A database migration tool for SQLAlchemy.
Project description
Alembic is a semi-experimental database migrations tool. A migrations tool offers the following functionality:
Can emit ALTER statements to a database in order to change the structure of tables and other constructs
Provides a system whereby “migration scripts” may be constructed; each script indicates a particular series of steps that can “upgrade” a target database to a new version, and optionally a series of steps that can “downgrade” similarly, doing the same steps in reverse.
Allows the scripts to execute in some sequential manner.
The goals of Alembic are:
Very open ended and transparent configuration and operation. A new Alembic environment is generated from a set of templates which is selected among a set of options when setup first occurs. The templates then deposit a series of scripts that define fully how database connectivity is established and how migration scripts are invoked; the migration scripts themselves are generated from a template within that series of scripts. The scripts can then be further customized to define exactly how databases will be interacted with and what structure new migration files should take.
Full support for transactional DDL. The default scripts ensure that all migrations occur within a transaction - for those databases which support this (Postgresql, Microsoft SQL Server), migrations can be tested with no need to manually undo changes upon failure.
Minimalist script construction. Basic operations like renaming tables/columns, adding/removing columns, changing column attributes can be performed through one line commands like alter_column(), rename_table(), add_constraint(). There is no need to recreate full SQLAlchemy Table structures for simple operations like these - the functions themselves generate minimalist schema structures behind the scenes to achieve the given DDL sequence.
Full support for migrations generated as SQL scripts. Those of us who work in corporate environments know that direct access to DDL commands on a production database is a rare privilege, and DBAs want textual SQL scripts. Alembic’s usage model and commands are oriented towards being able to run a series of migrations into a textual output file as easily as it runs them directly to a database. Care must be taken in this mode to not invoke other operations that rely upon in-memory SELECTs of rows - Alembic tries to provide helper constructs like bulk_insert() to help with data-oriented operations that are compatible with script-based DDL.
Non-linear versioning. Scripts are given UUID identifiers similarly to a DVCS, and the linkage of one script to the next is achieved via markers within the scripts themselves. Through this open-ended mechanism, branches containing other migration scripts can be merged - the linkages can be manually edited within the script files to create the new sequence.
Provide a library of ALTER constructs that can be used by any SQLAlchemy application. The DDL constructs build upon SQLAlchemy’s own DDLElement base and can be used standalone by any application or script.
Don’t break our necks over SQLite’s inability to ALTER things. If you’re using SQLite, you really should build a system of dumping your data and importing it back, as this backend simply does not support the migrations use case. Alembic has no issue talking to SQLite of course but most ALTER statements won’t work.
Alembic is working at a rudimentary level and includes a little bit of support for Postgresql and Microsoft SQL Server. As of yet the documentation hasn’t been written - this is really the only thing left before an early release can be put out.
