 375, in get_sqlachemy_type
    if issubclass(field.type_, str):
TypeError: issubclass() arg 1 must be a class

已确认Alembic配置正常且能连接数据库（首次空白修订成功更新数据库），若省略table=True参数则生成空白修订。请问该错误的原因是什么？

核心错误原因 #

Compra类中data字段的类型注解错误：

data: Type[date]

这里误用了Type[date]（表示date类本身的类型），但SQLModel需要的是字段值的类型注解date（表示字段存储date类型的实例）。当SQLModel尝试将Type[date]映射为SQLAlchemy类型时，执行issubclass(field.type_, str)时，field.type_是Type[date]而非合法的类对象，因此抛出TypeError。

额外修正点 #

Fornecedor类的Relationship中back_populates值不匹配：
原代码中Fornecedor.compras的back_populates设为"fornecedores"，但Compra类中对应的关联字段是fornecedor，需改为：

compras: List[Compra] = Relationship(back_populates="fornecedor")

修复后的完整代码 #

from datetime import date
from typing import Optional, List
from sqlmodel import SQLModel, Field, Relationship


class Compra(SQLModel, table=True):
    com_id: Optional[int] = Field(default=None, primary_key=True)
    data: date  # 移除Type[]修正类型注解
    item: str
    valor: float
    forn_id: Optional[int] = Field(default=None, foreign_key="fornecedor.forn_id")

    fornecedor: Optional["Fornecedor"] = Relationship(back_populates="compras")


class Fornecedor(SQLModel, table=True):
    forn_id: Optional[int] = Field(default=None, primary_key=True)
    nome: str
    rating: int = 5

    compras: List[Compra] = Relationship(back_populates="fornecedor")  # 修正back_populates值

内容的提问来源于stack exchange，提问作者André Carvalho