Alembic is a useful tool to manage migration, but can’t surpass local engine implementation
of SQL. As
SQLite doesn’t manage schema modifications such as
whole migration system of IRMA won’t support it. The preferred database engine is
You can still use SQLite, but you will be on your own for migrations.
Please note that most of the manipulations on this can and sometimes will alter your data.
If you are not sure about what you are doing, and even if you are sure, make backup.
Database migrations are managed in the frontend and brain IRMA components.
The files/directories used are:
All the commands below will assert to be executed on top of this file system,
as Alembic needs the
alembic.ini configuration file.
You could also use the
Alembic manage a ‘revision’ for each database evolution. These revisions are used to upgrade or
downgrade the database schema.
... shows the current revision of the database.
The command to get the history of the latest alembic migrations is:
$ alembic history --verbose
Create database from scratch with Alembic
Configuration and creating database
Alembic will use the information in the
[sqldb] section of the configuration files
conf/brain.ini for the repositories of
the frontend or the brain components). Make sure they are accurate.
The database must already exist. This step is quite simple, the SQL command usually being:
sql$ CREATE DATABASE <db_name>;
Update your schema with Alembic
If you use a virtualenv, activate it. Then enter:
Alembic applies each revision one after the other. At the end of the process, if no error
occurs, your database should be updated.
You can update the database one revision at a time, or up to a specific revision. See the
revisions section for further information.
If you already have a database WITHOUT Alembic
Alembic stores its current revision number in database. If your database doesn’t have this
information, you are very likely to encounter errors when using Alembic, as it will try to
create already existing tables.
The easiest solution is to destroy your database and go for a fresh install.
Although, if you don’t want to lose your data, you could update the Alembic information
You will need to:
Get the exact current Alembic revision of your database. Each migration file has a
Revision ID in its header. Investigate the successive revisions to know which one
matches your current database state.
Once you known your Alembic revision, run:
$ alembic stamp <your_alembic_revision_number>
Your database is now synchronized with Alembic! You should be able to use Alembic to
upgrade/downgrade your database now. Be aware that if the revision number you provided is
false, you could encounter massive errors while attempting to upgrade/downgrade your
Generating a new revision
Creating a new revision can be done with the command:
$ alembic revision -m <revision_message>
This command produces a new
<hash>_<revision_message>.py file in the
directory. This file contains two functions
downgrade, respectively used
to upgrade the database to the revision, or downgrade from it. These two functions are empty
and must be completed with the desired modifications (see the
A revision could be produced automatically, from database metadata defined in the IRMA SQL
objects description through
sqlalchemy, with the command:
$ alembic revision --autogenerate -m <revision_message>
These SQL objects are defined in:
frontend/models/sqlobjects.py for the frontend,
brain/models/sqlobjects.py for the brain.
Alembic scripts in IRMA repositories are already configured to use metadata defined in these
files. You should be able to use the
--autogenerate option without further modifications.
IRMA configuration allows to prefix table names through configuration. Our revision files
use the function
<frontend_or_brain>/config/parser.py:prefix_table_name to generate table
names rather than keeping alembic-generated plain string names. A good practice would be
to keep using this function in revision files.
Alembic easily detects changes such as adding/removing columns, but could be blind on thin,
inner modifications. Re-reading the auto-generated script is a strongly recommended step
before actually performing the migration.
See the alembic documentation
for more information.
Database modifications using
ALTER_COLUMN (such as changing the type of a column) can’t
be performed on
SQLite databases. Be aware of this limitation if you absolutely want
to use migration scripts with this SQL engine.
Migrating between revisions
Once the revision is properly described, the migration is performed with:
Alembic allows to migrate the database to any revision, relatively to the current revision
or absolutely. Several examples:
$ alembic upgrade +4
$ alembic downgrade base
$ alembic upgrade <revision_number>+3
Tips and tricks
Don’t trust Alembic too much. It is nothing more than a tool, without any comprehension
on the code. Cautiously read the revision scripts it generates.
Database migration is hardly ever a painless step. Be sure to:
- save your data before performing a migration,
- test your application after the migration to ensure its compatibility with the new data
PostgreSQL database, the
Float type is tolerated but the real type name used
by the database is
Real. It means that SQL objects described in
Float columns will be properly applied in database, but at each autogenerate revision,
alembic will see
Real type in database, against
Float type in the code metadata,
and so will perform each time a useless
This problem could be avoided (with
PostgreSQL) by declaring
Real instead of
See this page for more
PostgreSQL numeric types.
Alembic can’t directly deal with many somehow complex operations, such as type migration
with no trivial cast. In these cases, the operation must be manually described with a raw
SQL command (which could be database-dependent).
For instance, alembic can’t perform the migration from
> alembic.alter_column('table', 'column',
... because of an error
a column "column" cannot be cast automatically to type timestamp
with time zone.
A proper migration for
PostgreSQL would be (in
> alembic.execute('ALTER TABLE "table" ALTER COLUMN "column" TYPE TIMESTAMP WITHOUT TIME ZONE USING to_timestamp(column)')
And the reverse code to downgrade the migration could be:
> alembic.execute('ALTER TABLE "table" ALTER COLUMN "column" TYPE REAL USING extract(epoch from column)')
Rather than managing migrations directly with Alembic, we could generate SQL migration
revision to be used directly on database with the command:
$ alembic upgrade <revision> --sql > migration.sql
Deleting a revision R is simple:
- downgrade the database to the revision before R-1 the revision you want to delete;
- if any, edit the script of the following revision R+1 and update the
variable to match the revision number of revision R-1;
- delete the script of the revision R you want to delete;
- upgrade your database.
The deleted revision want be applied any more.