Migration Operations¶
Migration files are composed of one or more Operation
s, objects that
declaratively record what the migration should do to your database.
Django also uses these Operation
objects to work out what your models
looked like historically, and to calculate what changes you’ve made to
your models since the last migration so it can automatically write
your migrations; that’s why they’re declarative, as it means Django can
easily load them all into memory and run through them without touching
the database to work out what your project should look like.
There are also more specialized Operation
objects which are for things like
data migrations and for advanced manual database
manipulation. You can also write your own Operation
classes if you want
to encapsulate a custom change you commonly make.
If you need an empty migration file to write your own Operation
objects
into, use python manage.py makemigrations --empty yourappname
, but be aware
that manually adding schema-altering operations can confuse the migration
autodetector and make resulting runs of makemigrations
output
incorrect code.
All of the core Django operations are available from the
django.db.migrations.operations
module.
For introductory material, see the migrations topic guide.
Schema Operations¶
CreateModel
¶
Creates a new model in the project history and a corresponding table in the database to match it.
name
is the model name, as would be written in the models.py
file.
fields
is a list of 2-tuples of (field_name, field_instance)
.
The field instance should be an unbound field (so just
models.CharField(...)
, rather than a field taken from another model).
options
is an optional dictionary of values from the model’s Meta
class.
bases
is an optional list of other classes to have this model inherit from;
it can contain both class objects as well as strings in the format
"appname.ModelName"
if you want to depend on another model (so you inherit
from the historical version). If it’s not supplied, it defaults to inheriting
from the standard models.Model
.
managers
takes a list of 2-tuples of (manager_name, manager_instance)
.
The first manager in the list will be the default manager for this model during
migrations.
DeleteModel
¶
Deletes the model from the project history and its table from the database.
RenameModel
¶
Renames the model from an old name to a new one.
You may have to manually add this if you change the model’s name and quite a few of its fields at once; to the autodetector, this will look like you deleted a model with the old name and added a new one with a different name, and the migration it creates will lose any data in the old table.
AlterUniqueTogether
¶
Changes the model’s set of unique constraints (the
unique_together
option on the Meta
subclass).
AlterIndexTogether
¶
Changes the model’s set of custom indexes (the
index_together
option on the Meta
subclass).
AlterOrderWithRespectTo
¶
Makes or deletes the _order
column needed for the
order_with_respect_to
option on the Meta
subclass.
AlterModelOptions
¶
Stores changes to miscellaneous model options (settings on a model’s Meta
)
like permissions
and verbose_name
. Does not affect the database, but
persists these changes for RunPython
instances to use. options
should be a dictionary mapping option names to values.
AlterModelManagers
¶
Alters the managers that are available during migrations.
AddField
¶
Adds a field to a model. model_name
is the model’s name, name
is
the field’s name, and field
is an unbound Field instance (the thing
you would put in the field declaration in models.py
- for example,
models.IntegerField(null=True)
.
The preserve_default
argument indicates whether the field’s default
value is permanent and should be baked into the project state (True
),
or if it is temporary and just for this migration (False
) - usually
because the migration is adding a non-nullable field to a table and needs
a default value to put into existing rows. It does not affect the behavior
of setting defaults in the database directly - Django never sets database
defaults and always applies them in the Django ORM code.
Warning
On older databases, adding a field with a default value may cause a full rewrite of the table. This happens even for nullable fields and may have a negative performance impact. To avoid that, the following steps should be taken.
- Add the nullable field without the default value and run the
makemigrations
command. This should generate a migration with anAddField
operation. - Add the default value to your field and run the
makemigrations
command. This should generate a migration with anAlterField
operation.
RemoveField
¶
Removes a field from a model.
Bear in mind that when reversed, this is actually adding a field to a model. The operation is reversible (apart from any data loss, which is irreversible) if the field is nullable or if it has a default value that can be used to populate the recreated column. If the field is not nullable and does not have a default value, the operation is irreversible.
AlterField
¶
Alters a field’s definition, including changes to its type,
null
, unique
,
db_column
and other field attributes.
The preserve_default
argument indicates whether the field’s default
value is permanent and should be baked into the project state (True
),
or if it is temporary and just for this migration (False
) - usually
because the migration is altering a nullable field to a non-nullable one and
needs a default value to put into existing rows. It does not affect the
behavior of setting defaults in the database directly - Django never sets
database defaults and always applies them in the Django ORM code.
Note that not all changes are possible on all databases - for example, you
cannot change a text-type field like models.TextField()
into a number-type
field like models.IntegerField()
on most databases.
AddIndex
¶
Creates an index in the database table for the model with model_name
.
index
is an instance of the Index
class.
RemoveIndex
¶
Removes the index named name
from the model with model_name
.
RenameIndex
¶
-
class
RenameIndex
(model_name, new_name, old_name=None, old_fields=None)¶
Renames an index in the database table for the model with model_name
.
Exactly one of old_name
and old_fields
can be provided. old_fields
is an iterable of the strings, often corresponding to fields of
index_together
.
On databases that don’t support an index renaming statement (SQLite and MariaDB < 10.5.2), the operation will drop and recreate the index, which can be expensive.
AddConstraint
¶
Creates a constraint in the database table for
the model with model_name
.
Special Operations¶
RunSQL
¶
Allows running of arbitrary SQL on the database - useful for more advanced features of database backends that Django doesn’t support directly.
sql
, and reverse_sql
if provided, should be strings of SQL to run on
the database. On most database backends (all but PostgreSQL), Django will
split the SQL into individual statements prior to executing them.
Warning
On PostgreSQL and SQLite, only use BEGIN
or COMMIT
in your SQL in
non-atomic migrations, to avoid breaking
Django’s transaction state.
You can also pass a list of strings or 2-tuples. The latter is used for passing queries and parameters in the same way as cursor.execute(). These three operations are equivalent:
migrations.RunSQL("INSERT INTO musician (name) VALUES ('Reinhardt');")
migrations.RunSQL([("INSERT INTO musician (name) VALUES ('Reinhardt');", None)])
migrations.RunSQL([("INSERT INTO musician (name) VALUES (%s);", ['Reinhardt'])])
If you want to include literal percent signs in the query, you have to double them if you are passing parameters.
The reverse_sql
queries are executed when the migration is unapplied. They
should undo what is done by the sql
queries. For example, to undo the above
insertion with a deletion:
migrations.RunSQL(
sql=[("INSERT INTO musician (name) VALUES (%s);", ['Reinhardt'])],
reverse_sql=[("DELETE FROM musician where name=%s;", ['Reinhardt'])],
)
If reverse_sql
is None
(the default), the RunSQL
operation is
irreversible.
The state_operations
argument allows you to supply operations that are
equivalent to the SQL in terms of project state. For example, if you are
manually creating a column, you should pass in a list containing an AddField
operation here so that the autodetector still has an up-to-date state of the
model. If you don’t, when you next run makemigrations
, it won’t see any
operation that adds that field and so will try to run it again. For example:
migrations.RunSQL(
"ALTER TABLE musician ADD COLUMN name varchar(255) NOT NULL;",
state_operations=[
migrations.AddField(
'musician',
'name',
models.CharField(max_length=255),
),
],
)
The optional hints
argument will be passed as **hints
to the
allow_migrate()
method of database routers to assist them in making
routing decisions. See Hints for more details on
database hints.
The optional elidable
argument determines whether or not the operation will
be removed (elided) when squashing migrations.
-
RunSQL.
noop
¶ Pass the
RunSQL.noop
attribute tosql
orreverse_sql
when you want the operation not to do anything in the given direction. This is especially useful in making the operation reversible.
RunPython
¶
Runs custom Python code in a historical context. code
(and reverse_code
if supplied) should be callable objects that accept two arguments; the first is
an instance of django.apps.registry.Apps
containing historical models that
match the operation’s place in the project history, and the second is an
instance of SchemaEditor
.
The reverse_code
argument is called when unapplying migrations. This
callable should undo what is done in the code
callable so that the
migration is reversible. If reverse_code
is None
(the default), the
RunPython
operation is irreversible.
The optional hints
argument will be passed as **hints
to the
allow_migrate()
method of database routers to assist them in making a
routing decision. See Hints for more details on
database hints.
The optional elidable
argument determines whether or not the operation will
be removed (elided) when squashing migrations.
You are advised to write the code as a separate function above the Migration
class in the migration file, and pass it to RunPython
. Here’s an example of
using RunPython
to create some initial objects on a Country
model:
from django.db import migrations
def forwards_func(apps, schema_editor):
# We get the model from the versioned app registry;
# if we directly import it, it'll be the wrong version
Country = apps.get_model("myapp", "Country")
db_alias = schema_editor.connection.alias
Country.objects.using(db_alias).bulk_create([
Country(name="USA", code="us"),
Country(name="France", code="fr"),
])
def reverse_func(apps, schema_editor):
# forwards_func() creates two Country instances,
# so reverse_func() should delete them.
Country = apps.get_model("myapp", "Country")
db_alias = schema_editor.connection.alias
Country.objects.using(db_alias).filter(name="USA", code="us").delete()
Country.objects.using(db_alias).filter(name="France", code="fr").delete()
class Migration(migrations.Migration):
dependencies = []
operations = [
migrations.RunPython(forwards_func, reverse_func),
]
This is generally the operation you would use to create data migrations, run custom data updates and alterations, and anything else you need access to an ORM and/or Python code for.
Much like RunSQL
, ensure that if you change schema inside here you’re
either doing it outside the scope of the Django model system (e.g. triggers)
or that you use SeparateDatabaseAndState
to add in operations that will
reflect your changes to the model state - otherwise, the versioned ORM and
the autodetector will stop working correctly.
By default, RunPython
will run its contents inside a transaction on
databases that do not support DDL transactions (for example, MySQL and
Oracle). This should be safe, but may cause a crash if you attempt to use
the schema_editor
provided on these backends; in this case, pass
atomic=False
to the RunPython
operation.
On databases that do support DDL transactions (SQLite and PostgreSQL),
RunPython
operations do not have any transactions automatically added
besides the transactions created for each migration. Thus, on PostgreSQL, for
example, you should avoid combining schema changes and RunPython
operations
in the same migration or you may hit errors like OperationalError: cannot
ALTER TABLE "mytable" because it has pending trigger events
.
If you have a different database and aren’t sure if it supports DDL
transactions, check the django.db.connection.features.can_rollback_ddl
attribute.
If the RunPython
operation is part of a non-atomic migration, the operation will only be executed in a transaction
if atomic=True
is passed to the RunPython
operation.
Warning
RunPython
does not magically alter the connection of the models for you;
any model methods you call will go to the default database unless you
give them the current database alias (available from
schema_editor.connection.alias
, where schema_editor
is the second
argument to your function).
SeparateDatabaseAndState
¶
A highly specialized operation that lets you mix and match the database (schema-changing) and state (autodetector-powering) aspects of operations.
It accepts two lists of operations. When asked to apply state, it will use the
state_operations
list (this is a generalized version of RunSQL
’s
state_operations
argument). When asked to apply changes to the database, it
will use the database_operations
list.
If the actual state of the database and Django’s view of the state get out of
sync, this can break the migration framework, even leading to data loss. It’s
worth exercising caution and checking your database and state operations
carefully. You can use sqlmigrate
and dbshell
to check
your database operations. You can use makemigrations
, especially
with --dry-run
, to check your state
operations.
For an example using SeparateDatabaseAndState
, see
Changing a ManyToManyField to use a through model.
Writing your own¶
Operations have a relatively simple API, and they’re designed so that you can
easily write your own to supplement the built-in Django ones. The basic
structure of an Operation
looks like this:
from django.db.migrations.operations.base import Operation
class MyCustomOperation(Operation):
# If this is False, it means that this operation will be ignored by
# sqlmigrate; if true, it will be run and the SQL collected for its output.
reduces_to_sql = False
# If this is False, Django will refuse to reverse past this operation.
reversible = False
def __init__(self, arg1, arg2):
# Operations are usually instantiated with arguments in migration
# files. Store the values of them on self for later use.
pass
def state_forwards(self, app_label, state):
# The Operation should take the 'state' parameter (an instance of
# django.db.migrations.state.ProjectState) and mutate it to match
# any schema changes that have occurred.
pass
def database_forwards(self, app_label, schema_editor, from_state, to_state):
# The Operation should use schema_editor to apply any changes it
# wants to make to the database.
pass
def database_backwards(self, app_label, schema_editor, from_state, to_state):
# If reversible is True, this is called when the operation is reversed.
pass
def describe(self):
# This is used to describe what the operation does in console output.
return "Custom Operation"
@property
def migration_name_fragment(self):
# Optional. A filename part suitable for automatically naming a
# migration containing this operation, or None if not applicable.
return "custom_operation_%s_%s" % (self.arg1, self.arg2)
You can take this template and work from it, though we suggest looking at the
built-in Django operations in django.db.migrations.operations
- they cover
a lot of the example usage of semi-internal aspects of the migration framework
like ProjectState
and the patterns used to get historical models, as well
as ModelState
and the patterns used to mutate historical models in
state_forwards()
.
Some things to note:
You don’t need to learn too much about
ProjectState
to write migrations; just know that it has anapps
property that gives access to an app registry (which you can then callget_model
on).database_forwards
anddatabase_backwards
both get two states passed to them; these represent the difference thestate_forwards
method would have applied, but are given to you for convenience and speed reasons.If you want to work with model classes or model instances from the
from_state
argument indatabase_forwards()
ordatabase_backwards()
, you must render model states using theclear_delayed_apps_cache()
method to make related models available:def database_forwards(self, app_label, schema_editor, from_state, to_state): # This operation should have access to all models. Ensure that all models are # reloaded in case any are delayed. from_state.clear_delayed_apps_cache() ...
to_state
in the database_backwards method is the older state; that is, the one that will be the current state once the migration has finished reversing.You might see implementations of
references_model
on the built-in operations; this is part of the autodetection code and does not matter for custom operations.
Warning
For performance reasons, the Field
instances in
ModelState.fields
are reused across migrations. You must never change
the attributes on these instances. If you need to mutate a field in
state_forwards()
, you must remove the old instance from
ModelState.fields
and add a new instance in its place. The same is true
for the Manager
instances in
ModelState.managers
.
As an example, let’s make an operation that loads PostgreSQL extensions (which contain some of PostgreSQL’s more exciting features). Since there’s no model state changes, all it does is run one command:
from django.db.migrations.operations.base import Operation
class LoadExtension(Operation):
reversible = True
def __init__(self, name):
self.name = name
def state_forwards(self, app_label, state):
pass
def database_forwards(self, app_label, schema_editor, from_state, to_state):
schema_editor.execute("CREATE EXTENSION IF NOT EXISTS %s" % self.name)
def database_backwards(self, app_label, schema_editor, from_state, to_state):
schema_editor.execute("DROP EXTENSION %s" % self.name)
def describe(self):
return "Creates extension %s" % self.name
@property
def migration_name_fragment(self):
return "create_extension_%s" % self.name