13.2.8. Database migrations

When you change a database structure between revisions and versions, you have to create database migrations. Migrations change a database structure from one state to another.

NextGIS Web provides some tools to work with database migrations. These tools were inspired by alembic library with the additional support of multiple components and dependencies between migrations.

Under the hood, it’s based on git-like tree structure with branches for each component separately. Single migration is like git’s commit which can be applied (forward operation) or reverted (rewind operation).


Migrations framework doesn’t support automatic migration creation at this moment. You might expect this, but now you have to write SQL scripts with your own hands. But it may change in the future. Existing components

For example, you have component foo which model definition had looked like this:

class FooModel(Base):
    __tablename__ = 'foo_model'
    id = db.Column(db.Integer, primary_key=True)
    some = db.Column(db.Unicode)

You had decided to add a new field “other” and changed its definition to this:

class FooModel(Base):
    __tablename__ = 'foo_model'
    id = db.Column(db.Integer, primary_key=True)
    some = db.Column(db.Unicode)
    other = db.Column(db.Unicode)

Now you have to create the migration, which adds the other column to the corresponding table. You can do this as follows:

$ nextgisweb migration create foo "Add other column"
Migration [foo:2c12ca17] created:
* migration/2c12ca17-add-other-column.fw.sql
* migration/2c12ca17-add-other-column.rw.sql

This command will generate two files, where you should put SQL instruction. The first one 2c12ca17-add-other-column.fw.sql is for forwarding migration, which adds the column. The second one 2c12ca17-add-other-column.rw.sql is rewinding migration, which drops the column and brings a database state to the previous.

You have to put SQL instructions into these files. Don’t remove migrations metadata in the first lines of files and remove a generated placeholder. For example:

Listing 13.3. File 2c12ca17-add-other-column.fw.sql
/*** {
    "revision": "2c12ca17", "parents": ["00000000"],
    "date": "2021-01-09T21:57:55",
    "message": "Add other column"
} ***/

ALTER TABLE foo_model ADD COLUMN other text;
Listing 13.4. File 2c12ca17-add-other-column.rw.sql
/*** { "revision": "2c12ca17" } ***/

ALTER TABLE foo_model DROP COLUMN other;

Now you can apply it using:

$ nextgisweb migration forward --no-dry-run foo:2c12ca17

Or undo it with:

$ nextgisweb migration rewind --no-dry-run foo:2c12ca17


Each nextgisweb migration * command which changes a database state doesn’t change without --no-dry-run option. You can use commands without this option to review what command will do.

To review the current database state you can use nextgisweb migration status command. It will show this migration:

$ nextgisweb migration status
A |    | Migration                      | Message
+ |    | foo:00000000                   | Initial migration
+ | FR | foo:2c12ca17                   | Add other column

After that, you have to commit these files to git repository. The command nextgisweb migration upgrade will apply them during the standard upgrade process. New components

When you create a new NextGIS Web component, you shouldn’t create the initial migration. Migration framework detects the presence of metadata and automatically creates initial migration with revision id 00000000.

For example, you’ve created the component bar with a model like this:

class BarModel(Base):
    __tablename__ = 'bar_model'
    id = db.Column(db.Integer, primary_key=True)
    some = db.Column(db.Unicode)

Now you can ask NextGIS Web to create tables for this component:


Previously, you could use nextgisweb initialize_db, but now you should use nextgisweb migration install.

$ nextgisweb migration install --no-dry-run bar

Moreover, you can ask to drop tables for this (or any other component) with the following command:

$ nextgisweb migration uninstall --no-dry-run bar

Thus nextgisweb migration install and nextgisweb migration uninstall commands provide the way to install and uninstall components. Python migrations

Sometimes it’s hard or impossible to write migration logic using SQL only. In this case, it’s possible to use Python module as a migration. So, a migration module can be created this way:

$ nextgisweb migration create --format python foo "Add another column"
Migration [foo:2e89a6fc] created:
* migration/2e89a6fc-add-another-column.py

It works the same way as SQL migrations, but forward and rewind migrations are both in one file.

""" {
    "revision": "2e89a6fc", "parents": ["2c12ca17"],
    "date": "2021-01-09T22:47:12",
    "message": "Add another column"
} """

def forward(ctx):
    pass  # TODO: Write code here and remove this placeholder line!

def rewind(ctx):
    pass  # TODO: Write code here and remove this placeholder line!

The global env object can be accessed by ctx.env attribute. But keep in mind that migration code is executed on outdated database structure. It means that you shouldn’t use SQLAlchemy models there.