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Docs for yoyo-migrations (v8.2.0)
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README.md
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yoyo-migrations.rst

Yoyo database migrations

Source repository and issue tracker

Yoyo is a database schema migration tool. Migrations are written as SQL files or Python scripts that define a list of migration steps. They can be as simple as this:

# file: migrations/0001.create-foo.py
from yoyo import step
steps = [
   step(
       "CREATE TABLE foo (id INT, bar VARCHAR(20), PRIMARY KEY (id))",
       "DROP TABLE foo"
   )
]

Installation and project setup

Install yoyo-migrations from PyPI:

pip install yoyo-migrations

Initialize yoyo for your project, supplying a database connection string and migrations directory name, for example:

yoyo init --database sqlite:///mydb.sqlite3 migrations

This will create a new, empty directory called migrations and install a yoyo.ini configuration file in the current directory. The configuration file will contain any database credentials supplied on the command line. If you do not wish this to happen, then omit the --database argument from the command.

Create a new migration by running yoyo new. By default, a Python format file is generated, use --sql if you prefer SQL format:

yoyo new --sql

An editor will open with a template migration file. Add a comment explaining what the migration does followed by the SQL commands, for example:

-- Create table foo
-- depends:

CREATE TABLE foo (
     a int
);

Save and exit, and the new migration file will be created. Check your migration has been created with yoyo list and apply it with yoyo apply:

$ yoyo list
$ yoyo apply

Command line usage

You can see the list of available commands by running:

.. command-output:: yoyo --help

You can check options for any command with yoyo <command> --help

yoyo new

Start a new migration. yoyo new will create a new migration file and opens it your configured editor.

By default a Python formation migration will be created. To use the simpler SQL format, specify --sql.

yoyo new -m "Add column to foo"
yoyo new --sql

yoyo list

List available migrations. Each migration will be prefixed with one of U (unapplied) or A (applied).

yoyo apply

Apply migrations to the target database. By default this will prompt you for each unapplied migration. To turn off prompting use --batch or specify batch_mode = on in yoyo.ini.

yoyo rollback

By default this will prompt you for each applied migration, starting with the most recently applied.

If you wish to rollback a single migration, specify the migration with the -r/--revision flag. Note that this will also cause any migrations that depend on the selected migration to be rolled back.

yoyo reapply

Reapply (ie rollback then apply again) migrations. As with yoyo rollback, you can select a target migration with -r/--revision

yoyo develop

Apply any unapplied migrations without prompting.

If there are no unapplied migrations, rollback and reapply the most recent migration. Use yoyo develop -n <n> to act on just the n most recently applied migrations.

yoyo mark

Mark one or more migrations as applied, without actually applying them.

yoyo unmark

Unmark one or more migrations as unapplied, without actually rolling them back.

Connecting to a database

Database connections are specified using a URL, for example:

yoyo list --database postgresql://scott:tiger@localhost/mydatabase

The protocol part of the URL (the part before ://) is used to specify the backend. Yoyo provides the following core backends:

And these backends have been contributed and are bundled with yoyo:

How other parts of the URL are interpreted depends on the underlying backend and the DB-API driver used. The host part especially tends to be interpreted differently by drivers. A few of the more important differences are listed below.

MySQL connections

mysqlclient and pymysql have different ways to interpret the host part of the connection URL:

  • With mysqlclient (mysql+mysqldb://), setting the host to localhost or leaving it empty causes the driver to attempt a local unix socket connection.
  • In pymysql (mysql://), the driver will attempt a tcp connection in both cases. Specify a unix socket connection with the unix_socket option (eg ?unix_socket=/tmp/mysql.sock)

To enable SSL, specify ?ssl=1 and the following options as required:

  • sslca
  • sslcapath
  • sslcert
  • sslkey
  • sslcipher

These options correspond to the ca, capath, cert, key and cipher options used by mysql_ssl_set.

Example configurations:

# MySQL: Network database connection
database = mysql://scott:tiger@localhost/mydatabase

# MySQL: unix socket connection
database = mysql://scott:tiger@/mydatabase?unix_socket=/tmp/mysql.sock

# MySQL with the MySQLdb driver (instead of pymysql)
database = mysql+mysqldb://scott:tiger@localhost/mydatabase

# MySQL with SSL/TLS enabled
database = mysql+mysqldb://scott:tiger@localhost/mydatabase?ssl=yes&sslca=/path/to/cert

PostgreSQL connections

The psycopg family of drivers will use a unix socket if the host is left empty (or the value of PGHOST if this is set in your environment). Otherwise it will attempt a tcp connection to the specified host.

To force a unix socket connection leave the host part of the URL empty and provide a host option that points to the directory containing the socket (eg postgresql:///mydb?host=/path/to/socket/).

The postgresql backends also allow a custom schema to be selected by specifying a schema option, eg postgresql://…/mydatabase?schema=myschema.

Example configurations:

database = postgresql://scott:tiger@localhost/mydatabase

# unix socket connection
database = postgresql://scott:tiger@/mydatabase

# unix socket at a non-standard location and port number
database = postgresql://scott:tiger@/mydatabase?host=/var/run/postgresql&port=5434

# PostgreSQL with psycopg 3 driver
database = postgresql+psycopg://scott:tiger@localhost/mydatabase

# Changing the default schema
database = postgresql://scott:tiger@/mydatabase?schema=some_schema

SQLite connections

The SQLite backend ignores everything in the connection URL except the database name, which should be a filename, or the special value :memory: for an in-memory database.

3 slashes are required to specify a relative path:

sqlite:///mydb.sqlite

and 4 for an absolute path on unix-like platforms:

sqlite:////home/user/mydb.sqlite

Password security

You can specify your database username and password either as part of the database connection string on the command line (exposing your database password in the process list) or in a configuration file where other users may be able to read it.

The -p or --prompt-password flag causes yoyo to prompt for a password, helping prevent your credentials from being leaked.

Migration files

The migrations directory contains a series of migration scripts. Each migration script is a Python (.py) or SQL file (.sql).

The name of each file without the extension is used as the migration's unique identifier. You may include migrations from multiple sources, but identifiers are assumed to be globally unique, so it's wise to choose a unique prefix for you project (eg <project-name>-0001-migration.sql) or use the yoyo new command to generate a suitable filename.

Migrations scripts are run in dependency then filename order.

Each migration file is run in a single transaction where this is supported by the database.

Yoyo creates tables in your target database to track which migrations have been applied. By default these are:

  • _yoyo_migration
  • _yoyo_log
  • _yoyo_version
  • yoyo_lock

Migrations as Python scripts

A migration script written in Python has the following structure:

#
# file: migrations/0001_create_foo.py
#
from yoyo import step

__depends__ = {"0000.initial-schema"}

steps = [
  step(
      "CREATE TABLE foo (id INT, bar VARCHAR(20), PRIMARY KEY (id))",
      "DROP TABLE foo",
  ),
  step(
      "ALTER TABLE foo ADD COLUMN baz INT NOT NULL"
  )
]

The step function may take up to 3 arguments:

  • apply: an SQL query (or Python function, see below) to apply the migration step.
  • rollback: (optional) an SQL query (or Python function) to rollback the migration step.
  • ignore_errors: (optional, one of "apply", "rollback" or "all") causes yoyo to ignore database errors in either the apply stage, rollback stage or both.

Migration steps as Python functions

If SQL is not flexible enough, you may supply a Python function as either or both of the apply or rollback arguments of step. Each function should take a database connection as its only argument:

#
# file: migrations/0001_create_foo.py
#
from yoyo import step

def apply_step(conn):
    cursor = conn.cursor()
    cursor.execute(
        # query to perform the migration
    )

def rollback_step(conn):
    cursor = conn.cursor()
    cursor.execute(
        # query to undo the above
    )

steps = [
  step(apply_step, rollback_step)
]

Dependencies

Migrations may declare dependencies on other migrations via the __depends__ attribute:

#
# file: migrations/0002.modify-foo.py
#
__depends__ = {'0000.initial-schema', '0001.create-foo'}

steps = [
  # migration steps
]

If you use the yoyo new command the __depends__ attribute will be auto populated for you.

Migrations as SQL scripts

An SQL migration script files should be named <migration-name>.sql and contain the one or more SQL statements required to apply the migration.

--
-- file: migrations/0001.create-foo.sql
--
CREATE TABLE foo (id INT, bar VARCHAR(20), PRIMARY KEY (id));

SQL rollback steps should be saved in a separate file named <migration-name>.rollback.sql:

--
-- file: migrations/0001.create-foo.rollback.sql
--
DROP TABLE foo;

Dependencies

A structured SQL comment may be used to specify dependencies as a space separated list:

-- depends: 0000.initial-schema 0001.create-foo

ALTER TABLE foo ADD baz INT;

Post-apply hook

It can be useful to have a script that is run after every successful migration. For example you could use this to update database permissions or re-create views.

To do this, create a special migration file called post-apply.py or post-apply.sql. This file should have the same format as any other migration file.

Configuration file

Yoyo looks for a configuration file named yoyo.ini in the current working directory or any ancestor directory.

If no configuration file is found yoyo will prompt you to create one, populated from the current command line arguments.

Using a configuration file saves repeated typing, avoids your database username and password showing in process listings and lessens the risk of accidentally running migrations against the wrong database (ie by re-running an earlier yoyo entry in your command history when you have moved to a different directory).

If you do not want a config file to be loaded add the --no-config-file parameter to the command line options.

The configuration file may contain the following options:

[DEFAULT]

# List of migration source directories. "%(here)s" is expanded to the
# full path of the directory containing this ini file.
sources = %(here)s/migrations %(here)s/lib/module/migrations

# Target database
database = postgresql://scott:tiger@localhost/mydb

# Verbosity level. Goes from 0 (least verbose) to 3 (most verbose)
verbosity = 3

# Disable interactive features
batch_mode = on

# Editor to use when starting new migrations
# "{}" is expanded to the filename of the new migration
editor = /usr/local/bin/vim -f {}

# An arbitrary command to run after a migration has been created
# "{}" is expanded to the filename of the new migration
post_create_command = hg add {}

# A prefix to use for generated migration filenames
prefix = myproject_

Config file inheritance and includes

The special %inherit and %include directives allow config file inheritance and inclusion:

#
# file: yoyo-defaults.ini
#
[DEFAULT]
sources = %(here)s/migrations

#
# file: yoyo.ini
#
[DEFAULT]

; Inherit settings from yoyo-defaults.ini
;
; Settings in inherited files are processed first and may be overridden by
; settings in this file
%inherit = yoyo-defaults.ini

; Include settings from yoyo-local.ini
;
; Included files are processed after this file and may override the settings
; in this file
%include = yoyo-local.ini


; Use '?' to avoid raising an error if the file does not exist
%inherit = ?yoyo-defaults.ini

database = sqlite:///%(here)s/mydb.sqlite

Substitutions and environment variables

The special variable %(here)s will be substituted with the directory name of the config file.

Environment variables can be substituted with the same syntax, eg %(HOME)s.

Substitutions are case-insensitive so for example %(HOME)s and %(home)s will both refer to the same variable.

Migration sources

Yoyo reads migration scripts from the directories specified in the sources config option. Paths may include glob patterns, for example:

[DEFAULT]
sources =
    %(here)s/migrations
    %(here)s/src/*/migrations

You may also read migrations from installed python packages, by supplying a path in the special form package:<package-name>:<path-to-migrations-dir>, for example:

[DEFAULT]
sources = package:myapplication:data/migrations

Transactions

Each migration runs in a separate transaction. Savepoints are used to isolate steps within each migration.

If an error occurs during a step and the step has ignore_errors set, then that individual step will be rolled back and execution will pick up from the next step. If ignore_errors is not set then the entire migration will be rolled back and execution stopped.

Note that some databases (eg MySQL) do not support rollback on DDL statements (eg CREATE ... and ALTER ... statements). For these databases you may need to manually intervene to reset the database state should errors occur in your migration.

Using group allows you to nest steps, giving you control of where rollbacks happen. For example:

group([
  step("ALTER TABLE employees ADD tax_code TEXT"),
  step("CREATE INDEX tax_code_idx ON employees (tax_code)")
], ignore_errors='all')
step("UPDATE employees SET tax_code='C' WHERE pay_grade < 4")
step("UPDATE employees SET tax_code='B' WHERE pay_grade >= 6")
step("UPDATE employees SET tax_code='A' WHERE pay_grade >= 8")

Disabling transactions

Disable transaction handling within a migration by setting __transactional__ = False, eg:

__transactional__ = False

step("CREATE DATABASE mydb", "DROP DATABASE mydb")

Or for SQL migrations:

-- transactional: false

CREATE DATABASE mydb

This feature is only tested against the PostgreSQL and SQLite backends.

PostgreSQL

In PostgreSQL it is an error to run certain statements inside a transaction block. These include:

CREATE DATABASE ...
ALTER TYPE ... ADD VALUE

Using __transactional__ = False allows you to run these within a migration

SQLite

In SQLite, the default transactional behavior may prevent other tools from accessing the database for the duration of the migration. Using __transactional__ = False allows you to work around this limitation.

Calling Yoyo from Python code

The following example shows how to apply migrations from inside python code:

from yoyo import read_migrations
from yoyo import get_backend

backend = get_backend('postgresql://myuser@localhost/mydatabase')
migrations = read_migrations('path/to/migrations')

with backend.lock():

    # Apply any outstanding migrations
    backend.apply_migrations(backend.to_apply(migrations))

    # Rollback all migrations
    backend.rollback_migrations(backend.to_rollback(migrations))
.. toctree::
   :maxdepth: 2
   :caption: Contents:

Adding custom backends

Backends are discovered using Python importlib.metadata entry points.

To add a custom backend, create a python package containing a subclass of :class:`yoyo.backends.base.DatabaseBackend` and configure it in the package metadata (typically in setup.cfg), for example:

[options.entry_points]

yoyo.backends =
    mybackend = mypackage:MyBackend

Use the backend by specifying 'mybackend' as the driver protocol:

.. code:: sh

 yoyo apply --database my_backend://...

Contributing

Report an issue

Use the yoyo-migrations issue tracker to report issues.

There is also a mailing list where you can post questions or suggestions.

Pull requests

Yoyo-migrations is developed on sourcehut and uses a mailing list to review commits for inclusion into the project.

To send commits to the mailing list:

  1. Clone the repository: hg clone https://hg.sr.ht/~olly/yoyo
  2. Take care to commit your work in logically separate changes. Use hg commit -i to commit your work in logically separate changes. Make sure each commit has a meaningful message.
  3. When you are ready to send your commits, use hg config --edit to add the following lines to your user Mercurial configuration file:
[extensions]
patchbomb =

[email]
from = Your Name <[email protected]>
method = smtp

[smtp]
host = mail.example.org
port = 587
tls = smtps
username = [email protected]

Then use hg config --local to add the following lines to the repository configuration file:

[email]
to = <~olly/[email protected]>
  1. Run hg mail -o to send your commits by email. This command will send all your commits; if you want to send just a subset, refer to the hg email docs.

For more detailed instructions, see here: https://man.sr.ht/hg.sr.ht/email.md

Mailing list

The mailing list archives can be found here: https://lists.sr.ht/~olly/yoyo.

Changelog

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