---
title: "模型"
version: 2.1
locale: zh-hans
source: https://docs.djangoproject.com/zh-hans/2.1/topics/db/models/
canonical: https://djangodocs.dev/zh-hans/2.1/topics/db/models/
---
# 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`](/zh-hans/2.1/ref/models/instances/#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 [进行查询](/zh-hans/2.1/topics/db/queries/)。

## 快速上手

这个样例模型定义了一个 `Person`, 其拥有 `first_name` 和 `last_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_name` 和 `last_name` [是模型的字段](#fields)。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：

```sql
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](/zh-hans/2.1/topics/settings/).

## 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`](/zh-hans/2.1/ref/settings/#std-setting-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',
    #...
]
```

当你向 [`INSTALLED_APPS`](/zh-hans/2.1/ref/settings/#std-setting-INSTALLED_APPS) 添加新的应用的时候，请务必运行:djadmin:manage.py migrate \<migrate\>，此外你也可以先使用以下命令先进行迁移 [`manage.py makemigrations`](/zh-hans/2.1/ref/django-admin/#django-admin-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

模型中每一个字段都应该是相应类的实例， Django 利用这些字段类来实现下面这些功能。

- 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。

### 字段选项

每一种字段都需要指定一些特定的参数（参考 model field reference\<model-field-types\> ） 例如： :class:\`~django.db.models.CharField （以及它的子类）需要接收一个 [`max_length`](/zh-hans/2.1/ref/models/fields/#django.db.models.CharField.max_length) 参数，用以指定数据库存储数据时用的 `VARCHAR` 大小。

一些可选的参数是通用的，可以用于任何字段类型，详情请见 :ref:reference\<common-model-field-options\> \` ，下面介绍一部分经常用到的通用参数：

**[`null`](/zh-hans/2.1/ref/models/fields/#django.db.models.Field.null)**

  如果设置为 `True` ， 当该字段为空时，Django会将数据库中该字段设置为 `NULL` 。默认为 `False` 。

**[`blank`](/zh-hans/2.1/ref/models/fields/#django.db.models.Field.blank)**

  如果设置为 `True` ，该字段允许为空。默认为 `False` 。

  注意该选项与 `False` 不同， [`null`](/zh-hans/2.1/ref/models/fields/#django.db.models.Field.null) 选项仅仅是数据库层面的设置，然而 [`blank`](/zh-hans/2.1/ref/models/fields/#django.db.models.Field.blank) 是涉及表单验证方面。如果一个字段设置为 [`blank=True`](/zh-hans/2.1/ref/models/fields/#django.db.models.Field.blank) ，在进行表单验证时，接收的数据该字段值允许为空，而设置为 [`blank=False`](/zh-hans/2.1/ref/models/fields/#django.db.models.Field.blank) 时，不允许为空。

**[`choices`](/zh-hans/2.1/ref/models/fields/#django.db.models.Field.choices)**

  该参数接收一个可迭代的列表或元组（基本单位为二元组）。如果指定了该参数，在实例化该模型时，该字段只能取选项列表中的值。

  一个选项列表：

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

  每个二元组的第一个值会储存在数据库中，而第二个值将只会用于显示作用。

  对于一个模型实例，要获取该字段二元组中相对应的第二个值，使用 [`get_FOO_display()`](/zh-hans/2.1/ref/models/instances/#django.db.models.Model.get_FOO_display) 方法。例如：

  ```
  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`](/zh-hans/2.1/ref/models/fields/#django.db.models.Field.default)**

  该字段的默认值。可以是一个值或者是个可调用的对象，如果是个可调用对象，每次实例化模型时都会调用该对象。

**[`help_text`](/zh-hans/2.1/ref/models/fields/#django.db.models.Field.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`](/zh-hans/2.1/ref/models/fields/#django.db.models.Field.primary_key)**

  如果设置为 `True` ，将该字段设置为该模型的主键。

  在一个模型中，如果你没有对任何一个字段设置 [`primary_key=True`](/zh-hans/2.1/ref/models/fields/#django.db.models.Field.primary_key) 选项。 Django 会自动添加一个 [`IntegerField`](/zh-hans/2.1/ref/models/fields/#django.db.models.IntegerField) 字段，用于设置为主键，因此除非你想重写 Django 默认的主键设置行为，你可以不手动设置主键。详情请见 [自动设置主键](#automatic-primary-key-fields) 。

  主键字段是只可读的，如果你修改一个模型实例该字段的值并保存，你将等同于创建了一个新的模型实例。例如：

  ```
  from django.db import models

  class Fruit(models.Model):
      name = models.CharField(max_length=100, primary_key=True)
  ```

  ```pycon
  >>> fruit = Fruit.objects.create(name='Apple')
  >>> fruit.name = 'Pear'
  >>> fruit.save()
  >>> Fruit.objects.values_list('name', flat=True)
  <QuerySet ['Apple', 'Pear']>
  ```

**[`unique`](/zh-hans/2.1/ref/models/fields/#django.db.models.Field.unique)**

  如果设置为 `True`，这个字段必须在整个表中保持值唯一。

再次声明，以上只是一些通用参数的简略描述。你可以在 :ref:common model field option reference\<common-model-field-options\> \` 中找到完整的介绍。

### 自动设置主键

默认情况下， Django 会给每一个模型添加下面的字段：

```
id = models.AutoField(primary_key=True)
```

这是一个自增的主键。

如果你想指定设置为为主键的字段， 在你想要设置为主键的字段上设置 [`primary_key=True`](/zh-hans/2.1/ref/models/fields/#django.db.models.Field.primary_key) 选项。如果 Django 看到你显式的设置了 [`Field.primary_key`](/zh-hans/2.1/ref/models/fields/#django.db.models.Field.primary_key) ，将不会自动在表（模型）中添加 `id` 列。

每个模型都需要拥有一个设置了 [`primary_key=True`](/zh-hans/2.1/ref/models/fields/#django.db.models.Field.primary_key) 的字段（无论是显式的设置还是 Django 自动设置）。

### 备注名

除了  [`ForeignKey`](/zh-hans/2.1/ref/models/fields/#django.db.models.ForeignKey) ， [`ManyToManyField`](/zh-hans/2.1/ref/models/fields/#django.db.models.ManyToManyField) 和 [`OneToOneField`](/zh-hans/2.1/ref/models/fields/#django.db.models.OneToOneField) ，任何字段类型都接收一个可选的参数 [`verbose_name`](/zh-hans/2.1/ref/models/fields/#django.db.models.Field.verbose_name)  ，如果未指定该参数值， Django 会自动使用该字段的属性名作为该参数值，并且把下划线转换为空格。

在该例中：备注名为 `"person's first name"`:: 。

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

在该例中：备注名为 `"first name"`:: 。

```
first_name = models.CharField(max_length=30)
```

[`ForeignKey`](/zh-hans/2.1/ref/models/fields/#django.db.models.ForeignKey), [`ManyToManyField`](/zh-hans/2.1/ref/models/fields/#django.db.models.ManyToManyField) and [`OneToOneField`](/zh-hans/2.1/ref/models/fields/#django.db.models.OneToOneField) 接收的第一个参数为模型的类名，后面可以添加一个 [`verbose_name`](/zh-hans/2.1/ref/models/fields/#django.db.models.Field.verbose_name) 参数：

```
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",
)
```

一般情况下不需要将 [`verbose_name`](/zh-hans/2.1/ref/models/fields/#django.db.models.Field.verbose_name) 值首字母大写，必要时 Djanog 会自动把首字母转换为大写。

### 关联关系

显然，关系型数据库的强大之处在于各表之间的关联关系。 Django 提供了定义三种最常见的数据库关联关系的方法：多对一，多对多，一对一。

#### Many-to-one relationships

定义一个多对一的关联关系，使用 [`django.db.models.ForeignKey`](/zh-hans/2.1/ref/models/fields/#django.db.models.ForeignKey) 类。就和其他 [`Field`](/zh-hans/2.1/ref/models/fields/#django.db.models.Field) 字段类型一样，只需要在你模型中添加一个值为该类的属性。

[`ForeignKey`](/zh-hans/2.1/ref/models/fields/#django.db.models.ForeignKey) requires a positional argument: the class
to which the model is related.

例如，如果一个 `Car` 模型 有一个制造者 `Manufacturer` --就是说一个 `Manufacturer` 制造许多辆车，但是每辆车都属于某个特定的制造者-- 那么使用下面的方法定义这个关系：

```
from django.db import models

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

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

你也可以创建一个 [recursive relationships](/zh-hans/2.1/ref/models/fields/#recursive-relationships) 关系（一个模型与它本身有多对一的关系）和 :ref:relationships to models not yet defined \<lazy-relationships\> \` ；详情请见  :ref:the model field reference \<ref-foreignkey\> \` 。

建议设置 [`ForeignKey`](/zh-hans/2.1/ref/models/fields/#django.db.models.ForeignKey) 字段（上例中的 `manufacturer` ）名为想要关联的模型名，但是你也可以随意设置为你想要的名称，例如：

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

> **See also**
>
> [`ForeignKey`](/zh-hans/2.1/ref/models/fields/#django.db.models.ForeignKey) 字段还可以接收一些其他的参数，详见 [the model field reference](/zh-hans/2.1/ref/models/fields/#foreign-key-arguments) ，这些可选的参数可以更深入的规定光联关系的具体实现。
>
> For details on accessing backwards-related objects, see the
> [Following relationships backward example](/zh-hans/2.1/topics/db/queries/#backwards-related-objects).
>
> 如要查看相关示例代码，详见 :doc:Many-to-one relationship model example \</topics/db/examples/many\_to\_one\> \` 。

#### Many-to-many relationships

定义一个多对多的关联关系，使用 [`django.db.models.ManyToManyField`](/zh-hans/2.1/ref/models/fields/#django.db.models.ManyToManyField) 类。就和其他 [`Field`](/zh-hans/2.1/ref/models/fields/#django.db.models.Field) 字段类型一样，只需要在你模型中添加一个值为该类的属性。

[`ManyToManyField`](/zh-hans/2.1/ref/models/fields/#django.db.models.ManyToManyField) requires a positional argument: the
class to which the model is related.

例如：如果 `Pizza` 含有多种 ```Topping``（配料） -- 也就是一种 ``Topping``` 可能存在于多个 `Pizza` 中，并且每个 `Pizza`  含有多种 `Topping` --那么可以这样表示这种关系：

```
from django.db import models

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

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

和 [`ForeignKey`](/zh-hans/2.1/ref/models/fields/#django.db.models.ForeignKey) 类一样，你也可以创建 [recursive relationships](/zh-hans/2.1/ref/models/fields/#recursive-relationships) 关系（一个对象与他本身有着多对多的关系）和 [relationships to models not yet defined](/zh-hans/2.1/ref/models/fields/#lazy-relationships) 关系。

建议设置 [`ManyToManyField`](/zh-hans/2.1/ref/models/fields/#django.db.models.ManyToManyField) 字段（上例中的 `toppings` ）名为一个复数名词，表示所要光联的模型对象的集合。

对于多对多光联关系的两个模型，可以在任何一个模型中添加 [`ManyToManyField`](/zh-hans/2.1/ref/models/fields/#django.db.models.ManyToManyField) 字段，但只能选择一个模型设置该字段，即不能同时在两模型中添加该字段。

一般来讲，应该把 [`ManyToManyField`](/zh-hans/2.1/ref/models/fields/#django.db.models.ManyToManyField) 实例放到需要在表单中被编辑的对象中。在之前的例子中， `toppings` 被放在 `Pizza` 当中（而不是 `Topping` 中有指向 `pizzas` 的 [`ManyToManyField`](/zh-hans/2.1/ref/models/fields/#django.db.models.ManyToManyField) 实例 ）因为相较于配料被放在不同的披萨当中，披萨当中有很多种配料更加符合常理。按照先前说的，在编辑 `Pizza` 的表单时用户可以选择多种配料。

> **See also**
>
> 如要查看完整示例代码，详见 [Many-to-many relationship model example](/zh-hans/2.1/topics/db/examples/many_to_many/)。

[`ManyToManyField`](/zh-hans/2.1/ref/models/fields/#django.db.models.ManyToManyField) fields also accept a number of
extra arguments which are explained in [the model field reference](/zh-hans/2.1/ref/models/fields/#manytomany-arguments). These options help define how the relationship
should work; all are optional.

#### 在多对多(many-to-many)关系中添加添加额外的属性字段

如果你只是想要一个类似于记录披萨和配料之间混合和搭配的简单多对多关系，标准的 [`ManyToManyField`](/zh-hans/2.1/ref/models/fields/#django.db.models.ManyToManyField) 就足够你用了。然而，有的时候你可能会需要在两个模型的关系中记录更多的数据。

举例来讲，考虑一个需要跟踪音乐人属于哪个音乐组的应用程序。在人和他们所在的组之间有一个多对多关系，你可以使用 [`ManyToManyField`](/zh-hans/2.1/ref/models/fields/#django.db.models.ManyToManyField) 来代表这个关系。然而，你想要记录更多的信息在这样的所属关系当中，比如你想要记录某人是何时加入一个组的。

对于这些情况，Django允许你指定用于控制多对多关系的模型。你可以在中间模型当中添加而外的字段。在实例化 [`ManyToManyField`](/zh-hans/2.1/ref/models/fields/#django.db.models.ManyToManyField) 的时候使用 [`through`](/zh-hans/2.1/ref/models/fields/#django.db.models.ManyToManyField.through) 参数指定多对多关系使用哪个中间模型。对于我们举的音乐家的例子，代码如下：

```
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)
```

在设置中间模型的时候，你需要显式地为多对多关系中涉及的模型指定外键。这种显式声明定义了这两个模型之间的关系。

在中间模型当中有一些限制条件：

- 你的中间模型要么有且 *仅* 有一个指向源模型（我们例子当中的 `Group` ）的外键，要么你必须通过 [`ManyToManyField.through_fields`](/zh-hans/2.1/ref/models/fields/#django.db.models.ManyToManyField.through_fields) 参数在多个外键当中手动选择一个外键，如果有多个外健且没有用 `through_fields` 参数选择一个的话，会出现验证错误。对于指向目标模型（我们例子当中的 `Person` ）的外键也有同样的限制。
- 在一个用于描述模型当中自己指向自己的多对多关系的中间模型当中，可以有两个指向同一个模型的外健，但这两个外健分表代表多对多关系（不同）的两端。如果外健的个数 *超过* 两个，你必须和上面一样指定 `through_fields` 参数，要不然会出现验证错误。
- 在定义模型自己指向自己的多对多关系时，如果使用中间模型，你 *必须* 定义 [`symmetrical=False`](/zh-hans/2.1/ref/models/fields/#django.db.models.ManyToManyField.symmetrical) （查看 [the model field reference](/zh-hans/2.1/ref/models/fields/#manytomany-arguments)）。

现在你已经通过中间模型完成你的 [`ManyToManyField`](/zh-hans/2.1/ref/models/fields/#django.db.models.ManyToManyField) （例子中的\`\`Membership\`\`），可以开始创建一些多对多关系了。你通过实例化中间模型来创建关系：

```
>>> 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>]>
```

和一般的多对多字段不同，你 *不能* 使用 `add()`， `create()`， 或 `set()` 来创建关系。

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

为什么？你不能简单的在 `Person` 和 `Group` 之间创立关系 - 你需要指定 `Membership` 模型当中需要的所有关于此关系的细节信息。因此，在使用中间模型来定义多对多关系的时候这些方法无法使用。创立这种关系的唯一办法是创建中间模型的实例。

[`remove()`](/zh-hans/2.1/ref/models/relations/#django.db.models.fields.related.RelatedManager.remove) 方法也因为同样的原因无法使用。举例来讲，如果通过中间模型定义的自定义中间表没有确保二元祖 `(model1, model2)` 的唯一，`remove()` 在被调用的时候没有足够的信息来确定哪一个中间模型需要被删除。

```
>>> 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)
```

但是，[`clear()`](/zh-hans/2.1/ref/models/relations/#django.db.models.fields.related.RelatedManager.clear) 方法可以被用来移除一个实例的所有多对多关系：

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

一旦你通过创建中间模型创立了多对多关系，你可以执行查询。就和一般的多对多关系一样，你可以使用多对多关联模型的属性来执行查询：

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

当你使用中间模型的时候，你也可以查询他的属性：

```
# 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]>
```

如果你想访问一个关系的信息时你可以直接查询 `Membership` 模型：

```
>>> 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.'
```

另一种访问同样信息的方法是通过 `Person` 对象来查询 ref:many-to-many reverse relationship\<m2m-reverse-relationships\> ：

```
>>> 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

使用 [`OneToOneField`](/zh-hans/2.1/ref/models/fields/#django.db.models.OneToOneField) 来定义一对一关系。就像使用其他类型的 `Field` 一样：在模型属性中包含它。

当一个对象以某种方式“扩展”另一个对象时，这对该对象的主键非常有用。

[`OneToOneField`](/zh-hans/2.1/ref/models/fields/#django.db.models.OneToOneField) 需要一个位置参数：与模型相关的类。

例如，当你要建立一个有关“位置”信息的数据库时，你可能会包含通常的地址，电话等字段。接着，如果你想接着建立一个关于关于餐厅的数据库，除了将位置数据库当中的字段复制到 `Restaurant` 模型，你也可以将一个指向 `Place` [`OneToOneField`](/zh-hans/2.1/ref/models/fields/#django.db.models.OneToOneField) 放到 `Restaurant` 当中（因为餐厅“是一个”地点）；事实上，在处理这样的情况时最好使用 [inheritance](#model-inheritance) ，它隐含的包括了一个一对一关系。

和  [`ForeignKey`](/zh-hans/2.1/ref/models/fields/#django.db.models.ForeignKey) 一样，可以创建 [recursive relationship](/zh-hans/2.1/ref/models/fields/#recursive-relationships) 也可以创建 [references to as-yet undefined models](/zh-hans/2.1/ref/models/fields/#lazy-relationships) 。

> **See also**
>
> 点击文档 [One-to-one relationship model example](/zh-hans/2.1/topics/db/examples/one_to_one/) 来查看完整的例子。

[`OneToOneField`](/zh-hans/2.1/ref/models/fields/#django.db.models.OneToOneField) 字段还接受一个可选的 [`parent_link`](/zh-hans/2.1/ref/models/fields/#django.db.models.OneToOneField.parent_link) 参数。

[`OneToOneField`](/zh-hans/2.1/ref/models/fields/#django.db.models.OneToOneField) 类通常自动的成为模型的主键，这条规则现在不再使用了（然而你可以手动指定 [`primary_key`](/zh-hans/2.1/ref/models/fields/#django.db.models.Field.primary_key) 参数）。因此，现在可以在单个模型当中指定多个 [`OneToOneField`](/zh-hans/2.1/ref/models/fields/#django.db.models.OneToOneField) 字段。

### 跨文件模型

关联另一个应用中的模型是当然可以的。为了实现这一点，在定义模型的文件开头导入需要被关联的模型。接着，接着就可以在其他有需要的模型类当中关联它了。比如：

```
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,
    )
```

### 字段命名限制

Django在模型字段命名方面仅有两个限制。

1. 一个字段的名称不能是Python保留字，因为这回导致Python语法错误。比如：

   ```
   class Example(models.Model):
       pass = models.IntegerField() # 'pass' is a reserved word!
   ```
2. 一个字段名称不能包含连续的多个下划线，原因在于Django查询语法的工作方式。比如：

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

但是，这些限制是可以被解决的，因为字段名没要求和数据库列名一样。查看 [`db_column`](/zh-hans/2.1/ref/models/fields/#django.db.models.Field.db_column) 选项。

SQL保留字，例如 `join`， `where` 或 `select`， *是* 可以被用在模型字段名当中的，因为Django在对底层的SQL查询当中清洗了所有的数据库表名和字段名，通过使用特定数据库引擎的引用语法。

### 自定义的字段类型

如果已经存在的模型字段不能满足你的需求，或者你希望支持一些不太常见的数据库列类型，你可以创建自己的字段类。在 :doc:/howto/custom-model-fields 中提供了创建自己的字段的各方面内容。

## `Meta` 选项

使用内部 `Meta类` 来给模型赋予元数据，就像：

```
from django.db import models

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

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

模型的元数据是指“所有不是字段的东西”，比如排序选项（attr:~Options.ordering），数据库表名（[`db_table`](/zh-hans/2.1/ref/models/options/#django.db.models.Options.db_table)），或是人可读的单复数名（[`verbose_name`](/zh-hans/2.1/ref/models/options/#django.db.models.Options.verbose_name) 和 [`verbose_name_plural`](/zh-hans/2.1/ref/models/options/#django.db.models.Options.verbose_name_plural)）。都不是必须的，并且在模型当中添加 `Meta类` 也完全是可选的。

在 [model option reference](/zh-hans/2.1/ref/models/options/) 中列出了 `Meta` 可使用的全部选项。

## 模型属性

**`objects`**

  模型当中最重要的属性是 [`Manager`](/zh-hans/2.1/topics/db/managers/#django.db.models.Manager)。它是Django模型和数据库查询操作之间的接口，并且它被用作从数据库当中 [retrieve the instances](/zh-hans/2.1/topics/db/queries/#retrieving-objects)，如果没有指定自定义的 `Manager` 默认名称是 [`objects`](/zh-hans/2.1/ref/models/class/#django.db.models.Model.objects)。Manager只能通过模型类来访问，不能通过模型实例来访问。

## 模型方法

Define custom methods on a model to add custom "row-level" functionality to your
objects. Whereas [`Manager`](/zh-hans/2.1/topics/db/managers/#django.db.models.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](/zh-hans/2.1/glossary/#term-property).

The [model instance reference](/zh-hans/2.1/ref/models/instances/) has a complete list
of [methods automatically given to each model](/zh-hans/2.1/ref/models/instances/#model-instance-methods).
You can override most of these -- see [overriding predefined model methods](#overriding-predefined-model-methods),
below -- but there are a couple that you'll almost always want to define:

**[`__str__()`](/zh-hans/2.1/ref/models/instances/#django.db.models.Model.__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()`](/zh-hans/2.1/ref/models/instances/#django.db.models.Model.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](/zh-hans/2.1/ref/models/instances/#model-instance-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()`](/zh-hans/2.1/ref/models/instances/#django.db.models.Model.save) and
[`delete()`](/zh-hans/2.1/ref/models/instances/#django.db.models.Model.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()`](/zh-hans/2.1/ref/models/instances/#django.db.models.Model.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()`](/zh-hans/2.1/ref/models/instances/#django.db.models.Model.delete) method for an object is not
> necessarily called when [deleting objects in bulk using a
> QuerySet](/zh-hans/2.1/topics/db/queries/#topics-db-queries-delete) or as a result of a [`cascading
> delete`](/zh-hans/2.1/ref/models/fields/#django.db.models.ForeignKey.on_delete). To ensure customized
> delete logic gets executed, you can use
> [`pre_delete`](/zh-hans/2.1/ref/signals/#django.db.models.signals.pre_delete) and/or
> [`post_delete`](/zh-hans/2.1/ref/signals/#django.db.models.signals.post_delete) signals.
>
> Unfortunately, there isn't a workaround when
> [`creating`](/zh-hans/2.1/ref/models/querysets/#django.db.models.query.QuerySet.bulk_create) or
> [`updating`](/zh-hans/2.1/ref/models/querysets/#django.db.models.query.QuerySet.update) objects in bulk,
> since none of [`save()`](/zh-hans/2.1/ref/models/instances/#django.db.models.Model.save),
> [`pre_save`](/zh-hans/2.1/ref/signals/#django.db.models.signals.pre_save), and
> [`post_save`](/zh-hans/2.1/ref/signals/#django.db.models.signals.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](/zh-hans/2.1/topics/db/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`](/zh-hans/2.1/ref/models/instances/#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](#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](#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](#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](#meta-options)
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](#meta-options)
inner class you declared in the base class available as an
attribute. If a child class does not declare its own [Meta](#meta-options)
class, it will inherit the parent's [Meta](#meta-options). If the child wants to
extend the parent's [Meta](#meta-options) 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](#meta-options) class of an abstract base
class: before installing the [Meta](#meta-options) 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](#meta-options) 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](#meta-options)) 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`](/zh-hans/2.1/ref/models/fields/#django.db.models.ForeignKey.related_name) or
[`related_query_name`](/zh-hans/2.1/ref/models/fields/#django.db.models.ForeignKey.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`](/zh-hans/2.1/ref/models/fields/#django.db.models.ForeignKey.related_name) and
[`related_query_name`](/zh-hans/2.1/ref/models/fields/#django.db.models.ForeignKey.related_query_name)) each time.

To work around this problem, when you are using
[`related_name`](/zh-hans/2.1/ref/models/fields/#django.db.models.ForeignKey.related_name) or
[`related_query_name`](/zh-hans/2.1/ref/models/fields/#django.db.models.ForeignKey.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`](/zh-hans/2.1/ref/django-admin/#django-admin-migrate)).

If you don't specify a [`related_name`](/zh-hans/2.1/ref/models/fields/#django.db.models.ForeignKey.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`](/zh-hans/2.1/ref/models/fields/#django.db.models.ForeignKey.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.

### 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`](/zh-hans/2.1/ref/models/fields/#django.db.models.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`](/zh-hans/2.1/ref/models/fields/#django.db.models.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`](/zh-hans/2.1/ref/models/fields/#django.db.models.OneToOneField) with [`parent_link=True`](/zh-hans/2.1/ref/models/fields/#django.db.models.OneToOneField.parent_link) 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](#meta-options) class. All the [Meta](#meta-options) 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](#meta-options) class. However, there are a few limited cases where the child
inherits behavior from the parent: if the child does not specify an
[`ordering`](/zh-hans/2.1/ref/models/options/#django.db.models.Options.ordering) attribute or a
[`get_latest_by`](/zh-hans/2.1/ref/models/options/#django.db.models.Options.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`](/zh-hans/2.1/ref/models/fields/#django.db.models.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`](/zh-hans/2.1/ref/models/fields/#django.db.models.ForeignKey.related_name) value for
[`ForeignKey`](/zh-hans/2.1/ref/models/fields/#django.db.models.ForeignKey) and
[`ManyToManyField`](/zh-hans/2.1/ref/models/fields/#django.db.models.ManyToManyField) relations.  If you
are putting those types of relations on a subclass of the parent model, you
**must** specify the [`related_name`](/zh-hans/2.1/ref/models/fields/#django.db.models.ForeignKey.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`](/zh-hans/2.1/ref/models/fields/#django.db.models.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')`.

#### Specifying the parent link field

As mentioned, Django will automatically create a
[`OneToOneField`](/zh-hans/2.1/ref/models/fields/#django.db.models.OneToOneField) linking your child
class back to any non-abstract parent models. If you want to control the
name of the attribute linking back to the parent, you can create your
own [`OneToOneField`](/zh-hans/2.1/ref/models/fields/#django.db.models.OneToOneField) and set
[`parent_link=True`](/zh-hans/2.1/ref/models/fields/#django.db.models.OneToOneField.parent_link)
to indicate that your field is the link back to the parent class.

### Proxy models

When using [multi-table inheritance](#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`](/zh-hans/2.1/ref/models/options/#django.db.models.Options.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\>。

#### `QuerySet`s 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](/zh-hans/2.1/topics/db/managers/#custom-managers-and-inheritance) 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`](/zh-hans/2.1/ref/models/options/#django.db.models.Options.managed) attribute on a
model's `Meta` class.

With careful setting of [`Meta.db_table`](/zh-hans/2.1/ref/models/options/#django.db.models.Options.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](#meta-options)) appears in will be the one that is used; for example, this
means that if multiple parents contain a [Meta](#meta-options) 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`](/zh-hans/2.1/ref/models/fields/#django.db.models.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`](/zh-hans/2.1/ref/models/fields/#django.db.models.AutoField). This
requires using an explicit [`OneToOneField`](/zh-hans/2.1/ref/models/fields/#django.db.models.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`](/zh-hans/2.1/topics/db/managers/#django.db.models.Manager) may cause subtle bugs. See [custom
> managers and model inheritance](/zh-hans/2.1/topics/db/managers/#custom-managers-and-inheritance).

> **Note**
>
> Some fields define extra attributes on the model, e.g. a
> [`ForeignKey`](/zh-hans/2.1/ref/models/fields/#django.db.models.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`](/zh-hans/2.1/ref/models/fields/#django.db.models.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`](/zh-hans/2.1/ref/exceptions/#django.core.exceptions.FieldError) if you override
any model field in any ancestor model.

## Organizing models in a package

The [`manage.py startapp`](/zh-hans/2.1/ref/django-admin/#django-admin-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*

```python
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](/zh-hans/2.1/ref/models/)**
>
>   Covers all the model related APIs including model fields, related
>   objects, and `QuerySet`.