可用的Meta options(元数据设置)
(1)abstract
Options.abstract
抽象基类选项:abstract=True则该模型是一个抽象的基类
Abstract base classes
Abstract base classes are useful when you want to put some common information into a number of other models. You write your base class and put abstract=True in the Meta class. This model will then not be used to create any database table. Instead, when it is used as a base class for other models, its fields will be added to those of the child class.
An example:
from django.db import models
class CommonInfo(models.Model):
name = models.CharField(max_length=100)
age = models.PositiveIntegerField()
class Meta:
abstract = True
class Student(CommonInfo):
home_group = models.CharField(max_length=5)
The Student model will have three fields: name, age and home_group. The CommonInfo model cannot be used as a normal Django model, since it is an abstract base class. It does not generate a database table or have a manager, and cannot be instantiated or saved directly.
Fields inherited from abstract base classes can be overridden with another field or value, or be removed with None.
For many uses, this type of model inheritance will be exactly what you want. It provides a way to factor out common information at the Python level, while still only creating one database table per child model at the database level.
Meta inheritance
When an abstract base class is created, Django makes any Meta inner class you declared in the base class available as an attribute. If a child class does not declare its own Meta class, it will inherit the parent’s Meta. If the child wants to extend the parent’s Meta class, it can subclass it. For example:
from django.db import models
class CommonInfo(models.Model):
# ...
class Meta:
abstract = True
ordering = ['name']
class Student(CommonInfo):
# ...
class Meta(CommonInfo.Meta):
db_table = 'student_info'
Django does make one adjustment to the Meta class of an abstract base class: before installing the Meta attribute, it sets abstract=False. This means that children of abstract base classes don’t automatically become abstract classes themselves. Of course, you can make an abstract base class that inherits from another abstract base class. You just need to remember to explicitly set abstract=True each time.
Some attributes won’t make sense to include in the Meta class of an abstract base class. For example, including db_table would mean that all the child classes (the ones that don’t specify their own Meta) would use the same database table, which is almost certainly not what you want.
Be careful with related_name and related_query_name
If you are using related_name or related_query_name on a ForeignKey or ManyToManyField, you must always specify a unique reverse name and query name for the field. This would normally cause a problem in abstract base classes, since the fields on this class are included into each of the child classes, with exactly the same values for the attributes (including related_name and related_query_name) each time.
To work around this problem, when you are using related_name or related_query_name in an abstract base class (only), part of the value should contain '%(app_label)s' and '%(class)s'.
'%(class)s' is replaced by the lower-cased name of the child class that the field is used in.
'%(app_label)s' is replaced by the lower-cased name of the app the child class is contained within. Each installed application name must be unique and the model class names within each app must also be unique, therefore the resulting name will end up being different.
For example, given an app common/models.py:
from django.db import models
class Base(models.Model):
m2m = models.ManyToManyField(
OtherModel,
related_name="%(app_label)s_%(class)s_related",
related_query_name="%(app_label)s_%(class)ss",
)
class Meta:
abstract = True
class ChildA(Base):
pass
class ChildB(Base):
pass
Along with another app rare/models.py:
from common.models import Base
class ChildB(Base):
pass
The reverse name of the common.ChildA.m2m field will be common_childa_related and the reverse query name will be common_childas. The reverse name of the common.ChildB.m2m field will be common_childb_related and the reverse query name will be common_childbs. Finally, the reverse name of the rare.ChildB.m2m field will be rare_childb_related and the reverse query name will be rare_childbs. It’s up to you how you use the '%(class)s' and '%(app_label)s' portion to construct your related name or related query name but if you forget to use it, Django will raise errors when you perform system checks (or run migrate).
If you don’t specify a related_name attribute for a field in an abstract base class, the default reverse name will be the name of the child class followed by '_set', just as it normally would be if you’d declared the field directly on the child class. For example, in the above code, if the related_name attribute was omitted, the reverse name for the m2m field would be childa_set in the ChildA case and childb_set for the ChildB field.
(2)app_label(定义应用程序之外的模型)
Options.app_labe
例如:
app_label = 'myapp'
INSTALLED_APPS
Default: [] (Empty list)
A list of strings designating all applications that are enabled in this Django installation. Each string should be a dotted Python path to:
an application configuration class (preferred), or
a package containing an application.
Learn more about application configurations.
Use the application registry for introspection
Your code should never access INSTALLED_APPS directly. Use django.apps.apps instead.
Application names and labels must be unique in INSTALLED_APPS
Application names — the dotted Python path to the application package — must be unique. There is no way to include the same application twice, short of duplicating its code under another name.
Application labels — by default the final part of the name — must be unique too. For example, you can’t include both django.contrib.auth and myproject.auth. However, you can relabel an application with a custom configuration that defines a different label.
These rules apply regardless of whether INSTALLED_APPS references application configuration classes or application packages.
When several applications provide different versions of the same resource (template, static file, management command, translation), the application listed first in INSTALLED_APPS has
(3)base_manager_name
Options.base_manager_name
用于模型管理器名称
Base managers
Model._base_manager
Using managers for related object access
By default, Django uses an instance of the Model._base_manager manager class when accessing related objects (i.e. choice.question), not the _default_manager on the related object. This is because Django needs to be able to retrieve the related object, even if it would otherwise be filtered out (and hence be inaccessible) by the default manager.
If the normal base manager class (django.db.models.Manager) isn’t appropriate for your circumstances, you can tell Django which class to use by setting Meta.base_manager_name.
Base managers aren’t used when querying on related models. For example, if the Question model from the tutorial had a deleted field and a base manager that filters out instances with deleted=True, a queryset like Choice.objects.filter(question__name__startswith='What') would include choices related to deleted questions.
(4)db_table
Options.db_table
模型使用的数据表名称
例如:
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 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.
(5)db_tablespace
Options.db_tablespace
(6)default_manager_name
Options.default_manager_name
The name of the manager to use for the model’s _default_manager.
(7)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.
(8)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.
Example:
# 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.
Changed in Django 2.0:
Support for a list of fields was added.
(9)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 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
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.
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: Proxy models.
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.
Changed in Django 2.0:
Support for query expressions was added.
Warning
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, delete and change 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'). 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
New in Django 1.11.
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
Sets of field names that, taken together, must be unique:
unique_together = (("driver", "restaurant"),)
This is a tuple of tuples 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 tuple 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"]
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'.
