---
title: "模型"
version: 2.0
locale: zh-hans
source: https://docs.djangoproject.com/zh-hans/2.0/topics/db/models/
canonical: https://djangodocs.dev/zh-hans/2.0/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/2.0/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 [Making queries](/zh-hans/2.0/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/2.0/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/2.0/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',
#...
]
```
When you add new apps to [`INSTALLED_APPS`](/zh-hans/2.0/ref/settings/#std-setting-INSTALLED_APPS), be sure to run
[`manage.py migrate`](/zh-hans/2.0/ref/django-admin/#django-admin-migrate), optionally making migrations
for them first with [`manage.py makemigrations`](/zh-hans/2.0/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\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`](/zh-hans/2.0/ref/models/fields/#django.db.models.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\（如：\\）
- 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\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](/zh-hans/2.0/ref/models/fields/#model-field-types)). For example,
[`CharField`](/zh-hans/2.0/ref/models/fields/#django.db.models.CharField) (and its subclasses) require a
[`max_length`](/zh-hans/2.0/ref/models/fields/#django.db.models.CharField.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](/zh-hans/2.0/ref/models/fields/#common-model-field-options), but here's a quick summary of the most often-used
ones:
**[`null`](/zh-hans/2.0/ref/models/fields/#django.db.models.Field.null)**
If `True`, Django will store empty values as `NULL` in the database.
Default is `False`.
**[`blank`](/zh-hans/2.0/ref/models/fields/#django.db.models.Field.blank)**
If `True`, the field is allowed to be blank. Default is `False`.
Note that this is different than [`null`](/zh-hans/2.0/ref/models/fields/#django.db.models.Field.null).
[`null`](/zh-hans/2.0/ref/models/fields/#django.db.models.Field.null) is purely database-related, whereas
[`blank`](/zh-hans/2.0/ref/models/fields/#django.db.models.Field.blank) is validation-related. If a field has
[`blank=True`](/zh-hans/2.0/ref/models/fields/#django.db.models.Field.blank), form validation will
allow entry of an empty value. If a field has [`blank=False`](/zh-hans/2.0/ref/models/fields/#django.db.models.Field.blank), the field will be required.
**[`choices`](/zh-hans/2.0/ref/models/fields/#django.db.models.Field.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()`](/zh-hans/2.0/ref/models/instances/#django.db.models.Model.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`](/zh-hans/2.0/ref/models/fields/#django.db.models.Field.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`](/zh-hans/2.0/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/2.0/ref/models/fields/#django.db.models.Field.primary_key)**
If `True`, this field is the primary key for the model.
If you don't specify [`primary_key=True`](/zh-hans/2.0/ref/models/fields/#django.db.models.Field.primary_key) for
any fields in your model, Django will automatically add an
[`IntegerField`](/zh-hans/2.0/ref/models/fields/#django.db.models.IntegerField) to hold the primary key, so you don't need to set
[`primary_key=True`](/zh-hans/2.0/ref/models/fields/#django.db.models.Field.primary_key) on any of your fields
unless you want to override the default primary-key behavior. For more,
see [Automatic primary key fields](#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)
```
```pycon
>>> fruit = Fruit.objects.create(name='Apple')
>>> fruit.name = 'Pear'
>>> fruit.save()
>>> Fruit.objects.values_list('name', flat=True)
```
**[`unique`](/zh-hans/2.0/ref/models/fields/#django.db.models.Field.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](/zh-hans/2.0/ref/models/fields/#common-model-field-options).
### 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`](/zh-hans/2.0/ref/models/fields/#django.db.models.Field.primary_key) on one of your fields. If Django
sees you've explicitly set [`Field.primary_key`](/zh-hans/2.0/ref/models/fields/#django.db.models.Field.primary_key), it won't add the automatic
`id` column.
Each model requires exactly one field to have [`primary_key=True`](/zh-hans/2.0/ref/models/fields/#django.db.models.Field.primary_key) (either explicitly declared or automatically added).
### Verbose field names
Each field type, except for [`ForeignKey`](/zh-hans/2.0/ref/models/fields/#django.db.models.ForeignKey),
[`ManyToManyField`](/zh-hans/2.0/ref/models/fields/#django.db.models.ManyToManyField) and
[`OneToOneField`](/zh-hans/2.0/ref/models/fields/#django.db.models.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`](/zh-hans/2.0/ref/models/fields/#django.db.models.ForeignKey),
[`ManyToManyField`](/zh-hans/2.0/ref/models/fields/#django.db.models.ManyToManyField) and
[`OneToOneField`](/zh-hans/2.0/ref/models/fields/#django.db.models.OneToOneField) require the first argument to be a
model class, so use the [`verbose_name`](/zh-hans/2.0/ref/models/fields/#django.db.models.Field.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`](/zh-hans/2.0/ref/models/fields/#django.db.models.Field.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`](/zh-hans/2.0/ref/models/fields/#django.db.models.ForeignKey).
You use it just like any other [`Field`](/zh-hans/2.0/ref/models/fields/#django.db.models.Field) type: by
including it as a class attribute of your model.
[`ForeignKey`](/zh-hans/2.0/ref/models/fields/#django.db.models.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](/zh-hans/2.0/ref/models/fields/#recursive-relationships) (an
object with a many-to-one relationship to itself) and [relationships to
models not yet defined](/zh-hans/2.0/ref/models/fields/#lazy-relationships); see [the model field
reference](/zh-hans/2.0/ref/models/fields/#ref-foreignkey) for details.
It's suggested, but not required, that the name of a
[`ForeignKey`](/zh-hans/2.0/ref/models/fields/#django.db.models.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`](/zh-hans/2.0/ref/models/fields/#django.db.models.ForeignKey) fields accept a number of extra
> arguments which are explained in [the model field reference](/zh-hans/2.0/ref/models/fields/#foreign-key-arguments). 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](/zh-hans/2.0/topics/db/queries/#backwards-related-objects).
>
> For sample code, see the [Many-to-one relationship model example](/zh-hans/2.0/topics/db/examples/many_to_one/).
#### Many-to-many relationships
To define a many-to-many relationship, use
[`ManyToManyField`](/zh-hans/2.0/ref/models/fields/#django.db.models.ManyToManyField). You use it just like any other
[`Field`](/zh-hans/2.0/ref/models/fields/#django.db.models.Field) type: by including it as a class attribute of
your model.
[`ManyToManyField`](/zh-hans/2.0/ref/models/fields/#django.db.models.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`](/zh-hans/2.0/ref/models/fields/#django.db.models.ForeignKey), you can also create
[recursive relationships](/zh-hans/2.0/ref/models/fields/#recursive-relationships) (an object with a
many-to-many relationship to itself) and [relationships to models not yet
defined](/zh-hans/2.0/ref/models/fields/#lazy-relationships).
It's suggested, but not required, that the name of a
[`ManyToManyField`](/zh-hans/2.0/ref/models/fields/#django.db.models.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`](/zh-hans/2.0/ref/models/fields/#django.db.models.ManyToManyField), but you should only put it in one
of the models -- not both.
Generally, [`ManyToManyField`](/zh-hans/2.0/ref/models/fields/#django.db.models.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`](/zh-hans/2.0/ref/models/fields/#django.db.models.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](/zh-hans/2.0/topics/db/examples/many_to_many/) for a full example.
[`ManyToManyField`](/zh-hans/2.0/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/2.0/ref/models/fields/#manytomany-arguments). 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`](/zh-hans/2.0/ref/models/fields/#django.db.models.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`](/zh-hans/2.0/ref/models/fields/#django.db.models.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`](/zh-hans/2.0/ref/models/fields/#django.db.models.ManyToManyField) using the
[`through`](/zh-hans/2.0/ref/models/fields/#django.db.models.ManyToManyField.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`](/zh-hans/2.0/ref/models/fields/#django.db.models.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`](/zh-hans/2.0/ref/models/fields/#django.db.models.ManyToManyField.symmetrical) (see
[the model field reference](/zh-hans/2.0/ref/models/fields/#manytomany-arguments)).
Now that you have set up your [`ManyToManyField`](/zh-hans/2.0/ref/models/fields/#django.db.models.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()
]>
>>> ringo.group_set.all()
]>
>>> m2 = Membership.objects.create(person=paul, group=beatles,
... date_joined=date(1960, 8, 1),
... invite_reason="Wanted to form a band.")
>>> beatles.members.all()
, ]>
```
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()`](/zh-hans/2.0/ref/models/relations/#django.db.models.fields.related.RelatedManager.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()
, , ]>
>>> # This will not work because it cannot tell which membership to remove
>>> beatles.members.remove(ringo)
```
However, the [`clear()`](/zh-hans/2.0/ref/models/relations/#django.db.models.fields.related.RelatedManager.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()
```
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')
]>
```
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))
```
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](/zh-hans/2.0/topics/db/queries/#m2m-reverse-relationships) 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`](/zh-hans/2.0/ref/models/fields/#django.db.models.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`](/zh-hans/2.0/ref/models/fields/#django.db.models.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`](/zh-hans/2.0/ref/models/fields/#django.db.models.OneToOneField) to `Place` (because a
restaurant "is a" place; in fact, to handle this you'd typically use
[inheritance](#model-inheritance), which involves an implicit
one-to-one relation).
As with [`ForeignKey`](/zh-hans/2.0/ref/models/fields/#django.db.models.ForeignKey), a [recursive relationship](/zh-hans/2.0/ref/models/fields/#recursive-relationships) can be defined and [references to as-yet
undefined models](/zh-hans/2.0/ref/models/fields/#lazy-relationships) can be made.
> **See also**
>
> See the [One-to-one relationship model example](/zh-hans/2.0/topics/db/examples/one_to_one/) for a full example.
[`OneToOneField`](/zh-hans/2.0/ref/models/fields/#django.db.models.OneToOneField) fields also accept an optional
[`parent_link`](/zh-hans/2.0/ref/models/fields/#django.db.models.OneToOneField.parent_link) argument.
[`OneToOneField`](/zh-hans/2.0/ref/models/fields/#django.db.models.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`](/zh-hans/2.0/ref/models/fields/#django.db.models.Field.primary_key) argument if you like).
Thus, it's now possible to have multiple fields of type
[`OneToOneField`](/zh-hans/2.0/ref/models/fields/#django.db.models.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`](/zh-hans/2.0/ref/models/fields/#django.db.models.Field.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](/zh-hans/2.0/howto/custom-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`](/zh-hans/2.0/ref/models/options/#django.db.models.Options.ordering)), database table name ([`db_table`](/zh-hans/2.0/ref/models/options/#django.db.models.Options.db_table)), or
human-readable singular and plural names ([`verbose_name`](/zh-hans/2.0/ref/models/options/#django.db.models.Options.verbose_name) and
[`verbose_name_plural`](/zh-hans/2.0/ref/models/options/#django.db.models.Options.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](/zh-hans/2.0/ref/models/options/).
## Model attributes
**`objects`**
The most important attribute of a model is the
[`Manager`](/zh-hans/2.0/topics/db/managers/#django.db.models.Manager). It's the interface through which
database query operations are provided to Django models and is used to
[retrieve the instances](/zh-hans/2.0/topics/db/queries/#retrieving-objects) from the database. If no
custom `Manager` is defined, the default name is
[`objects`](/zh-hans/2.0/ref/models/class/#django.db.models.Model.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`](/zh-hans/2.0/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/2.0/glossary/#term-property).
The [model instance reference](/zh-hans/2.0/ref/models/instances/) has a complete list
of [methods automatically given to each model](/zh-hans/2.0/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/2.0/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/2.0/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/2.0/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/2.0/ref/models/instances/#django.db.models.Model.save) and
[`delete()`](/zh-hans/2.0/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/2.0/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/2.0/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/2.0/topics/db/queries/#topics-db-queries-delete) or as a result of a [`cascading
> delete`](/zh-hans/2.0/ref/models/fields/#django.db.models.ForeignKey.on_delete). To ensure customized
> delete logic gets executed, you can use
> [`pre_delete`](/zh-hans/2.0/ref/signals/#django.db.models.signals.pre_delete) and/or
> [`post_delete`](/zh-hans/2.0/ref/signals/#django.db.models.signals.post_delete) signals.
>
> Unfortunately, there isn't a workaround when
> [`creating`](/zh-hans/2.0/ref/models/querysets/#django.db.models.query.QuerySet.bulk_create) or
> [`updating`](/zh-hans/2.0/ref/models/querysets/#django.db.models.query.QuerySet.update) objects in bulk,
> since none of [`save()`](/zh-hans/2.0/ref/models/instances/#django.db.models.Model.save),
> [`pre_save`](/zh-hans/2.0/ref/signals/#django.db.models.signals.pre_save), and
> [`post_save`](/zh-hans/2.0/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/2.0/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/2.0/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/2.0/ref/models/fields/#django.db.models.ForeignKey.related_name) or
[`related_query_name`](/zh-hans/2.0/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/2.0/ref/models/fields/#django.db.models.ForeignKey.related_name) and
[`related_query_name`](/zh-hans/2.0/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/2.0/ref/models/fields/#django.db.models.ForeignKey.related_name) or
[`related_query_name`](/zh-hans/2.0/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/2.0/ref/django-admin/#django-admin-migrate)).
If you don't specify a [`related_name`](/zh-hans/2.0/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/2.0/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/2.0/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
```
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/2.0/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/2.0/ref/models/fields/#django.db.models.OneToOneField) with [`parent_link=True`](/zh-hans/2.0/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/2.0/ref/models/options/#django.db.models.Options.ordering) attribute or a
[`get_latest_by`](/zh-hans/2.0/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/2.0/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/2.0/ref/models/fields/#django.db.models.ForeignKey.related_name) value for
[`ForeignKey`](/zh-hans/2.0/ref/models/fields/#django.db.models.ForeignKey) and
[`ManyToManyField`](/zh-hans/2.0/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/2.0/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/2.0/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/2.0/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/2.0/ref/models/fields/#django.db.models.OneToOneField) and set
[`parent_link=True`](/zh-hans/2.0/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/2.0/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")
```
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\。
#### `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/2.0/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/2.0/ref/models/options/#django.db.models.Options.managed) attribute on a
model's `Meta` class.
With careful setting of [`Meta.db_table`](/zh-hans/2.0/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/2.0/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/2.0/ref/models/fields/#django.db.models.AutoField). This
requires using an explicit [`OneToOneField`](/zh-hans/2.0/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/2.0/topics/db/managers/#django.db.models.Manager) may cause subtle bugs. See [custom
> managers and model inheritance](/zh-hans/2.0/topics/db/managers/#custom-managers-and-inheritance).
> **Note**
>
> Some fields define extra attributes on the model, e.g. a
> [`ForeignKey`](/zh-hans/2.0/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/2.0/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/2.0/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/2.0/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*
```
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/2.0/ref/models/)**
>
> Covers all the model related APIs including model fields, related
> objects, and `QuerySet`.