Alembic: alter column type with USING

问题

I'm attempting to use alembic to convert a SQLAlchemy PostgreSQL ARRAY(Text) field to a BIT(varying=True) field for one of my table columns.

The column is currently defined as:

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

I want to change it to:

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

Changing column types doesn't seem to be supported by default, so I'm editing the alembic script by hand. This is what I have currently:

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())
    )

However, running this script gives the error:

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' {}

How can I change my script with the USING expression?

回答1:

Unfortunately you need to use raw SQL as alembic doesn't output a USING statement ever when changing types.

However, writing custom SQL for this is quite easy:

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

Of course you have to replace expr with an expression that converts the old data type to the new data type.

回答2:

Since version 0.8.8, alembic supports PostgreSQL's USING with the postgresql_using parameter:

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

来源：https://stackoverflow.com/questions/29069506/alembic-alter-column-type-with-using

标签

postgresql

sqlalchemy

alembic