Alembic SQLAlchemy自动生成MetaData错误

Question

我正在尝试使用运行alembic revision --autogenerate -m 'initial setup'的alembic创建一个迁移，但是我得到了错误：

失败:无法继续--自动生成选项；环境脚本/用户/paul/python/my_project/alembic/env.py不向上下文提供MetaData对象或对象序列。

我不知道为什么会出现这个错误，因为我已经在target_metadata文件中设置了env.py，并将文件夹添加到了路径中。我尝试从另一个没有问题的项目中复制alembic设置，但在这个项目上仍然有相同的错误。当我运行alembic current时，数据库将在没有问题的情况下被创建。

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env.py文件的正确路径，因此它似乎正在查找该文件，但没有看到target_metadata = SqlAlchemyBase.metadata行。

Answer 1

问题出在我的编辑身上。我关闭了env.py文件，重新打开了它，并按预期在那里进行了所有更改。然后，我决定看看如果我通过command+clicking从控制台中的错误消息(我正在使用VSCode)打开文件会发生什么，并且当我以这种方式打开它时，我的任何更改都不会发生。我再次尝试从侧文件资源管理器打开文件，并且我的更改在文件中，但是alembic仍然无法运行。我完全关闭了VSCode并重新打开它，在做了这些更改之后，我的文件和alembic都没有问题。

我以前从未遇到过这个问题，所以我不知道VSCode发生了什么，但是不管它重新启动了什么，它都解决了这个问题。

Answer 2

我也面临同样的错误。在我的例子中，问题是一个畸形的env.py。特别是，我的数据库定义如下：

# alembic.ini

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

虽然元数据定义不对齐，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"
}

Answer 3

在我的例子中，target_metadata在migrations/env.py文件中被设置为None。

相反，我应该使用：

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

将base定义在src.db.models文件中，如下所示：

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

base = declarative_base(bind=create_async_engine(DATABASE_URL))