Alembic: 使用USING修改列类型

Question

我试图使用 alembic 将一个 SQLAlchemy PostgreSQL ARRAY(Text) 字段转换为表中一列的 BIT(varying=True) 字段。

该列当前定义为：

cols = Column(ARRAY(TEXT), nullable=False, index=True)

我希望你能做翻译，把它改成中文：

cols = Column(BIT(varying=True), nullable=False, index=True)

默认情况下似乎不支持更改列类型，因此我正在手动编辑Alembic脚本。目前这就是我的代码：

def upgrade():
    op.alter_column(
        table_name='views',
        column_name='cols',
        nullable=False,
        type_=postgresql.BIT(varying=True)
    )


def downgrade():
    op.alter_column(
        table_name='views',
        column_name='cols',
        nullable=False,
        type_=postgresql.ARRAY(sa.Text())
    )

然而，运行这个脚本会出现错误：

Traceback (most recent call last):
  File "/home/home/.virtualenvs/deus_lex/bin/alembic", line 9, in <module>
    load_entry_point('alembic==0.7.4', 'console_scripts', 'alembic')()
  File "/home/home/.virtualenvs/deus_lex/local/lib/python2.7/site-packages/alembic/config.py", line 399, in main
    CommandLine(prog=prog).main(argv=argv)
  File "/home/home/.virtualenvs/deus_lex/local/lib/python2.7/site-packages/alembic/config.py", line 393, in main
    self.run_cmd(cfg, options)
  File "/home/home/.virtualenvs/deus_lex/local/lib/python2.7/site-packages/alembic/config.py", line 376, in run_cmd
    **dict((k, getattr(options, k)) for k in kwarg)
  File "/home/home/.virtualenvs/deus_lex/local/lib/python2.7/site-packages/alembic/command.py", line 165, in upgrade
    script.run_env()
  File "/home/home/.virtualenvs/deus_lex/local/lib/python2.7/site-packages/alembic/script.py", line 382, in run_env
    util.load_python_file(self.dir, 'env.py')
  File "/home/home/.virtualenvs/deus_lex/local/lib/python2.7/site-packages/alembic/util.py", line 242, in load_python_file
    module = load_module_py(module_id, path)
  File "/home/home/.virtualenvs/deus_lex/local/lib/python2.7/site-packages/alembic/compat.py", line 79, in load_module_py
    mod = imp.load_source(module_id, path, fp)
  File "./scripts/env.py", line 83, in <module>
    run_migrations_online()
  File "./scripts/env.py", line 76, in run_migrations_online
    context.run_migrations()
  File "<string>", line 7, in run_migrations
  File "/home/home/.virtualenvs/deus_lex/local/lib/python2.7/site-packages/alembic/environment.py", line 742, in run_migrations
    self.get_context().run_migrations(**kw)
  File "/home/home/.virtualenvs/deus_lex/local/lib/python2.7/site-packages/alembic/migration.py", line 305, in run_migrations
    step.migration_fn(**kw)
  File "/home/home/deus_lex/winslow/scripts/versions/2644864bf479_store_caselist_column_views_as_bits.py", line 24, in upgrade
    type_=postgresql.BIT(varying=True)
  File "<string>", line 7, in alter_column
  File "<string>", line 1, in <lambda>
  File "/home/home/.virtualenvs/deus_lex/local/lib/python2.7/site-packages/alembic/util.py", line 387, in go
    return fn(*arg, **kw)
  File "/home/home/.virtualenvs/deus_lex/local/lib/python2.7/site-packages/alembic/operations.py", line 470, in alter_column
    existing_autoincrement=existing_autoincrement
  File "/home/home/.virtualenvs/deus_lex/local/lib/python2.7/site-packages/alembic/ddl/impl.py", line 147, in alter_column
    existing_nullable=existing_nullable,
  File "/home/home/.virtualenvs/deus_lex/local/lib/python2.7/site-packages/alembic/ddl/impl.py", line 105, in _exec
    return conn.execute(construct, *multiparams, **params)
  File "/home/home/.virtualenvs/deus_lex/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 729, in execute
    return meth(self, multiparams, params)
  File "/home/home/.virtualenvs/deus_lex/local/lib/python2.7/site-packages/sqlalchemy/sql/ddl.py", line 69, in _execute_on_connection
    return connection._execute_ddl(self, multiparams, params)
  File "/home/home/.virtualenvs/deus_lex/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 783, in _execute_ddl
    compiled
  File "/home/home/.virtualenvs/deus_lex/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 958, in _execute_context
    context)
  File "/home/home/.virtualenvs/deus_lex/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1159, in _handle_dbapi_exception
    exc_info
  File "/home/home/.virtualenvs/deus_lex/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/home/.virtualenvs/deus_lex/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 951, in _execute_context
    context)
  File "/home/home/.virtualenvs/deus_lex/local/lib/python2.7/site-packages/sqlalchemy/engine/default.py", line 436, in do_execute
    cursor.execute(statement, parameters)
sqlalchemy.exc.ProgrammingError: (ProgrammingError) column "cols" cannot be cast automatically to type bit varying
HINT:  Specify a USING expression to perform the conversion.
 'ALTER TABLE views ALTER COLUMN cols TYPE BIT VARYING' {}

我该如何使用 USING 表达式来修改我的脚本？

- bard

Answer 1

自版本0.8.8起，alembic使用postgresql_using参数支持PostgreSQL的USING：

op.alter_column('views', 'cols', type_=postgresql.BIT(varying=True), postgresql_using='col_name::expr')

- Dag Høidahl

---

赞成将数据库迁移过程写进代码，以便能够轻松地在其他环境中重现数据库的状态。关于“USING”表达式的更多提示，目的是告诉Postgres如何将所有当前存储的值转换为新类型。例如，将整数列更改为varchar，您可以使用类似于“col_name::varchar(32)”的语法。 - Hartley Brody

这正是我现在正在寻找的。 :) - Martin Thorsen Ranang

应该是被接受的答案。真的很难找到关于postgresql_using的文档... - Nevenoe

这应该是被接受的答案。 - undefined

能否使用它将Redshift的FLOAT4转换为NUMERIC(3,1)，反之亦然？ - undefined

Answer 2

很不幸，由于alembic在更改类型时从不输出USING语句，因此您需要使用原始SQL。

不过，编写自定义SQL非常容易：

op.execute('ALTER TABLE views ALTER COLUMN cols TYPE bit varying USING expr')

当然，您需要将 expr 替换为一个表达式，该表达式将旧数据类型转换为新数据类型。

- ThiefMaster