为什么在删除列时 Flask-migrate 无法升级 [英] Why Flask-migrate cannot upgrade when drop column

问题描述

我正在使用 SqlAlchemy 和 Flask-migrate 进行数据库迁移.我已经成功init 数据库和upgrade 一次，但是当我删除我的表列之一时，我设法migrate 但是upgrade 给了我以下错误:

I am using SqlAlchemy and Flask-migrate for DB migration. I have successfully init the DB and upgrade once, but when I deleted one of my table column, I managed to migrate however upgrade gave me the following error:

sqlalchemy.exc.OperationalError: (sqlite3.OperationalError) near "DROP": syntax error [SQL: u'ALTER TABLE posts DROP COLUMN tags']

我的models.py有一部分

There is part of my models.py

class Post(db.Model):
    __tabelname__ = 'posts'
    id = db.Column(db.Integer, primary_key=True)
    body = db.Column(db.UnicodeText)
    # tags = db.Column(db.Unicode(32))
    # I deleted this field, upgrade give me error

    ....

我再次运行python manage.py db upgrade，错误改变了！

And I run python manage.py db upgrade again, the error changed!

(venv)ncp@ubuntu:~/manualscore$ python manage.py db upgrade
INFO  [alembic.migration] Context impl SQLiteImpl.
INFO  [alembic.migration] Will assume non-transactional DDL.
INFO  [alembic.migration] Running upgrade 555b78ffd5f -> 2e063b1b3164, add tag table
Traceback (most recent call last):
  File "manage.py", line 79, in <module>
    manager.run()
  File "/home/ncp/manualscore/venv/local/lib/python2.7/site-packages/flask_script/__init__.py", line 405, in run
    result = self.handle(sys.argv[0], sys.argv[1:])
  File "/home/ncp/manualscore/venv/local/lib/python2.7/site-packages/flask_script/__init__.py", line 384, in handle
    return handle(app, *positional_args, **kwargs)
  File "/home/ncp/manualscore/venv/local/lib/python2.7/site-packages/flask_script/commands.py", line 145, in handle
    return self.run(*args, **kwargs)
  File "/home/ncp/manualscore/venv/local/lib/python2.7/site-packages/flask_migrate/__init__.py", line 177, in upgrade
    command.upgrade(config, revision, sql=sql, tag=tag)
  File "/home/ncp/manualscore/venv/local/lib/python2.7/site-packages/alembic/command.py", line 165, in upgrade
    script.run_env()
  File "/home/ncp/manualscore/venv/local/lib/python2.7/site-packages/alembic/script.py", line 390, in run_env
    util.load_python_file(self.dir, 'env.py')
  File "/home/ncp/manualscore/venv/local/lib/python2.7/site-packages/alembic/util.py", line 243, in load_python_file
    module = load_module_py(module_id, path)
  File "/home/ncp/manualscore/venv/local/lib/python2.7/site-packages/alembic/compat.py", line 79, in load_module_py
    mod = imp.load_source(module_id, path, fp)
  File "migrations/env.py", line 72, in <module>
    run_migrations_online()
  File "migrations/env.py", line 65, in run_migrations_online
    context.run_migrations()
  File "<string>", line 7, in run_migrations
  File "/home/ncp/manualscore/venv/local/lib/python2.7/site-packages/alembic/environment.py", line 738, in run_migrations
    self.get_context().run_migrations(**kw)
  File "/home/ncp/manualscore/venv/local/lib/python2.7/site-packages/alembic/migration.py", line 309, in run_migrations
    step.migration_fn(**kw)
  File "/home/ncp/manualscore/migrations/versions/2e063b1b3164_add_tag_table.py", line 24, in upgrade
    sa.PrimaryKeyConstraint('id')
  File "<string>", line 7, in create_table
  File "/home/ncp/manualscore/venv/local/lib/python2.7/site-packages/alembic/operations.py", line 944, in create_table
    self.impl.create_table(table)
  File "/home/ncp/manualscore/venv/local/lib/python2.7/site-packages/alembic/ddl/impl.py", line 198, in create_table
    self._exec(schema.CreateTable(table))
  File "/home/ncp/manualscore/venv/local/lib/python2.7/site-packages/alembic/ddl/impl.py", line 122, in _exec
    return conn.execute(construct, *multiparams, **params)
  File "/home/ncp/manualscore/venv/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 914, in execute
    return meth(self, multiparams, params)
  File "/home/ncp/manualscore/venv/local/lib/python2.7/site-packages/sqlalchemy/sql/ddl.py", line 68, in _execute_on_connection
    return connection._execute_ddl(self, multiparams, params)
  File "/home/ncp/manualscore/venv/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 968, in _execute_ddl
    compiled
  File "/home/ncp/manualscore/venv/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1146, in _execute_context
    context)
  File "/home/ncp/manualscore/venv/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1339, in _handle_dbapi_exception
    exc_info
  File "/home/ncp/manualscore/venv/local/lib/python2.7/site-packages/sqlalchemy/util/compat.py", line 199, in raise_from_cause
    reraise(type(exception), exception, tb=exc_tb)
  File "/home/ncp/manualscore/venv/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1139, in _execute_context
    context)
  File "/home/ncp/manualscore/venv/local/lib/python2.7/site-packages/sqlalchemy/engine/default.py", line 442, in do_execute
    cursor.execute(statement, parameters)
sqlalchemy.exc.OperationalError: (sqlite3.OperationalError) table tags already exists [SQL: u'
CREATE TABLE tags (
	id INTEGER NOT NULL, 
	name VARCHAR(32), 
	post_id INTEGER, 
	PRIMARY KEY (id), 
	FOREIGN KEY(post_id) REFERENCES posts (id)
)

']

推荐答案

SQLite 不支持删除或更改列.但是，有一种方法可以通过在表级别进行更改来解决此问题:https://www.sqlite.org/lang_altertable.html

SQLite does not support dropping or altering columns. However, there is a way to work around this by making changes at the table level: https://www.sqlite.org/lang_altertable.html

对于 Alembic/Flask-Migrate 用户更有用的是，Alembic 的 batch_alter_table 上下文管理器让您可以以自然的方式指定更改，并做了一些创建新表 - 复制数据 - 删除旧表 - 重命名新表"的舞蹈使用 SQLite 时的场景.请参阅:http://alembic.zzzcomputing.com/en/latest/batch.html

And more usefully for Alembic/Flask-Migrate users, Alembic's batch_alter_table context manager lets you specify the changes in a natural way, and does a little "make new table - copy data - drop old table - rename new table" dance behind the scenes when using SQLite. See: http://alembic.zzzcomputing.com/en/latest/batch.html

因此迁移文件中的 upgrade() 函数应包含以下内容:

So the upgrade() function in your migration file should contain something like:

with op.batch_alter_table('posts') as batch_op:
    batch_op.drop_column('tags')

恐怕我不知道为什么第二次尝试升级时错误发生了变化.

I'm afraid I don't know why the error changed the second time you tried the upgrade.

正如 tkisme 所指出的，您还可以在 env.py 中配置 EnvironmentContext.configure.render_as_batch 标志，以便自动生成的迁移脚本将使用 batch_alter_table默认.请参阅:http://alembic.zzzcomputing.com/en/latest/batch.html#batch-mode-with-autogenerate

As tkisme points out, you can also configure the EnvironmentContext.configure.render_as_batch flag in env.py so that autogenerated migration scripts will use batch_alter_table by default. See: http://alembic.zzzcomputing.com/en/latest/batch.html#batch-mode-with-autogenerate

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