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Generating SQL Scripts (a.k.a. "Offline Mode")
==============================================
A major capability of Alembic is to generate migrations as SQL scripts, instead of running
them against the database - this is also referred to as *offline mode*.
This is a critical feature when working in large organizations
where access to DDL is restricted, and SQL scripts must be handed off to DBAs. Alembic makes
this easy via the ``--sql`` option passed to any ``upgrade`` or ``downgrade`` command. We
can, for example, generate a script that revises up to rev ``ae1027a6acf``::
$ alembic upgrade ae1027a6acf --sql
INFO [alembic.context] Context class PostgresqlContext.
INFO [alembic.context] Will assume transactional DDL.
BEGIN;
CREATE TABLE alembic_version (
version_num VARCHAR(32) NOT NULL
);
INFO [alembic.context] Running upgrade None -> 1975ea83b712
CREATE TABLE account (
id SERIAL NOT NULL,
name VARCHAR(50) NOT NULL,
description VARCHAR(200),
PRIMARY KEY (id)
);
INFO [alembic.context] Running upgrade 1975ea83b712 -> ae1027a6acf
ALTER TABLE account ADD COLUMN last_transaction_date TIMESTAMP WITHOUT TIME ZONE;
INSERT INTO alembic_version (version_num) VALUES ('ae1027a6acf');
COMMIT;
While the logging configuration dumped to standard error, the actual script was dumped to standard output -
so in the absence of further configuration (described later in this section), we'd at first be using output
redirection to generate a script::
$ alembic upgrade ae1027a6acf --sql > migration.sql
Getting the Start Version
--------------------------
Notice that our migration script started at the base - this is the default when using offline
mode, as no database connection is present and there's no ``alembic_version`` table to read from.
One way to provide a starting version in offline mode is to provide a range to the command line.
This is accomplished by providing the "version" in ``start:end`` syntax::
$ alembic upgrade 1975ea83b712:ae1027a6acf --sql > migration.sql
The ``start:end`` syntax is only allowed in offline mode; in "online" mode, the ``alembic_version``
table is always used to get at the current version.
It's also possible to have the ``env.py`` script retrieve the "last" version from
the local environment, such as from a local file. A scheme like this would basically
treat a local file in the same way ``alembic_version`` works::
if context.is_offline_mode():
version_file = os.path.join(os.path.dirname(config.config_file_name), "version.txt")
if os.path.exists(version_file):
current_version = open(version_file).read()
else:
current_version = None
context.configure(dialect_name=engine.name, starting_rev=current_version)
context.run_migrations()
end_version = context.get_revision_argument()
if end_version and end_version != current_version:
open(version_file, 'w').write(end_version)
Writing Migration Scripts to Support Script Generation
------------------------------------------------------
The challenge of SQL script generation is that the scripts we generate can't rely upon
any client/server database access. This means a migration script that pulls some rows
into memory via a ``SELECT`` statement will not work in ``--sql`` mode. It's also
important that the Alembic directives, all of which are designed specifically to work
in both "live execution" as well as "offline SQL generation" mode, are used.
Customizing the Environment
---------------------------
Users of the ``--sql`` option are encouraged to hack their ``env.py`` files to suit their
needs. The ``env.py`` script as provided is broken into two sections: ``run_migrations_online()``
and ``run_migrations_offline()``. Which function is run is determined at the bottom of the
script by reading :meth:`.EnvironmentContext.is_offline_mode`, which basically determines if the
``--sql`` flag was enabled.
For example, a multiple database configuration may want to run through each
database and set the output of the migrations to different named files - the :meth:`.EnvironmentContext.configure`
function accepts a parameter ``output_buffer`` for this purpose. Below we illustrate
this within the ``run_migrations_offline()`` function::
from alembic import context
import myapp
import sys
db_1 = myapp.db_1
db_2 = myapp.db_2
def run_migrations_offline():
"""Run migrations *without* a SQL connection."""
for name, engine, file_ in [
("db1", db_1, "db1.sql"),
("db2", db_2, "db2.sql"),
]:
context.configure(
url=engine.url,
transactional_ddl=False,
output_buffer=open(file_, 'w'))
context.execute("-- running migrations for '%s'" % name)
context.run_migrations(name=name)
sys.stderr.write("Wrote file '%s'" % file_)
def run_migrations_online():
"""Run migrations *with* a SQL connection."""
for name, engine in [
("db1", db_1),
("db2", db_2),
]:
connection = engine.connect()
context.configure(connection=connection)
try:
context.run_migrations(name=name)
session.commit()
except:
session.rollback()
raise
if context.is_offline_mode():
run_migrations_offline()
else:
run_migrations_online()
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