New in version 0.6

Migrations are the basic instrument needed to propagate changes you make to your models into your database schema.

While the Database class in Emmett can be initialized to automatically migrate your schema, under the default behaviour Emmett encourage the adoption of its migration system. In fact, while automatic migration can be handy for small applications that won't frequently change the structure of their models, whenever your application evolves and requires a lot of changes on the models, or you need a fine control on the migration process for the production side, this feature may produce unwanted side effects, as the migration data has to be stored in the databases directory of your application and migrations will be performed every time you push new code in your models.

The migration engine is instead based on revisions: this will use migration files containing the instructions to be performed on the database side and will store the current migration status on the database itself, fact that prevents inconsistencies on the migration status of your application if you are running the code from several machines.

Emmett provides different migration commands, that can be used in order to generate, apply and revert migrations on your database. Moreover, to avoid you the pain of writing a lot of migration code aside with your models, Emmett will automatically generate the migration scripts for you starting from your models' code.
In the next sections we will describe all of this using the bloggy application we saw in the tutorial chapter as an example.

Note: we strongly reccomend you to not enable automatic migrations on applications that run on production environments. The automatic migrations and the ones performed by the migration engine have some slight differences; while we will document operations supported by the second system, the detection performed by the automatic one depends on the pydal library, and are not officially supported by the Emmett development. If you need more informations about this you should check the web2py docs.

Generating your first migration

Once you have written your models, you will need to create the relevant tables on the database to start performing operations.

In the bloggy example we defined three models, and since we turned off automatic migrations, we need to create a migration that will add the relevant tables to the database. This can be quite immediate if you use the Emmett generation command:

$ emmett migrations generate -m "First migration"
> Generated migration for revision fe68547ce244

As you can see the generate command accepts a -m option which will be the message for the migration and also its name, and prints out the revision number for the generated migration. This number is unique and will identify the single migration.

Now if you look into the migrations folder of the bloggy application, you will find a 4ceb82ecd8e4_first_migration.py file, containing all the operations needed to create the tables defined by our models. Since we used the Auth module in bloggy, beside the tables for the User, Post and Comment models we will find also the operations that will create the ones for the authorization module as well:

"""First migration

Migration ID: fe68547ce244
Creation Date: 2017-03-09 16:31:24.333823


from emmett.orm import migrations

class Migration(migrations.Migration):
    revision = 'fe68547ce244'
    revises = None

    def up(self):
            migrations.Column('id', 'id'),
            migrations.Column('created_at', 'datetime'),
            migrations.Column('updated_at', 'datetime'),
            migrations.Column('email', 'string', length=255),
            migrations.Column('password', 'password', length=512),
            migrations.Column('registration_key', 'string', default='', length=512),
            migrations.Column('reset_password_key', 'string', default='', length=512),
            migrations.Column('registration_id', 'string', default='', length=512),
            migrations.Column('first_name', 'string', notnull=True, length=128),
            migrations.Column('last_name', 'string', notnull=True, length=128))
            migrations.Column('id', 'id'),
            migrations.Column('created_at', 'datetime'),
            migrations.Column('updated_at', 'datetime'),
            migrations.Column('role', 'string', default='', length=255),
            migrations.Column('description', 'text'))
            migrations.Column('id', 'id'),
            migrations.Column('created_at', 'datetime'),
            migrations.Column('updated_at', 'datetime'),
            migrations.Column('user', 'reference users', ondelete='CASCADE'),
            migrations.Column('auth_group', 'reference auth_groups', ondelete='CASCADE'))
            migrations.Column('id', 'id'),
            migrations.Column('created_at', 'datetime'),
            migrations.Column('updated_at', 'datetime'),
            migrations.Column('name', 'string', default='default', notnull=True, length=512),
            migrations.Column('table_name', 'string', length=512),
            migrations.Column('record_id', 'integer', default=0),
            migrations.Column('auth_group', 'reference auth_groups', ondelete='CASCADE'))
            migrations.Column('id', 'id'),
            migrations.Column('created_at', 'datetime'),
            migrations.Column('updated_at', 'datetime'),
            migrations.Column('client_ip', 'string', length=512),
            migrations.Column('origin', 'string', default='auth', notnull=True, length=512),
            migrations.Column('description', 'text', default='', notnull=True),
            migrations.Column('user', 'reference users', ondelete='CASCADE'))
            migrations.Column('id', 'id'),
            migrations.Column('title', 'string', length=512),
            migrations.Column('text', 'text'),
            migrations.Column('date', 'datetime'),
            migrations.Column('user', 'reference users', ondelete='CASCADE'))
            migrations.Column('id', 'id'),
            migrations.Column('text', 'text'),
            migrations.Column('date', 'datetime'),
            migrations.Column('user', 'reference users', ondelete='CASCADE'),
            migrations.Column('post', 'reference posts', ondelete='CASCADE'))

    def down(self):

As you can see, every migration file contains a Migration class that has the revision attribute, used to indentify the revision number of the migration, and the revises attribute, that in our case is None. This attribute will specify the prior revision used to generate the migration: since we just created the first migration for the application, we don't have any other migration to use as a base.

The Migration class also has two methods, the up and down ones, that will specify the operations that should be run when we apply the migration and the ones that should be performed when we want to rollback the migration changes and return to the previous state. In this case, we have seven create_table operations for the up section, and the seven opposite drop_table operations in the down ones.

create_table and drop_table are directives of the Emmett migration engine. We will see that the Migration class have several methods like these that can be used to alter tables and columns in your database.

Running your fist migration

Once we generated our first migration, we need to run it in order to apply the canges to the database. We can use the up command of Emmett migrations:

$ emmett migrations up
> Performing upgrades against sqlite://dummy.db
> Performing upgrade: <base> -> 4ceb82ecd8e4 (head), First migration
> Adding revision 4ceb82ecd8e4 to schema
> Succesfully upgraded to revision 4ceb82ecd8e4: First migration

As you can see, the command prints out some information regarding the operations it runs: it says on which database the operations are performed, which migrations are used to upgrade the database and which revision is stored on the schema. In this case the migration was performed successfully, in fact we can check the current revision of the database:

$ emmett migrations status
> Current revision(s) for sqlite://dummy.db
4ceb82ecd8e4 (head)

If somehow something were wrong with the migration, the up command would notifiy it and rollback the migration, and the status command would displayed a different revision.

Running a second migration

Whenever you make changes to the application models, you need to create a migration that will modify your database accordingly. Considering the bloggy example again, if we write down another model:

class Tag(Model):
    name = Field()

and add it to our Database instance:

db.define_models(Post, Comment, Tag)

we can run the generate command again:

$ emmett migrations generate -m "Add tags"
> Generated migration for revision 4dee31071bf8

and we will find a 4dee31071bf8_add_tags.py file in the migrations folder:

"""Add tags

Migration ID: 4dee31071bf8
Revises: 4ceb82ecd8e4
Creation Date: 2016-01-24 14:43:16.963860


from emmett.orm import migrations

class Migration(migrations.Migration):
    revision = '4dee31071bf8'
    revises = '4ceb82ecd8e4'

    def up(self):
            migrations.Column('id', 'id'),
            migrations.Column('name', 'string', length=512))

    def down(self):

As you can see the engine found that the change to perform is just the one of creating the tags table. Now we can upgrade our database to this revision as we did for the first migration:

$ emmett migrations up
> Performing upgrades against sqlite://dummy.db
> Performing upgrade: 4ceb82ecd8e4 -> 4dee31071bf8 (head), Add tags
> Updating schema revision from 4ceb82ecd8e4 to 4dee31071bf8
> Succesfully upgraded to revision 4dee31071bf8: Add tags

As we increase the number of migrations for our application, the history command can be useful to check out them as an ordered list:

$ emmett migrations history
> Migrations history
4ceb82ecd8e4 -> 4dee31071bf8 (head), Add tags
<base> -> 4ceb82ecd8e4, First migration

where the most recent is on top and the last one is the oldest one.

Downgrading migrations

Whenever you need to rollback to a previous revision of your schema, you can use the down command:

$ emmett migrations down -r base
> Performing downgrades against sqlite://dummy.db
> Performing downgrade: 4ceb82ecd8e4 -> 4dee31071bf8 (head), Add tags
> Updating schema revision from 4dee31071bf8 to 4ceb82ecd8e4
> Succesfully downgraded from revision 4dee31071bf8: Add tags
> Performing downgrade: <base> -> 4ceb82ecd8e4, First migration
> Removing revision 4ceb82ecd8e4 from schema
> Succesfully downgraded from revision 4ceb82ecd8e4: First migration

The -r option is required and has to be the revision to be downgraded. Whenever you specify a revision identifier that is not the current head state, Emmett will downgrade every revision applied after the revision you specified and the one you specified too.
When this parameter is set to base, Emmett will use the first migration in history: in our case we returned the database back to the beginning. In fact, if we run the status command:

$ emmett migrations status
> Current revision(s) for sqlite://dummy.db
No revision state found on the schema.

we can see that no revision is loaded on it.

Generation and changes detection

Unfortunately, the migration generator shipped with Emmett is not able to detect all the changes you might make on the models. Here we list what the generator can detect and what it cannot, but, please, consider that this list may change on new releases.

The generator detects:

  • tables additions, removals
  • columns additions, removals
  • type, nullable and default changes on columns

While it won't detect:

  • tables renaming
  • columns renaming
  • unique constraints changes on columns

The changes unsupported by the generation should be performed using the appropriate operations when available or writing down the SQL code manually and using the db._adapter.execute() command.

Empty migrations and operations

Aside with the generated migrations, you can obviously generate new empty migration files and write down the operations on your own. In fact, if you run the new command:

$ emmett migrations new -m "Custom migration"
> Created new migration with revision 57f23e051fa3

you will find out a 57f23e051fa3_custom_migration.py file inside your migrations folder:

"""Custom migration

Migration ID: 57f23e051fa3
Revises: 4dee31071bf8
Creation Date: 2016-01-24 19:17:24.773448


from emmett.orm import migrations

class Migration(migrations.Migration):
    revision = '57f23e051fa3'
    revises = '4dee31071bf8'

    def up(self):

    def down(self):

As you can see is just like the generated migrations, excepting the fact that the up and down methods are empty. You can now write down your migration code, using the first method to declare the changes you need to perform, and the second one to define the operations needed to rollback to the current state. The next paragrahs describe the instance methods available on the Migration class in order to migrate your database structure.


The create_table method will, indeed, create a new table in your database. It accepts the name of the table as the first parameter and a list of columns:

    migrations.Column('id', 'id'),
    migrations.Column('name', 'string', length=512))

The Column class behaves quite like the Field one, except that it has the column name as first parameter. Also the types are quite different, since you will have boolean instead of bool and integer in place of int. The first column of a table should always be the one with name id and type id, since it's used by Emmett in order to perform operations on the tables.


The drop_table method is the opposite of the create_table one, since it will permanently remove a table and all its data from your database. It accepts just one parameter: the name of table to drop.



The add_column method will add an additional column to the involved table. Requires the name of the table as first parameter and a Column object as second argument.

self.add_column('tags', migrations.Column('count', 'integer', default=0))


The drop_column method will remove a column from the specified table. Needs two parameters: the name of the involved table and the name of the column to remove.

self.drop_column('tags', 'count')


The alter_column method allows you to make changes on a specific column. It needs the name of the table as first parameter, the name of the column as second parameter and then named parameters for the changes.

The supported changes are on type, default and nullable option. You can combine any of all these three options in a single alter_column command, but you should add to the command the existing values for the other options:

#: change notnull from True to False
self.alter_column('things', 'name',
#: remove default value
self.alter_column('things', 'value',
#: change type from 'string' to 'integer'
self.alter_column('things', 'count',

Just as a recap, here is the list of named arguments supported by the alter_column operation (all of them are set to None by default):

  • notnull
  • default
  • type
  • existing_notnull
  • existing_default
  • existing_type


New in version 0.7

The create_index method will, indeed, create a new index for a specific table in your database. It accepts the name of the index as first parameter, the name of the table as the second parameter and then the components and options for the index:

    'tablename_widx__indexname', 'tablename', fields=[], expressions=[], unique=False)

The fields attribute should be a list of columns to use to build the index, while the expressions one should be a list of sql strings.

Some DBMS also supports the where parameter, which should be a sql string representing the query for the conditional index.


New in version 0.7

The drop_index method is the opposite of the create_index one, since it will permanently remove an index from your database. It accepts two parameters: the name of the index to drop, and the name of the table on which the index is defined on.

self.drop_index('tablename_widx__indexname', 'tablename')

Custom operations

Sometimes you will need to access the database during the migration and run custom operation on it. This is a supported behavior, since the Migration class have a db attribute which is actually your database, and all the normal operations are supported:

def up(self):
    self.alter_column('things', 'value',
    db(db.things.value == 3).update(value=5)

As you can se we added a custom update command inside the upgrade function; and obviously you can add operations to the Emmett generated migrations too.

Using custom migration folders

New in version 1.3

When you need to manage multiple databases with migrations, or if you prefer a different folder for migrations storage, you can use the migrations_folder option in the database configuration:

from emmett.orm import Database

app.config.db1.migrations_folder = 'db1_migrations'
app.config.db2.migrations_folder = 'db2_migrations'

db1 = Database(app, app.config.db1)
db2 = Database(app, app.config.db2)

Set migration status manually

New in version 2.4

Sometimes you need to manually set the current revision schema in your database, without actually applying the involved migration(s). Some examples might be:

  • you handled migrations using a different system in the past and want to start using Emmett
  • you want to rewrite migrations, or condense several of them into a single one

In such cases, you can use the set command of the migrations engine:

$ emmett migrations status  
> Current revision(s) for sqlite://dummy.db

$ emmett migrations history               
> Migrations history:
8422706ae767 -> 69a284b840cf (head), Generated migration
9d6518b3cdc2 -> 8422706ae767, Generated migration
<base> -> 9d6518b3cdc2, First migration

$ emmett migrations set -r 69a284b840cf
> Setting revision to 69a284b840cf against sqlite://dummy.db
Do you want to continue? [y/N]: y
> Updating schema revision from 8422706ae767 to 69a284b840cf
> Succesfully set revision to 69a284b840cf: Generated migration

DBMS support

The Emmett migration engine is designed to support all the DBMS supported by pyDAL. Still, due to limitations in some engines, some operations are partially or totally not supported. In this section we will list these limitations for every DBMS.


The SQLite engine doesn't support ALTER TABLE operations that change the columns. As a direct consequence, any alter_column operation you will try to run from a migration will fail.