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The migrations in the alembic/versions contain the changes needed to migrate
from older Congress releases to newer versions. A migration occurs by executing
a script that details the changes needed to upgrade/downgrade the database. The
migration scripts are ordered so that multiple scripts can run sequentially to
update the database. The scripts are executed by Congress's migration wrapper
which uses the Alembic library to manage the migration.  Congress supports
migration from Juno or later.


If you are a deployer or developer and want to migrate from Juno to Kilo
or later you must first add version tracking to the database:

$ congress-db-manage --config-file /path/to/congress.conf stamp initial_db

You can then upgrade to the latest database version via:
$ congress-db-manage --config-file /path/to/congress.conf upgrade head

To check the current database version:
$ congress-db-manage --config-file /path/to/congress.conf current

To create a script to run the migration offline:
$ congress-db-manage --config-file /path/to/congress.conf upgrade head --sql

To run the offline migration between specific migration versions:
$ congress-db-manage --config-file /path/to/congress.conf \
 upgrade <start version>:<end version> --sql

Upgrade the database incrementally:
$ congress-db-manage --config-file /path/to/congress.conf \
 upgrade --delta <# of revs>

Downgrade the database by a certain number of revisions:
$ congress-db-manage --config-file /path/to/congress.conf \
 downgrade --delta <# of revs>


DEVELOPERS:
A database migration script is required when you submit a change to Congress
that alters the database model definition.  The migration script is a special
python file that includes code to update/downgrade the database to match the
changes in the model definition. Alembic will execute these scripts in order to
provide a linear migration path between revision. The congress-db-manage command
can be used to generate migration template for you to complete.  The operations
in the template are those supported by the Alembic migration library.

$ congress-db-manage --config-file /path/to/congress.conf \
revision  -m "description of revision" --autogenerate

This generates a prepopulated template with the changes needed to match the
database state with the models.  You should inspect the autogenerated template
to ensure that the proper models have been altered.

In rare circumstances, you may want to start with an empty migration template
and manually author the changes necessary for an upgrade/downgrade.  You can
create a blank file via:

$ congress-db-manage --config-file /path/to/congress.conf \
 revision -m "description of revision"

The migration timeline should remain linear so that there is a clear path when
upgrading/downgrading.  To verify that the timeline does branch, you can run
this command:
$ congress-db-manage --config-file /path/to/congress.conf check_migration

If the migration path does branch, you can find the branch point via:
$ congress-db-manage --config-file /path/to/congress.conf history
