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Migration Guide
---------------
Alembic comes with a CLI called ``alembic``.
Create a Migration
^^^^^^^^^^^^^^^^^^
Lets create our first migration::
$ alembic revision -m "add favourite_band field"
Generating
/your/gmg/path/mediagoblin/db/migrations/versions/1e3793de36a_add_favourite_band_field.py ... done
By default, migration files have two methods: ``upgrade`` and ``downgrade``.
Alembic will invoke these methods to apply the migrations to your current
database.
Now, we need to edit our newly created migration file
``1e3793de36a_add_favourite_band_field.py`` to add a new column ``favourite_band``
to ``core__users`` table::
def upgrade():
op.add_column('core__users', sa.Column('favourite_band', sa.Unicode(100)))
def downgrade():
op.drop_column('core__users', 'favourite_band')
.. note::
Alembic can also generate `automatic migrations <http://alembic.readthedocs.org/en/latest/tutorial.html#auto-generating-migrations>`__.
Then we can run ``gmg dbupdate`` to apply the new migration::
$ gmg dbupdate
INFO [alembic.migration] Context impl SQLiteImpl.
INFO [alembic.migration] Will assume non-transactional DDL.
INFO [alembic.migration] Running upgrade None -> 1e3793de36a, add favourite band field
If you want to revert that migration, simply run::
$ alembic downgrade -1
.. warning::
Currently, Alembic cannot do ``DROP COLUMN``, ``ALTER COLUMN`` etc.
operations in SQLite. Please see https://bitbucket.org/zzzeek/alembic/issue/21/column-renames-not-supported-on-sqlite
for detailed information.
Glossary
^^^^^^^^
* ``alembic.ini``: The Alembic configuration file. The ``alembic`` CLI will
look that file everytime it invaked.
* ``mediagoblin/db/migrations/versions/``: Alembic will add new migration files
to this directory.
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