当我希望通过升级中的Session对象更改数据时，如何测试alembic迁移？

Question

I am trying to get alembic to do a database migration for me. I have made changes to my model which add two new columns, which will have data that is calculated based on the current rows in the table. What I really want to do is figure out how to test what the migration is going to do before it actually does it. My understanding is the --sql argument would allow me to do that.

When I try to do the upgrade with --sql to test the migration before actually doing it, using the following command line:

alembic -c dev_alembic.ini upgrade --sql 3e0

I am receiving an error from alembic:

BEGIN TRANSACTION;

CREATE TABLE alembic_version (
    version_num VARCHAR(32) NOT NULL
);

GO

-- Running upgrade  -> 3e076afb70e1

ALTER TABLE sipendpoint ADD extension VARCHAR(50) NULL;

GO

ALTER TABLE sipendpoint ADD home_site VARCHAR(5) NULL;

GO

Traceback (most recent call last):
  File "/home/pgrace/venv/pyramid27/bin/alembic", line 9, in <module>
    load_entry_point('alembic==0.7.6', 'console_scripts', 'alembic')()
  File "/home/pgrace/venv/pyramid27/lib/python2.7/site-packages/alembic/config.py", line 439, in main
    CommandLine(prog=prog).main(argv=argv)
  File "/home/pgrace/venv/pyramid27/lib/python2.7/site-packages/alembic/config.py", line 433, in main
    self.run_cmd(cfg, options)
  File "/home/pgrace/venv/pyramid27/lib/python2.7/site-packages/alembic/config.py", line 416, in run_cmd
    **dict((k, getattr(options, k)) for k in kwarg)
  File "/home/pgrace/venv/pyramid27/lib/python2.7/site-packages/alembic/command.py", line 165, in upgrade
    script.run_env()
  File "/home/pgrace/venv/pyramid27/lib/python2.7/site-packages/alembic/script.py", line 390, in run_env
    util.load_python_file(self.dir, 'env.py')
  File "/home/pgrace/venv/pyramid27/lib/python2.7/site-packages/alembic/util.py", line 243, in load_python_file
    module = load_module_py(module_id, path)
  File "/home/pgrace/venv/pyramid27/lib/python2.7/site-packages/alembic/compat.py", line 79, in load_module_py
    mod = imp.load_source(module_id, path, fp)
  File "alembic/env.py", line 65, in <module>
    run_migrations_offline()
  File "alembic/env.py", line 44, in run_migrations_offline
    context.run_migrations()
  File "<string>", line 7, in run_migrations
  File "/home/pgrace/venv/pyramid27/lib/python2.7/site-packages/alembic/environment.py", line 738, in run_migrations
    self.get_context().run_migrations(**kw)
  File "/home/pgrace/venv/pyramid27/lib/python2.7/site-packages/alembic/migration.py", line 309, in run_migrations
    step.migration_fn(**kw)
  File "/home/pgrace/repo/stackcallrouterserver/alembic/versions/3e076afb70e1_add_extension_field_and_home_site_to_.py", line 48, in upgrade
    for entry in DBSession.query(SIPEndpoint):
  File "/home/pgrace/venv/pyramid27/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line 2515, in __iter__
    return self._execute_and_instances(context)
  File "/home/pgrace/venv/pyramid27/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line 2528, in _execute_and_instances
    close_with_result=True)
  File "/home/pgrace/venv/pyramid27/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line 2519, in _connection_from_session
    **kw)
  File "/home/pgrace/venv/pyramid27/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 882, in connection
    execution_options=execution_options)
  File "/home/pgrace/venv/pyramid27/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 887, in _connection_for_bind
    engine, execution_options)
  File "/home/pgrace/venv/pyramid27/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 344, in _connection_for_bind
    transaction = conn.begin()
AttributeError: 'MockConnection' object has no attribute 'begin'

Here is the migration file in question:

# revision identifiers, used by Alembic.
revision = '3e076afb70e1'
down_revision = None
branch_labels = None
depends_on = None

from alembic import op
import sqlalchemy as sa
from sqlalchemy.ext.declarative import declarative_base
import re
from scrserver.models import SIPEndpoint

from sqlalchemy.orm import (
    scoped_session,
    sessionmaker,
    )

DBSession = scoped_session(sessionmaker(bind=op.get_bind()))
Base = declarative_base()
Base.metadata.bind = op.get_bind()

def sitecheck(x):
    try:
        return {
                '1.2.3.4': "FOO",
                '5.6.7.8': "BAR",
                '9.10.11.12': "BAZ"
                }
    except KeyError:
        return None

def upgrade():
    ### commands auto generated by Alembic - please adjust! ###
    op.add_column('sipendpoint', sa.Column('extension', sa.String(length=50), nullable=True))
    op.add_column('sipendpoint', sa.Column('home_site', sa.String(length=5), nullable=True))

    ### end Alembic commands ###


    for entry in DBSession.query(SIPEndpoint):
        update=sa.update(SIPEndpoint).where(SIPEndpoint.addr==entry.addr).values(extension=entry.addr)
        op.execute(update)


def downgrade():
    ### commands auto generated by Alembic - please adjust! ###
    op.drop_column('sipendpoint', 'home_site')
    op.drop_column('sipendpoint', 'extension')
    ### end Alembic commands ###

Answer 1

you cannot run this:

for entry in DBSession.query(SIPEndpoint):

in --sql mode, because that is a SELECT statement. There is no database with which to SELECT from in --sql mode.

There is no way to do a "dry run" of a script that wishes to have a conversation with the database, eg a script that relies on retrieving data from the database. --sql mode is limited to just those commands that require no interaction at all, eg CREATE, DROP and ALTER statements, and to an extremely limited degree, INSERT, UPDATE and DELETE statements, which must be emitted in very special ways such that they don't attempt to request information about the operation. For INSERT, the bulk insert style is recommended as it is designed to be compatible with --sql mode.