Migration files are composed of one or more Operations, 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, just 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.
CreateModel(name, fields, options=None, bases=None)
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 takes 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 just inheriting from the standard models.Model.
DeleteModel(name)
Deletes the model from the project history and its table from the database.
RenameModel(old_name, new_name)
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.
AlterModelTable(name, table)
Changes the model’s table name (the db_table option on the Meta subclass)
AlterUniqueTogether(name, unique_together)
Changes the model’s set of unique constraints (the unique_together option on the Meta subclass)
AlterIndexTogether(name, index_together)
Changes the model’s set of custom indexes (the index_together option on the Meta subclass)
AddField(model_name, name, field, preserve_default=True)
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 effect the behavior of setting defaults in the database directly - Django never sets database defaults, and always applies them in the Django ORM code.
RemoveField(model_name, name)
Removes a field from a model.
Bear in mind that when reversed this is actually adding a field to a model; if the field is not nullable this may make this operation irreversible (apart from any data loss, which of course is irreversible).
AlterField(model_name, name, field)
Alters a field’s definition, including changes to its type, null, unique, db_column and other field attributes.
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.
RenameField(model_name, old_name, new_name)
Changes a field’s name (and, unless db_column is set, its column name).
RunSQL(sql, reverse_sql=None, state_operations=None, multiple=False)
Allows runnning of arbitrary SQL on the database - useful for more advanced features of database backends that Django doesn’t support directly, like partial indexes.
sql, and reverse_sql if provided, should be strings of SQL to run on the database. They will be passed to the database as a single SQL statement unless multiple is set to True, in which case they will be split into separate statements manually by the operation before being passed through.
In some extreme cases, the built-in statement splitter may not be able to split correctly, in which case you should manually split the SQL into multiple calls to RunSQL.
The state_operations argument is so you can 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 (otherwise, when you next run makemigrations, it won’t see any operation that adds that field and so will try to run it again).
RunPython(code, reverse_code=None)
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.
You are advised to write the code as a separate function above the Migration class in the migration file, and just pass it to RunPython.
SeparateDatabaseAndState(database_operations=None, state_operations=None)
A highly specialized operation that let you mix and match the database (schema-changing) and state (autodetector-powering) aspects of operations.
It accepts two list of operations, and when asked to apply state will use the state list, and when asked to apply changes to the database will use the database list. Do not use this operation unless you’re very sure you know what you’re doing.
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"
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’re easy to read and 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.
Some things to note:
As a simple example, let’s make an operation that loads PostgreSQL extensions (which contain some of PostgreSQL’s more exciting features). It’s simple enough; there’s no model state changes, and 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
Sep 05, 2014