python - Alembic 尝试在创建表之后而不是之前删除表

Question

我正在尝试与 Alembic 一起迁移。

环境.py：

from logging.config import fileConfig

from sqlalchemy import engine_from_config
from sqlalchemy import pool

from alembic import context

import sys
import os

sys.path.insert(0, os.getcwd())
print(sys.path)
from appli.Base import Base

print(Base.metadata.sorted_tables)


# 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 = Base.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,
        dialect_opts={"paramstyle": "named"},
    )

    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()

alembic.ini：

# 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 = mysql+pymysql://root@localhost/test_migration


[post_write_hooks]
# post_write_hooks defines scripts or Python functions that are run
# on newly generated revision scripts.  See the documentation for further
# detail and examples

# format using "black" - use the console_scripts runner, against the "black" entrypoint
# hooks=black
# black.type=console_scripts
# black.entrypoint=black
# black.options=-l 79

# 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

我首先用它初始化数据库：

from sqlalchemy import create_engine
import pymysql
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, Integer, String, ForeignKey, UniqueConstraint

Base = declarative_base()
engine = create_engine('mysql+pymysql://root@localhost/test_migration')

#The axis on which the tests are executed
class AXIS(Base):
    __tablename__ = 'Axis'
    id_axis        = Column(Integer, primary_key = True)
    name_axis      = Column(String(10), nullable=False)

我在版本升级中得到了这一点。一切都是对的。

def upgrade():
    # ### commands auto generated by Alembic - please adjust! ###
    op.create_table('Axis',
    sa.Column('id_axis', sa.Integer(), nullable=False),
    sa.Column('name_axis', sa.String(length=10), nullable=False),
    sa.PrimaryKeyConstraint('id_axis')
    )
    # ### end Alembic commands ###

接下来，我将名为 test 的列添加到名为 axis 的表中以测试迁移：

#The axis on which the tests are executed
class AXIS(Base):
    __tablename__ = 'Axis'
    id_axis        = Column(Integer, primary_key = True)
    name_axis      = Column(String(10), nullable=False)
    test           = Column(String(10), nullable=False)

然后我们得到一个新的版本文件：

def upgrade():
    # ### commands auto generated by Alembic - please adjust! ###
    op.create_table('Axis',
    sa.Column('id_axis', sa.Integer(), nullable=False),
    sa.Column('name_axis', sa.String(length=10), nullable=False),
    sa.Column('test', sa.String(length=10), nullable=False),
    sa.PrimaryKeyConstraint('id_axis')
    )
    op.drop_table('axis')
    # ### end Alembic commands ###

但我注意到 alembic 首先尝试创建表（已经存在），然后尝试删除它们。要使升级生效，您必须在创建之前移动 drop。

为什么创建后会掉线？如果正常，请给我解释一下。如果不正常，我的错误可能是什么原因？

谢谢您的帮助 ！

标签： pythonsqlalchemyalembic