Alembic SQLAlchemy 自动生成元数据错误

Question

我正在尝试创建一个运行 alembic 的迁移

alembic revision --autogenerate -m 'initial setup'

但我收到错误：

失败：无法继续使用--autogenerate选项；环境脚本 /Users/paul/python/my_project/alembic/env.py 不向上下文提供元数据对象或对象序列。

我无法弄清楚为什么会收到此错误，因为我已在 env.py 文件中设置了 target_metadata，并将文件夹添加到路径中。我尝试从我的另一个没有问题的项目复制 alembic 设置，但在这个项目上我仍然遇到相同的错误。当我运行

alembic current

时，数据库会毫无问题地创建。

我的 alembic.ini 文件：

# A generic, single database configuration.

[alembic]
# path to migration scripts
script_location = alembic

# template used to generate migration files
# file_template = %%(rev)s_%%(slug)s

# timezone to use when rendering the date
# within the migration file as well as the filename.
# string value is passed to dateutil.tz.gettz()
# leave blank for localtime
# timezone =

# max length of characters to apply to the
# "slug" field
# truncate_slug_length = 40

# set to 'true' to run the environment during
# the 'revision' command, regardless of autogenerate
# revision_environment = false

# set to 'true' to allow .pyc and .pyo files without
# a source .py file to be detected as revisions in the
# versions/ directory
# sourceless = false

# version location specification; this defaults
# to alembic/versions.  When using multiple version
# directories, initial revisions must be specified with --version-path
# version_locations = %(here)s/bar %(here)s/bat alembic/versions

# the output encoding used when revision files
# are written from script.py.mako
# output_encoding = utf-8

sqlalchemy.url = sqlite:///./src/db/data.sqlite


# Logging configuration
[loggers]
keys = root,sqlalchemy,alembic

[handlers]
keys = console

[formatters]
keys = generic

[logger_root]
level = WARN
handlers = console
qualname =

[logger_sqlalchemy]
level = WARN
handlers =
qualname = sqlalchemy.engine

[logger_alembic]
level = INFO
handlers =
qualname = alembic

[handler_console]
class = StreamHandler
args = (sys.stderr,)
level = NOTSET
formatter = generic

[formatter_generic]
format = %(levelname)-5.5s [%(name)s] %(message)s
datefmt = %H:%M:%S

我的 env.py 文件：

import sys

from logging.config import fileConfig
from pathlib import Path

from sqlalchemy import engine_from_config
from sqlalchemy import pool

from alembic import context

folder = Path(__file__).resolve().parents[1]
sys.path.insert(0, str(folder))

import src.data.__all_models
from src.data.modelbase import SqlAlchemyBase

# this is the Alembic Config object, which provides
# access to the values within the .ini file in use.
config = context.config

# Interpret the config file for Python logging.
# This line sets up loggers basically.
fileConfig(config.config_file_name)

# add your model's MetaData object here
# for 'autogenerate' support
# from myapp import mymodel
# target_metadata = mymodel.Base.metadata
target_metadata = SqlAlchemyBase.metadata

# other values from the config, defined by the needs of env.py,
# can be acquired:
# my_important_option = config.get_main_option("my_important_option")
# ... etc.


def run_migrations_offline():
    """Run migrations in 'offline' mode.

    This configures the context with just a URL
    and not an Engine, though an Engine is acceptable
    here as well.  By skipping the Engine creation
    we don't even need a DBAPI to be available.

    Calls to context.execute() here emit the given string to the
    script output.

    """
    url = config.get_main_option("sqlalchemy.url")
    context.configure(
        url=url, target_metadata=target_metadata, literal_binds=True
    )

    with context.begin_transaction():
        context.run_migrations()


def run_migrations_online():
    """Run migrations in 'online' mode.

    In this scenario we need to create an Engine
    and associate a connection with the context.

    """
    connectable = engine_from_config(
        config.get_section(config.config_ini_section),
        prefix="sqlalchemy.",
        poolclass=pool.NullPool,
    )

    with connectable.connect() as connection:
        context.configure(
            connection=connection, target_metadata=target_metadata
        )

        with context.begin_transaction():
            context.run_migrations()


if context.is_offline_mode():
    run_migrations_offline()
else:
    run_migrations_online()

我的 modelbase.py 文件：

import sqlalchemy.ext.declarative as dec


SqlAlchemyBase = dec.declarative_base()

错误消息显示了 env.py 文件的正确路径，因此似乎找到了该文件，但没有看到

target_metadata = SqlAlchemyBase.metadata

行。

Answer 1

在我的例子中，

target_metadata

在

None

文件中设置为

migrations/env.py

。

我应该使用：

from src.db.models import base
target_metadata = base.metadata

与

base

在我的

src.db.models

文件中定义如下：

from sqlalchemy.ext.asyncio import create_async_engine
from sqlalchemy.ext.declarative import declarative_base

base = declarative_base(bind=create_async_engine(DATABASE_URL))

Answer 2

就我而言，愚蠢的是，我在

metadata

的开头正确添加了

env.py

，但是在蒸馏器启动后会自动生成一行。写着

的行

target_metadata = None

我删除了这一行并使用了我自己的元数据

from mypkg import Base
target_metadata = Base.metadata

此后，alembic 迁移成功。

Answer 3

问题出在我的编辑身上。我关闭了 env.py 文件，重新打开它，所有更改都按预期进行。然后我决定看看如果我通过命令打开文件并从控制台中的错误消息中单击它（我使用的是 VSCode），并且当我以这种方式打开它时，我的任何更改都没有发生。我尝试再次从侧面文件资源管理器打开文件，我的更改位于文件中，但 alembic 仍然无法运行。我完全关闭 VSCode 并重新打开它，之后所做的更改就在我的文件中，alembic 运行没有问题。

我以前从未遇到过这个问题，所以我不知道 VSCode 发生了什么，但无论它重新启动它都解决了问题。

Answer 4

我遇到了同样的错误。我的案例中的问题是格式错误的

env.py

。特别是，我的数据库定义如下：

# alembic.ini

[alembic]
...
databases = db1, db2
...

虽然元数据定义未对齐，例如：

#env.py

target_metadata = {
    "wrong_name_1": model_db1.Base.metadata,  # Key should be "db1"
    "wrong_name_2": model_db2.Base.metadata   # Key should be "db2"
}