Model Meta options

This document explains all the possible metadata options that you can give your model in its internal class Meta.

Available Meta options

abstract

Options.``abstract

If abstract = True, this model will be an abstract base class.

app_label

Options.``app_label

If a model is defined outside of an application in INSTALLED_APPS, it must declare which app it belongs to:

app_label = 'myapp'

If you want to represent a model with the format app_label.object_name or app_label.model_name you can use model._meta.label or model._meta.label_lower respectively.

base_manager_name

Options.``base_manager_name

The attribute name of the manager, for example, 'objects', to use for the model’s _base_manager.

db_table

Options.``db_table

The name of the database table to use for the model:

db_table = 'music_album'

Table names

To save you time, Django automatically derives the name of the database table from the name of your model class and the app that contains it. A model’s database table name is constructed by joining the model’s “app label” — the name you used in manage.py startapp — to the model’s class name, with an underscore between them.

For example, if you have an app bookstore (as created by manage.py startapp bookstore), a model defined as class Book will have a database table named bookstore_book.

To override the database table name, use the db_table parameter in class Meta.

If your database table name is an SQL reserved word, or contains characters that aren’t allowed in Python variable names — notably, the hyphen — that’s OK. Django quotes column and table names behind the scenes.

Use lowercase table names for MariaDB and MySQL

It is strongly advised that you use lowercase table names when you override the table name via db_table, particularly if you are using the MySQL backend. See the MySQL notes for more details.

Table name quoting for Oracle

In order to meet the 30-char limitation Oracle has on table names, and match the usual conventions for Oracle databases, Django may shorten table names and turn them all-uppercase. To prevent such transformations, use a quoted name as the value for db_table:

db_table = '"name_left_in_lowercase"'

Such quoted names can also be used with Django’s other supported database backends; except for Oracle, however, the quotes have no effect. See the Oracle notes for more details.

db_tablespace

Options.``db_tablespace

The name of the database tablespace to use for this model. The default is the project’s DEFAULT_TABLESPACE setting, if set. If the backend doesn’t support tablespaces, this option is ignored.

default_manager_name

Options.``default_manager_name

The name of the manager to use for the model’s _default_manager.

default_related_name

Options.``default_related_name

The name that will be used by default for the relation from a related object back to this one. The default is <model_name>_set.

This option also sets related_query_name.

As the reverse name for a field should be unique, be careful if you intend to subclass your model. To work around name collisions, part of the name should contain '%(app_label)s' and '%(model_name)s', which are replaced respectively by the name of the application the model is in, and the name of the model, both lowercased. See the paragraph on related names for abstract models.

get_latest_by

Options.``get_latest_by

The name of a field or a list of field names in the model, typically DateField, DateTimeField, or IntegerField. This specifies the default field(s) to use in your model Manager’s latest() and earliest() methods.

举例:

# Latest by ascending order_date.
get_latest_by = "order_date"
# Latest by priority descending, order_date ascending.
get_latest_by = ['-priority', 'order_date']

See the latest() docs for more.

managed

Options.``managed

Defaults to True, meaning Django will create the appropriate database tables in migrate or as part of migrations and remove them as part of a flush management command. That is, Django manages the database tables’ lifecycles.

If False, no database table creation, modification, or deletion operations will be performed for this model. This is useful if the model represents an existing table or a database view that has been created by some other means. This is the only difference when managed=False. All other aspects of model handling are exactly the same as normal. This includes

1. Adding an automatic primary key field to the model if you don’t declare it. To avoid confusion for later code readers, it’s recommended to specify all the columns from the database table you are modeling when using unmanaged models.

2. If a model with managed=False contains a ManyToManyField that points to another unmanaged model, then the intermediate table for the many-to-many join will also not be created. However, the intermediary table between one managed and one unmanaged model will be created.

   If you need to change this default behavior, create the intermediary table as an explicit model (with managed set as needed) and use the ManyToManyField.through attribute to make the relation use your custom model.

For tests involving models with managed=False, it’s up to you to ensure the correct tables are created as part of the test setup.

If you’re interested in changing the Python-level behavior of a model class, you could use managed=False and create a copy of an existing model. However, there’s a better approach for that situation: 代理模型.

order_with_respect_to

Options.``order_with_respect_to

Makes this object orderable with respect to the given field, usually a ForeignKey. This can be used to make related objects orderable with respect to a parent object. For example, if an Answer relates to a Question object, and a question has more than one answer, and the order of answers matters, you’d do this:

from django.db import models
class Question(models.Model):
    text = models.TextField()
    # ...
class Answer(models.Model):
    question = models.ForeignKey(Question, on_delete=models.CASCADE)
    # ...
    class Meta:
        order_with_respect_to = 'question'

When order_with_respect_to is set, two additional methods are provided to retrieve and to set the order of the related objects: get_RELATED_order() and set_RELATED_order(), where RELATED is the lowercased model name. For example, assuming that a Question object has multiple related Answer objects, the list returned contains the primary keys of the related Answer objects:

>>> question = Question.objects.get(id=1)
>>> question.get_answer_order()
[1, 2, 3]

The order of a Question object’s related Answer objects can be set by passing in a list of Answer primary keys:

>>> question.set_answer_order([3, 1, 2])

The related objects also get two methods, get_next_in_order() and get_previous_in_order(), which can be used to access those objects in their proper order. Assuming the Answer objects are ordered by id:

>>> answer = Answer.objects.get(id=2)
>>> answer.get_next_in_order()
<Answer: 3>
>>> answer.get_previous_in_order()
<Answer: 1>

order_with_respect_to implicitly sets the ordering option

Internally, order_with_respect_to adds an additional field/database column named _order and sets the model’s ordering option to this field. Consequently, order_with_respect_to and ordering cannot be used together, and the ordering added by order_with_respect_to will apply whenever you obtain a list of objects of this model.

Changing order_with_respect_to

Because order_with_respect_to adds a new database column, be sure to make and apply the appropriate migrations if you add or change order_with_respect_to after your initial migrate.

ordering

Options.``ordering

The default ordering for the object, for use when obtaining lists of objects:

ordering = ['-order_date']

This is a tuple or list of strings and/or query expressions. Each string is a field name with an optional “-“ prefix, which indicates descending order. Fields without a leading “-“ will be ordered ascending. Use the string “?” to order randomly.

For example, to order by a pub_date field ascending, use this:

ordering = ['pub_date']

To order by pub_date descending, use this:

ordering = ['-pub_date']

To order by pub_date descending, then by author ascending, use this:

ordering = ['-pub_date', 'author']

You can also use query expressions. To order by author ascending and make null values sort last, use this:

from django.db.models import F
ordering = [F('author').asc(nulls_last=True)]

Default ordering also affects aggregation queries but this won’t be the case starting in Django 3.1.

警告

Ordering is not a free operation. Each field you add to the ordering incurs a cost to your database. Each foreign key you add will implicitly include all of its default orderings as well.

If a query doesn’t have an ordering specified, results are returned from the database in an unspecified order. A particular ordering is guaranteed only when ordering by a set of fields that uniquely identify each object in the results. For example, if a name field isn’t unique, ordering by it won’t guarantee objects with the same name always appear in the same order.

permissions

Options.``permissions

Extra permissions to enter into the permissions table when creating this object. Add, change, delete, and view permissions are automatically created for each model. This example specifies an extra permission, can_deliver_pizzas:

permissions = [('can_deliver_pizzas', 'Can deliver pizzas')]

This is a list or tuple of 2-tuples in the format (permission_code, human_readable_permission_name).

default_permissions

Options.``default_permissions

Defaults to ('add', 'change', 'delete', 'view'). You may customize this list, for example, by setting this to an empty list if your app doesn’t require any of the default permissions. It must be specified on the model before the model is created by migrate in order to prevent any omitted permissions from being created.

proxy

Options.``proxy

If proxy = True, a model which subclasses another model will be treated as a proxy model.

required_db_features

Options.``required_db_features

List of database features that the current connection should have so that the model is considered during the migration phase. For example, if you set this list to ['gis_enabled'], the model will only be synchronized on GIS-enabled databases. It’s also useful to skip some models when testing with several database backends. Avoid relations between models that may or may not be created as the ORM doesn’t handle this.

required_db_vendor

Options.``required_db_vendor

Name of a supported database vendor that this model is specific to. Current built-in vendor names are: sqlite, postgresql, mysql, oracle. If this attribute is not empty and the current connection vendor doesn’t match it, the model will not be synchronized.

select_on_save

Options.``select_on_save

Determines if Django will use the pre-1.6 django.db.models.Model.save() algorithm. The old algorithm uses SELECT to determine if there is an existing row to be updated. The new algorithm tries an UPDATE directly. In some rare cases the UPDATE of an existing row isn’t visible to Django. An example is the PostgreSQL ON UPDATE trigger which returns NULL. In such cases the new algorithm will end up doing an INSERT even when a row exists in the database.

Usually there is no need to set this attribute. The default is False.

See django.db.models.Model.save() for more about the old and new saving algorithm.

indexes

Options.``indexes

A list of indexes that you want to define on the model:

from django.db import models
class Customer(models.Model):
    first_name = models.CharField(max_length=100)
    last_name = models.CharField(max_length=100)
    class Meta:
        indexes = [
            models.Index(fields=['last_name', 'first_name']),
            models.Index(fields=['first_name'], name='first_name_idx'),
        ]

unique_together

Options.``unique_together

Use UniqueConstraint with the constraints option instead.

UniqueConstraint provides more functionality than unique_together. unique_together may be deprecated in the future.

Sets of field names that, taken together, must be unique:

unique_together = [['driver', 'restaurant']]

This is a list of lists that must be unique when considered together. It’s used in the Django admin and is enforced at the database level (i.e., the appropriate UNIQUE statements are included in the CREATE TABLE statement).

For convenience, unique_together can be a single list when dealing with a single set of fields:

unique_together = ['driver', 'restaurant']

A ManyToManyField cannot be included in unique_together. (It’s not clear what that would even mean!) If you need to validate uniqueness related to a ManyToManyField, try using a signal or an explicit through model.

The ValidationError raised during model validation when the constraint is violated has the unique_together error code.

index_together

Options.``index_together

Use the indexes option instead.

The newer indexes option provides more functionality than index_together. index_together may be deprecated in the future.

Sets of field names that, taken together, are indexed:

index_together = [
    ["pub_date", "deadline"],
]

This list of fields will be indexed together (i.e. the appropriate CREATE INDEX statement will be issued.)

For convenience, index_together can be a single list when dealing with a single set of fields:

index_together = ["pub_date", "deadline"]

constraints

Options.``constraints

New in Django 2.2.

A list of constraints that you want to define on the model:

from django.db import models
class Customer(models.Model):
    age = models.IntegerField()
    class Meta:
        constraints = [
            models.CheckConstraint(check=models.Q(age__gte=18), name='age_gte_18'),
        ]

verbose_name

Options.``verbose_name

A human-readable name for the object, singular:

verbose_name = "pizza"

If this isn’t given, Django will use a munged version of the class name: CamelCase becomes camel case.

verbose_name_plural

Options.``verbose_name_plural

The plural name for the object:

verbose_name_plural = "stories"

If this isn’t given, Django will use verbose_name + "s".

Read-only Meta attributes

label

Options.``label

Representation of the object, returns app_label.object_name, e.g. 'polls.Question'.

label_lower

Options.``label_lower

Representation of the model, returns app_label.model_name, e.g. 'polls.question'.