Django2.0手册:模型



模型是您的数据唯一而且准确的信息来源。它包含您正在储存的数据的重要字段和行为。一般来说,每一个模型都映射一个数据库表。

基础:

  • 每个模型都是一个 Python 的类,这些类继承 django.db.models.Model
  • 模型类的每个属性都相当于一个数据库的字段。
  • 综上诉说,Django 给你一个自动生成访问数据库的 API;请参阅 Making queries

快速上手¶

这个样例模型定义了一个 Person, 其拥有 first_namelast_name:

from django.db import models

class Person(models.Model):
    first_name = models.CharField(max_length=30)
    last_name = models.CharField(max_length=30)

first_namelast_name 是模型的字段。每个字段都被指定为一个类属性,并且每个属性映射为一个数据库列。

上面的 Person 模型会创建一个如下的数据库表:

CREATE TABLE myapp_person (
    "id" serial NOT NULL PRIMARY KEY,
    "first_name" varchar(30) NOT NULL,
    "last_name" varchar(30) NOT NULL
);

一些技术上的说明:

  • 该表的名称 “myapp_person” 是自动从某些模型元数据中派生出来,但可以被改写。有关更多详细信息,请参阅:表命名
  • 一个 id 字段会被自动添加,但是这种行为可以被改写。请参阅:默认主键字段
  • The CREATE TABLE SQL in this example is formatted using PostgreSQL
    syntax, but it’s worth noting Django uses SQL tailored to the database
    backend specified in your settings file.

使用模型¶

一旦你定义了你的模型,你需要告诉 Django 你准备*使用*这些模型。你需要修改设置文件中的 INSTALLED_APPS ,在这个设置中添加包含你 models.py 文件的模块的名字。

例如,如果模型位于你项目中的“myapp.models“中( 此包结构使用:djadmin:manage.py startapp`命令创建),:setting:`INSTALLED_APPS 应设置如下:

INSTALLED_APPS = [
    #...
    'myapp',
    #...
]

When you add new apps to INSTALLED_APPS, be sure to run
manage.py migrate, optionally making migrations
for them first with manage.py makemigrations.

字段¶

模型中最重要的、并且也是唯一必须的是数据库的字段定义。字段在类中定义。定义字段名时应小心避免使用与 models API</ref/models/instances>冲突的名称, 如 “clean`, save, or delete“等.

举例:

from django.db import models

class Musician(models.Model):
    first_name = models.CharField(max_length=50)
    last_name = models.CharField(max_length=50)
    instrument = models.CharField(max_length=100)

class Album(models.Model):
    artist = models.ForeignKey(Musician, on_delete=models.CASCADE)
    name = models.CharField(max_length=100)
    release_date = models.DateField()
    num_stars = models.IntegerField()

字段类型¶

Each field in your model should be an instance of the appropriate
Field class. Django uses the field class types to
determine a few things:

  • 字段类型用以指定数据库数据类型(如:INTEGER, VARCHAR, TEXT
  • 默认的HTML表单输入框</ref/forms/widgets>(如:<input type=”text”><select>)
  • 用于Django admin和自动生成表单的基本验证。

Django内置了多种字段类型;你可以在模型字段参考<model-field-types>中看到完整列表。如果Django内置类型不能满足你的需求,你可以很轻松地编写自定义的字段类型;见:doc:/howto/custom-model-fields

Field options¶

Each field takes a certain set of field-specific arguments (documented in the
model field reference). For example,
CharField (and its subclasses) require a
max_length argument which specifies the size
of the VARCHAR database field used to store the data.

There’s also a set of common arguments available to all field types. All are
optional. They’re fully explained in the reference, but here’s a quick summary of the most often-used
ones:

null
If True, Django will store empty values as NULL in the database.
Default is False.
blank

If True, the field is allowed to be blank. Default is False.

Note that this is different than null.
null is purely database-related, whereas
blank is validation-related. If a field has
blank=True, form validation will
allow entry of an empty value. If a field has blank=False, the field will be required.

choices

An iterable (e.g., a list or tuple) of 2-tuples to use as choices for
this field. If this is given, the default form widget will be a select box
instead of the standard text field and will limit choices to the choices
given.

A choices list looks like this:

YEAR_IN_SCHOOL_CHOICES = (
    ('FR', 'Freshman'),
    ('SO', 'Sophomore'),
    ('JR', 'Junior'),
    ('SR', 'Senior'),
    ('GR', 'Graduate'),
)

The first element in each tuple is the value that will be stored in the
database. The second element is displayed by the field’s form widget.

Given a model instance, the display value for a field with choices can
be accessed using the get_FOO_display()
method. For example:

from django.db import models

class Person(models.Model):
    SHIRT_SIZES = (
        ('S', 'Small'),
        ('M', 'Medium'),
        ('L', 'Large'),
    )
    name = models.CharField(max_length=60)
    shirt_size = models.CharField(max_length=1, choices=SHIRT_SIZES)
>>> p = Person(name="Fred Flintstone", shirt_size="L")
>>> p.save()
>>> p.shirt_size
'L'
>>> p.get_shirt_size_display()
'Large'
default
The default value for the field. This can be a value or a callable
object. If callable it will be called every time a new object is
created.
help_text
Extra “help” text to be displayed with the form widget. It’s useful for
documentation even if your field isn’t used on a form.
primary_key

If True, this field is the primary key for the model.

If you don’t specify primary_key=True for
any fields in your model, Django will automatically add an
IntegerField to hold the primary key, so you don’t need to set
primary_key=True on any of your fields
unless you want to override the default primary-key behavior. For more,
see Automatic primary key fields.

The primary key field is read-only. If you change the value of the primary
key on an existing object and then save it, a new object will be created
alongside the old one. For example:

from django.db import models

class Fruit(models.Model):
    name = models.CharField(max_length=100, primary_key=True)
>>> fruit = Fruit.objects.create(name='Apple')
>>> fruit.name = 'Pear'
>>> fruit.save()
>>> Fruit.objects.values_list('name', flat=True)
<QuerySet ['Apple', 'Pear']>
unique
If True, this field must be unique throughout the table.

Again, these are just short descriptions of the most common field options. Full
details can be found in the common model field option reference.

Automatic primary key fields¶

By default, Django gives each model the following field:

id = models.AutoField(primary_key=True)

This is an auto-incrementing primary key.

If you’d like to specify a custom primary key, just specify
primary_key=True on one of your fields. If Django
sees you’ve explicitly set Field.primary_key, it won’t add the automatic
id column.

Each model requires exactly one field to have primary_key=True (either explicitly declared or automatically added).

Verbose field names¶

Each field type, except for ForeignKey,
ManyToManyField and
OneToOneField, takes an optional first positional
argument — a verbose name. If the verbose name isn’t given, Django will
automatically create it using the field’s attribute name, converting underscores
to spaces.

In this example, the verbose name is "person's first name":

first_name = models.CharField("person's first name", max_length=30)

In this example, the verbose name is "first name":

first_name = models.CharField(max_length=30)

ForeignKey,
ManyToManyField and
OneToOneField require the first argument to be a
model class, so use the verbose_name keyword argument:

poll = models.ForeignKey(
    Poll,
    on_delete=models.CASCADE,
    verbose_name="the related poll",
)
sites = models.ManyToManyField(Site, verbose_name="list of sites")
place = models.OneToOneField(
    Place,
    on_delete=models.CASCADE,
    verbose_name="related place",
)

The convention is not to capitalize the first letter of the
verbose_name. Django will automatically capitalize the first
letter where it needs to.

Relationships¶

Clearly, the power of relational databases lies in relating tables to each
other. Django offers ways to define the three most common types of database
relationships: many-to-one, many-to-many and one-to-one.

Many-to-one relationships

To define a many-to-one relationship, use django.db.models.ForeignKey.
You use it just like any other Field type: by
including it as a class attribute of your model.

ForeignKey requires a positional argument: the class
to which the model is related.

For example, if a Car model has a Manufacturer — that is, a
Manufacturer makes multiple cars but each Car only has one
Manufacturer — use the following definitions:

from django.db import models

class Manufacturer(models.Model):
    # ...
    pass

class Car(models.Model):
    manufacturer = models.ForeignKey(Manufacturer, on_delete=models.CASCADE)
    # ...

You can also create recursive relationships (an
object with a many-to-one relationship to itself) and relationships to
models not yet defined
; see the model field
reference
for details.

It’s suggested, but not required, that the name of a
ForeignKey field (manufacturer in the example
above) be the name of the model, lowercase. You can, of course, call the field
whatever you want. For example:

class Car(models.Model):
    company_that_makes_it = models.ForeignKey(
        Manufacturer,
        on_delete=models.CASCADE,
    )
    # ...

See also

ForeignKey fields accept a number of extra
arguments which are explained in the model field reference. These options help define how the relationship
should work; all are optional.

For details on accessing backwards-related objects, see the
Following relationships backward example.

For sample code, see the Many-to-one relationship model example.

Many-to-many relationships

To define a many-to-many relationship, use
ManyToManyField. You use it just like any other
Field type: by including it as a class attribute of
your model.

ManyToManyField requires a positional argument: the
class to which the model is related.

For example, if a Pizza has multiple Topping objects — that is, a
Topping can be on multiple pizzas and each Pizza has multiple toppings
— here’s how you’d represent that:

from django.db import models

class Topping(models.Model):
    # ...
    pass

class Pizza(models.Model):
    # ...
    toppings = models.ManyToManyField(Topping)

As with ForeignKey, you can also create
recursive relationships (an object with a
many-to-many relationship to itself) and relationships to models not yet
defined
.

It’s suggested, but not required, that the name of a
ManyToManyField (toppings in the example above)
be a plural describing the set of related model objects.

It doesn’t matter which model has the
ManyToManyField, but you should only put it in one
of the models — not both.

Generally, ManyToManyField instances should go in
the object that’s going to be edited on a form. In the above example,
toppings is in Pizza (rather than Topping having a pizzas
ManyToManyField ) because it’s more natural to think
about a pizza having toppings than a topping being on multiple pizzas. The way
it’s set up above, the Pizza form would let users select the toppings.

See also

See the Many-to-many relationship model example for a full example.

ManyToManyField fields also accept a number of
extra arguments which are explained in the model field reference. These options help define how the relationship
should work; all are optional.

Extra fields on many-to-many relationships

When you’re only dealing with simple many-to-many relationships such as
mixing and matching pizzas and toppings, a standard
ManyToManyField is all you need. However, sometimes
you may need to associate data with the relationship between two models.

For example, consider the case of an application tracking the musical groups
which musicians belong to. There is a many-to-many relationship between a person
and the groups of which they are a member, so you could use a
ManyToManyField to represent this relationship.
However, there is a lot of detail about the membership that you might want to
collect, such as the date at which the person joined the group.

For these situations, Django allows you to specify the model that will be used
to govern the many-to-many relationship. You can then put extra fields on the
intermediate model. The intermediate model is associated with the
ManyToManyField using the
through argument to point to the model
that will act as an intermediary. For our musician example, the code would look
something like this:

from django.db import models

class Person(models.Model):
    name = models.CharField(max_length=128)

    def __str__(self):
        return self.name

class Group(models.Model):
    name = models.CharField(max_length=128)
    members = models.ManyToManyField(Person, through='Membership')

    def __str__(self):
        return self.name

class Membership(models.Model):
    person = models.ForeignKey(Person, on_delete=models.CASCADE)
    group = models.ForeignKey(Group, on_delete=models.CASCADE)
    date_joined = models.DateField()
    invite_reason = models.CharField(max_length=64)

When you set up the intermediary model, you explicitly specify foreign
keys to the models that are involved in the many-to-many relationship. This
explicit declaration defines how the two models are related.

There are a few restrictions on the intermediate model:

  • Your intermediate model must contain one – and only one – foreign key
    to the source model (this would be Group in our example), or you must
    explicitly specify the foreign keys Django should use for the relationship
    using ManyToManyField.through_fields.
    If you have more than one foreign key and through_fields is not
    specified, a validation error will be raised. A similar restriction applies
    to the foreign key to the target model (this would be Person in our
    example).
  • For a model which has a many-to-many relationship to itself through an
    intermediary model, two foreign keys to the same model are permitted, but
    they will be treated as the two (different) sides of the many-to-many
    relationship. If there are more than two foreign keys though, you
    must also specify through_fields as above, or a validation error
    will be raised.
  • When defining a many-to-many relationship from a model to
    itself, using an intermediary model, you must use
    symmetrical=False (see
    the model field reference).

Now that you have set up your ManyToManyField to use
your intermediary model (Membership, in this case), you’re ready to start
creating some many-to-many relationships. You do this by creating instances of
the intermediate model:

>>> ringo = Person.objects.create(name="Ringo Starr")
>>> paul = Person.objects.create(name="Paul McCartney")
>>> beatles = Group.objects.create(name="The Beatles")
>>> m1 = Membership(person=ringo, group=beatles,
...     date_joined=date(1962, 8, 16),
...     invite_reason="Needed a new drummer.")
>>> m1.save()
>>> beatles.members.all()
<QuerySet [<Person: Ringo Starr>]>
>>> ringo.group_set.all()
<QuerySet [<Group: The Beatles>]>
>>> m2 = Membership.objects.create(person=paul, group=beatles,
...     date_joined=date(1960, 8, 1),
...     invite_reason="Wanted to form a band.")
>>> beatles.members.all()
<QuerySet [<Person: Ringo Starr>, <Person: Paul McCartney>]>

Unlike normal many-to-many fields, you can’t use add(), create(),
or set() to create relationships:

>>> # The following statements will not work
>>> beatles.members.add(john)
>>> beatles.members.create(name="George Harrison")
>>> beatles.members.set([john, paul, ringo, george])

Why? You can’t just create a relationship between a Person and a Group
– you need to specify all the detail for the relationship required by the
Membership model. The simple add, create and assignment calls
don’t provide a way to specify this extra detail. As a result, they are
disabled for many-to-many relationships that use an intermediate model.
The only way to create this type of relationship is to create instances of the
intermediate model.

The remove() method is
disabled for similar reasons. For example, if the custom through table defined
by the intermediate model does not enforce uniqueness on the
(model1, model2) pair, a remove() call would not provide enough
information as to which intermediate model instance should be deleted:

>>> Membership.objects.create(person=ringo, group=beatles,
...     date_joined=date(1968, 9, 4),
...     invite_reason="You've been gone for a month and we miss you.")
>>> beatles.members.all()
<QuerySet [<Person: Ringo Starr>, <Person: Paul McCartney>, <Person: Ringo Starr>]>
>>> # This will not work because it cannot tell which membership to remove
>>> beatles.members.remove(ringo)

However, the clear()
method can be used to remove all many-to-many relationships for an instance:

>>> # Beatles have broken up
>>> beatles.members.clear()
>>> # Note that this deletes the intermediate model instances
>>> Membership.objects.all()
<QuerySet []>

Once you have established the many-to-many relationships by creating instances
of your intermediate model, you can issue queries. Just as with normal
many-to-many relationships, you can query using the attributes of the
many-to-many-related model:

# Find all the groups with a member whose name starts with 'Paul'
>>> Group.objects.filter(members__name__startswith='Paul')
<QuerySet [<Group: The Beatles>]>

As you are using an intermediate model, you can also query on its attributes:

# Find all the members of the Beatles that joined after 1 Jan 1961
>>> Person.objects.filter(
...     group__name='The Beatles',
...     membership__date_joined__gt=date(1961,1,1))
<QuerySet [<Person: Ringo Starr]>

If you need to access a membership’s information you may do so by directly
querying the Membership model:

>>> ringos_membership = Membership.objects.get(group=beatles, person=ringo)
>>> ringos_membership.date_joined
datetime.date(1962, 8, 16)
>>> ringos_membership.invite_reason
'Needed a new drummer.'

Another way to access the same information is by querying the
many-to-many reverse relationship from a
Person object:

>>> ringos_membership = ringo.membership_set.get(group=beatles)
>>> ringos_membership.date_joined
datetime.date(1962, 8, 16)
>>> ringos_membership.invite_reason
'Needed a new drummer.'

One-to-one relationships

To define a one-to-one relationship, use
OneToOneField. You use it just like any other
Field type: by including it as a class attribute of your model.

This is most useful on the primary key of an object when that object “extends”
another object in some way.

OneToOneField requires a positional argument: the
class to which the model is related.

For example, if you were building a database of “places”, you would
build pretty standard stuff such as address, phone number, etc. in the
database. Then, if you wanted to build a database of restaurants on
top of the places, instead of repeating yourself and replicating those
fields in the Restaurant model, you could make Restaurant have
a OneToOneField to Place (because a
restaurant “is a” place; in fact, to handle this you’d typically use
inheritance, which involves an implicit
one-to-one relation).

As with ForeignKey, a recursive relationship can be defined and references to as-yet
undefined models
can be made.

See also

See the One-to-one relationship model example for a full example.

OneToOneField fields also accept an optional
parent_link argument.

OneToOneField classes used to automatically become
the primary key on a model. This is no longer true (although you can manually
pass in the primary_key argument if you like).
Thus, it’s now possible to have multiple fields of type
OneToOneField on a single model.

Models across files¶

It’s perfectly OK to relate a model to one from another app. To do this, import
the related model at the top of the file where your model is defined. Then,
just refer to the other model class wherever needed. For example:

from django.db import models
from geography.models import ZipCode

class Restaurant(models.Model):
    # ...
    zip_code = models.ForeignKey(
        ZipCode,
        on_delete=models.SET_NULL,
        blank=True,
        null=True,
    )

Field name restrictions¶

Django places only two restrictions on model field names:

  1. A field name cannot be a Python reserved word, because that would result
    in a Python syntax error. For example:

    class Example(models.Model):
        pass = models.IntegerField() # 'pass' is a reserved word!
    
  2. A field name cannot contain more than one underscore in a row, due to
    the way Django’s query lookup syntax works. For example:

    class Example(models.Model):
        foo__bar = models.IntegerField() # 'foo__bar' has two underscores!
    

These limitations can be worked around, though, because your field name doesn’t
necessarily have to match your database column name. See the
db_column option.

SQL reserved words, such as join, where or select, are allowed as
model field names, because Django escapes all database table names and column
names in every underlying SQL query. It uses the quoting syntax of your
particular database engine.

Custom field types¶

If one of the existing model fields cannot be used to fit your purposes, or if
you wish to take advantage of some less common database column types, you can
create your own field class. Full coverage of creating your own fields is
provided in 编写自定义 model fields.

Meta options¶

Give your model metadata by using an inner class Meta, like so:

from django.db import models

class Ox(models.Model):
    horn_length = models.IntegerField()

    class Meta:
        ordering = ["horn_length"]
        verbose_name_plural = "oxen"

Model metadata is “anything that’s not a field”, such as ordering options
(ordering), database table name (db_table), or
human-readable singular and plural names (verbose_name and
verbose_name_plural). None are required, and adding class
Meta
to a model is completely optional.

A complete list of all possible Meta options can be found in the model
option reference
.

Model attributes¶

objects
The most important attribute of a model is the
Manager. It’s the interface through which
database query operations are provided to Django models and is used to
retrieve the instances from the database. If no
custom Manager is defined, the default name is
objects. Managers are only accessible via
model classes, not the model instances.

Model methods¶

Define custom methods on a model to add custom “row-level” functionality to your
objects. Whereas Manager methods are intended to do
“table-wide” things, model methods should act on a particular model instance.

This is a valuable technique for keeping business logic in one place — the
model.

For example, this model has a few custom methods:

from django.db import models

class Person(models.Model):
    first_name = models.CharField(max_length=50)
    last_name = models.CharField(max_length=50)
    birth_date = models.DateField()

    def baby_boomer_status(self):
        "Returns the person's baby-boomer status."
        import datetime
        if self.birth_date < datetime.date(1945, 8, 1):
            return "Pre-boomer"
        elif self.birth_date < datetime.date(1965, 1, 1):
            return "Baby boomer"
        else:
            return "Post-boomer"

    @property
    def full_name(self):
        "Returns the person's full name."
        return '%s %s' % (self.first_name, self.last_name)

The last method in this example is a property.

The model instance reference has a complete list
of methods automatically given to each model.
You can override most of these — see overriding predefined model methods,
below — but there are a couple that you’ll almost always want to define:

__str__()

A Python “magic method” that returns a string representation of any
object. This is what Python and Django will use whenever a model
instance needs to be coerced and displayed as a plain string. Most
notably, this happens when you display an object in an interactive
console or in the admin.

You’ll always want to define this method; the default isn’t very helpful
at all.

get_absolute_url()

This tells Django how to calculate the URL for an object. Django uses
this in its admin interface, and any time it needs to figure out a URL
for an object.

Any object that has a URL that uniquely identifies it should define this
method.

Overriding predefined model methods¶

There’s another set of model methods that
encapsulate a bunch of database behavior that you’ll want to customize. In
particular you’ll often want to change the way save() and
delete() work.

You’re free to override these methods (and any other model method) to alter
behavior.

A classic use-case for overriding the built-in methods is if you want something
to happen whenever you save an object. For example (see
save() for documentation of the parameters it accepts):

from django.db import models

class Blog(models.Model):
    name = models.CharField(max_length=100)
    tagline = models.TextField()

    def save(self, *args, **kwargs):
        do_something()
        super().save(*args, **kwargs)  # Call the "real" save() method.
        do_something_else()

You can also prevent saving:

from django.db import models

class Blog(models.Model):
    name = models.CharField(max_length=100)
    tagline = models.TextField()

    def save(self, *args, **kwargs):
        if self.name == "Yoko Ono's blog":
            return # Yoko shall never have her own blog!
        else:
            super().save(*args, **kwargs)  # Call the "real" save() method.

It’s important to remember to call the superclass method — that’s
that super().save(*args, **kwargs) business — to ensure
that the object still gets saved into the database. If you forget to
call the superclass method, the default behavior won’t happen and the
database won’t get touched.

It’s also important that you pass through the arguments that can be
passed to the model method — that’s what the *args, **kwargs bit
does. Django will, from time to time, extend the capabilities of
built-in model methods, adding new arguments. If you use *args,
**kwargs
in your method definitions, you are guaranteed that your
code will automatically support those arguments when they are added.

Overridden model methods are not called on bulk operations

Note that the delete() method for an object is not
necessarily called when deleting objects in bulk using a
QuerySet
or as a result of a cascading
delete
. To ensure customized
delete logic gets executed, you can use
pre_delete and/or
post_delete signals.

Unfortunately, there isn’t a workaround when
creating or
updating objects in bulk,
since none of save(),
pre_save, and
post_save are called.

Executing custom SQL¶

Another common pattern is writing custom SQL statements in model methods and
module-level methods. For more details on using raw SQL, see the documentation
on using raw SQL.

模型继承¶

模型继承在 Django 中与普通类继承在 Python 中的工作方式几乎完全相同, 但也仍应遵循本页开头的内容. 这意味着其基类应该继承自 django.db.models.Model .

The only decision you have to make is whether you want the parent models to be
models in their own right (with their own database tables), or if the parents
are just holders of common information that will only be visible through the
child models.

There are three styles of inheritance that are possible in Django.

  1. Often, you will just want to use the parent class to hold information that
    you don’t want to have to type out for each child model. This class isn’t
    going to ever be used in isolation, so Abstract base classes are
    what you’re after.
  2. If you’re subclassing an existing model (perhaps something from another
    application entirely) and want each model to have its own database table,
    Multi-table inheritance is the way to go.
  3. Finally, if you only want to modify the Python-level behavior of a model,
    without changing the models fields in any way, you can use
    Proxy models.

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.

Multi-table inheritance¶

The second type of model inheritance supported by Django is when each model in
the hierarchy is a model all by itself. Each model corresponds to its own
database table and can be queried and created individually. The inheritance
relationship introduces links between the child model and each of its parents
(via an automatically-created OneToOneField).
For example:

from django.db import models

class Place(models.Model):
    name = models.CharField(max_length=50)
    address = models.CharField(max_length=80)

class Restaurant(Place):
    serves_hot_dogs = models.BooleanField(default=False)
    serves_pizza = models.BooleanField(default=False)

All of the fields of Place will also be available in Restaurant,
although the data will reside in a different database table. So these are both
possible:

>>> Place.objects.filter(name="Bob's Cafe")
>>> Restaurant.objects.filter(name="Bob's Cafe")

If you have a Place that is also a Restaurant, you can get from the
Place object to the Restaurant object by using the lower-case version
of the model name:

>>> p = Place.objects.get(id=12)
# If p is a Restaurant object, this will give the child class:
>>> p.restaurant
<Restaurant: ...>

However, if p in the above example was not a Restaurant (it had been
created directly as a Place object or was the parent of some other class),
referring to p.restaurant would raise a Restaurant.DoesNotExist
exception.

The automatically-created OneToOneField on
Restaurant that links it to Place looks like this:

place_ptr = models.OneToOneField(
    Place, on_delete=models.CASCADE,
    parent_link=True,
)

You can override that field by declaring your own
OneToOneField with parent_link=True on Restaurant.

Meta and multi-table inheritance

In the multi-table inheritance situation, it doesn’t make sense for a child
class to inherit from its parent’s Meta class. All the Meta options
have already been applied to the parent class and applying them again would
normally only lead to contradictory behavior (this is in contrast with the
abstract base class case, where the base class doesn’t exist in its own
right).

So a child model does not have access to its parent’s Meta class. However, there are a few limited cases where the child
inherits behavior from the parent: if the child does not specify an
ordering attribute or a
get_latest_by attribute, it will inherit
these from its parent.

If the parent has an ordering and you don’t want the child to have any natural
ordering, you can explicitly disable it:

class ChildModel(ParentModel):
    # ...
    class Meta:
        # Remove parent's ordering effect
        ordering = []

Inheritance and reverse relations

Because multi-table inheritance uses an implicit
OneToOneField to link the child and
the parent, it’s possible to move from the parent down to the child,
as in the above example. However, this uses up the name that is the
default related_name value for
ForeignKey and
ManyToManyField relations. If you
are putting those types of relations on a subclass of the parent model, you
must specify the related_name
attribute on each such field. If you forget, Django will raise a validation
error.

For example, using the above Place class again, let’s create another
subclass with a ManyToManyField:

class Supplier(Place):
    customers = models.ManyToManyField(Place)

This results in the error:

Reverse query name for 'Supplier.customers' clashes with reverse query
name for 'Supplier.place_ptr'.

HINT: Add or change a related_name argument to the definition for
'Supplier.customers' or 'Supplier.place_ptr'.

Adding related_name to the customers field as follows would resolve the
error: models.ManyToManyField(Place, related_name='provider').

Proxy models¶

When using multi-table inheritance, a new
database table is created for each subclass of a model. This is usually the
desired behavior, since the subclass needs a place to store any additional
data fields that are not present on the base class. Sometimes, however, you
only want to change the Python behavior of a model — perhaps to change the
default manager, or add a new method.

This is what proxy model inheritance is for: creating a proxy for the
original model. You can create, delete and update instances of the proxy model
and all the data will be saved as if you were using the original (non-proxied)
model. The difference is that you can change things like the default model
ordering or the default manager in the proxy, without having to alter the
original.

Proxy models are declared like normal models. You tell Django that it’s a
proxy model by setting the proxy attribute of
the Meta class to True.

For example, suppose you want to add a method to the Person model. You can do it like this:

from django.db import models

class Person(models.Model):
    first_name = models.CharField(max_length=30)
    last_name = models.CharField(max_length=30)

class MyPerson(Person):
    class Meta:
        proxy = True

    def do_something(self):
        # ...
        pass

The MyPerson class operates on the same database table as its parent
Person class. In particular, any new instances of Person will also be
accessible through MyPerson, and vice-versa:

>>> p = Person.objects.create(first_name="foobar")
>>> MyPerson.objects.get(first_name="foobar")
<MyPerson: foobar>

你仍然可以使用一个代理模型来定义模型的默认排序方法。你也许不会想一直对“Persion”进行排序,但是通常情况下用代理模型根据“last_name”属性进行排序。这很简单:

class OrderedPerson(Person):
    class Meta:
        ordering = ["last_name"]
        proxy = True

Now normal Person queries will be unordered
and OrderedPerson queries will be ordered by last_name.

代理模型继承“Meta”属性:ref:和普通模型使用同样的方法<meta-and-multi-table-inheritance>

QuerySets still return the model that was requested

There is no way to have Django return, say, a MyPerson object whenever you
query for Person objects. A queryset for Person objects will return
those types of objects. The whole point of proxy objects is that code relying
on the original Person will use those and your own code can use the
extensions you included (that no other code is relying on anyway). It is not
a way to replace the Person (or any other) model everywhere with something
of your own creation.

Base class restrictions

一个代理模型必须仅能继承一个非抽象模型类。你不能继承多个非抽象模型类,因为代理模型无法提供不同数据表的任何行间连接。一个代理模型可以继承任意数量的抽象模型类,假如他们*没有*定义任何的模型字段。一个代理模型也可以继承任意数量的代理模型,只需他们共享同一个非抽象父类。

代理模型管理器

If you don’t specify any model managers on a proxy model, it inherits the
managers from its model parents. If you define a manager on the proxy model,
it will become the default, although any managers defined on the parent
classes will still be available.

Continuing our example from above, you could change the default manager used
when you query the Person model like this:

from django.db import models

class NewManager(models.Manager):
    # ...
    pass

class MyPerson(Person):
    objects = NewManager()

    class Meta:
        proxy = True

If you wanted to add a new manager to the Proxy, without replacing the
existing default, you can use the techniques described in the custom
manager
documentation: create a base class
containing the new managers and inherit that after the primary base class:

# Create an abstract class for the new manager.
class ExtraManagers(models.Model):
    secondary = NewManager()

    class Meta:
        abstract = True

class MyPerson(Person, ExtraManagers):
    class Meta:
        proxy = True

通常情况下,你可能不需要这么做。然而,你需要的时候,这也是可以的。

Differences between proxy inheritance and unmanaged models

Proxy model inheritance might look fairly similar to creating an unmanaged
model, using the managed attribute on a
model’s Meta class.

With careful setting of Meta.db_table you could create an unmanaged model that
shadows an existing model and adds Python methods to it. However, that would be
very repetitive and fragile as you need to keep both copies synchronized if you
make any changes.

On the other hand, proxy models are intended to behave exactly like the model
they are proxying for. They are always in sync with the parent model since they
directly inherit its fields and managers.

The general rules are:

  1. If you are mirroring an existing model or database table and don’t want
    all the original database table columns, use Meta.managed=False.
    That option is normally useful for modeling database views and tables
    not under the control of Django.
  2. If you are wanting to change the Python-only behavior of a model, but
    keep all the same fields as in the original, use Meta.proxy=True.
    This sets things up so that the proxy model is an exact copy of the
    storage structure of the original model when data is saved.

Multiple inheritance¶

Just as with Python’s subclassing, it’s possible for a Django model to inherit
from multiple parent models. Keep in mind that normal Python name resolution
rules apply. The first base class that a particular name (e.g. Meta) appears in will be the one that is used; for example, this
means that if multiple parents contain a Meta class,
only the first one is going to be used, and all others will be ignored.

Generally, you won’t need to inherit from multiple parents. The main use-case
where this is useful is for “mix-in” classes: adding a particular extra
field or method to every class that inherits the mix-in. Try to keep your
inheritance hierarchies as simple and straightforward as possible so that you
won’t have to struggle to work out where a particular piece of information is
coming from.

Note that inheriting from multiple models that have a common id primary
key field will raise an error. To properly use multiple inheritance, you can
use an explicit AutoField in the base models:

class Article(models.Model):
    article_id = models.AutoField(primary_key=True)
    ...

class Book(models.Model):
    book_id = models.AutoField(primary_key=True)
    ...

class BookReview(Book, Article):
    pass

Or use a common ancestor to hold the AutoField. This
requires using an explicit OneToOneField from each
parent model to the common ancestor to avoid a clash between the fields that
are automatically generated and inherited by the child:

class Piece(models.Model):
    pass

class Article(Piece):
    article_piece = models.OneToOneField(Piece, on_delete=models.CASCADE, parent_link=True)
    ...

class Book(Piece):
    book_piece = models.OneToOneField(Piece, on_delete=models.CASCADE, parent_link=True)
    ...

class BookReview(Book, Article):
    pass

Field name “hiding” is not permitted¶

In normal Python class inheritance, it is permissible for a child class to
override any attribute from the parent class. In Django, this isn’t usually
permitted for model fields. If a non-abstract model base class has a field
called author, you can’t create another model field or define
an attribute called author in any class that inherits from that base class.

This restriction doesn’t apply to model fields inherited from an abstract
model. Such fields may be overridden with another field or value, or be removed
by setting field_name = None.

Warning

Model managers are inherited from abstract base classes. Overriding an
inherited field which is referenced by an inherited
Manager may cause subtle bugs. See custom
managers and model inheritance
.

Note

Some fields define extra attributes on the model, e.g. a
ForeignKey defines an extra attribute with
_id appended to the field name, as well as related_name and
related_query_name on the foreign model.

These extra attributes cannot be overridden unless the field that defines
it is changed or removed so that it no longer defines the extra attribute.

Overriding fields in a parent model leads to difficulties in areas such as
initializing new instances (specifying which field is being initialized in
Model.__init__) and serialization. These are features which normal Python
class inheritance doesn’t have to deal with in quite the same way, so the
difference between Django model inheritance and Python class inheritance isn’t
arbitrary.

This restriction only applies to attributes which are
Field instances. Normal Python attributes
can be overridden if you wish. It also only applies to the name of the
attribute as Python sees it: if you are manually specifying the database
column name, you can have the same column name appearing in both a child and
an ancestor model for multi-table inheritance (they are columns in two
different database tables).

Django will raise a FieldError if you override
any model field in any ancestor model.

Organizing models in a package¶

The manage.py startapp command creates an application
structure that includes a models.py file. If you have many models,
organizing them in separate files may be useful.

To do so, create a models package. Remove models.py and create a
myapp/models/ directory with an __init__.py file and the files to
store your models. You must import the models in the __init__.py file.

For example, if you had organic.py and synthetic.py in the models
directory:

myapp/models/__init__.py
from .organic import Person
from .synthetic import Robot

Explicitly importing each model rather than using from .models import *
has the advantages of not cluttering the namespace, making code more readable,
and keeping code analysis tools useful.

See also

The Models Reference
Covers all the model related APIs including model fields, related
objects, and QuerySet.