Django2.0手册:Migration Operations



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.

For introductory material, see the migrations topic guide.

Schema Operations¶

CreateModel¶

class CreateModel(name, fields, options=None, bases=None, managers=None)[source]

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 just
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¶

class DeleteModel(name)[source]

Deletes the model from the project history and its table from the database.

RenameModel¶

class RenameModel(old_name, new_name)[source]

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¶

class AlterModelTable(name, table)[source]

Changes the model’s table name (the db_table
option on the Meta subclass).

AlterUniqueTogether¶

class AlterUniqueTogether(name, unique_together)[source]

Changes the model’s set of unique constraints (the
unique_together option on the Meta
subclass).

AlterIndexTogether¶

class AlterIndexTogether(name, index_together)[source]

Changes the model’s set of custom indexes (the
index_together option on the Meta
subclass).

AlterOrderWithRespectTo¶

class AlterOrderWithRespectTo(name, order_with_respect_to)[source]

Makes or deletes the _order column needed for the
order_with_respect_to option on the Meta
subclass.

AlterModelOptions¶

class AlterModelOptions(name, options)[source]

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¶

class AlterModelManagers(name, managers)[source]

Alters the managers that are available during migrations.

AddField¶

class AddField(model_name, name, field, preserve_default=True)[source]

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.

RemoveField¶

class RemoveField(model_name, name)[source]

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 of course 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¶

class AlterField(model_name, name, field, preserve_default=True)[source]

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.

RenameField¶

class RenameField(model_name, old_name, new_name)[source]

Changes a field’s name (and, unless db_column
is set, its column name).

AddIndex¶

class AddIndex(model_name, index)[source]
New in Django 1.11.

Creates an index in the database table for the model with model_name.
index is an instance of the Index class.

RemoveIndex¶

class RemoveIndex(model_name, name)[source]
New in Django 1.11.

Removes the index named name from the model with model_name.

Special Operations¶

RunSQL¶

class RunSQL(sql, reverse_sql=None, state_operations=None, hints=None, elidable=False)[source]

Allows running 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. On most database backends (all but PostgreSQL), Django will
split the SQL into individual statements prior to executing them. This
requires installing the sqlparse Python library.

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, so
you can reverse the changes done in the forwards queries:

migrations.RunSQL(
    [("INSERT INTO musician (name) VALUES (%s);", ['Reinhardt'])],
    [("DELETE FROM musician where name=%s;", ['Reinhardt'])],
)

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). 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 to sql or reverse_sql when you
want the operation not to do anything in the given direction. This is
especially useful in making the operation reversible.

RunPython¶

class RunPython(code, reverse_code=None, atomic=None, hints=None, elidable=False)[source]

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.

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 just 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.

If you’re upgrading from South, this is basically the South pattern as an
operation – one or two methods for forwards and backwards, with an ORM and
schema operations available. Most of the time, you should be able to translate
the orm.Model or orm["appname", "Model"] references from South directly
into apps.get_model("appname", "Model") references here and leave most of
the rest of the code unchanged for data migrations. However, apps will only
have references to models in the current app unless migrations in other apps
are added to the migration’s dependencies.

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).

static RunPython.noop()[source]

Pass the RunPython.noop method to code or reverse_code when
you want the operation not to do anything in the given direction. This is
especially useful in making the operation reversible.

SeparateDatabaseAndState¶

class SeparateDatabaseAndState(database_operations=None, state_operations=None)[source]

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.

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"

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, 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 just write simple
    migrations; just know that it has an apps property that gives access to
    an app registry (which you can then call get_model on).

  • database_forwards and database_backwards both get two states passed
    to them; these just represent the difference the state_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 in database_forwards() or
    database_backwards(), you must render model states using the
    clear_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()
        ...
    
    New in Django 1.11:

    This requirement and the clear_delayed_apps_cache() method is new.

  • 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 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