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?
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.