- 有了这一切，Django给你自动生成数据库的访问API；详见 Making queries。
from django.db import models class Person(models.Model): first_name = models.CharField(max_length=30) last_name = models.CharField(max_length=30)
last_name 是模型的字段 fields_ 。 每个字段被指定为一个类的属性，
CREATE TABLE myapp_person ( "id" serial NOT NULL PRIMARY KEY, "first_name" varchar(30) NOT NULL, "last_name" varchar(30) NOT NULL );
INSTALLED_APPS = ( #... 'myapp', #... )
不要选择与 models API 冲突的名字命名字段。
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) name = models.CharField(max_length=100) release_date = models.DateField() num_stars = models.IntegerField()
- 用于渲染表单字段的小部件 widget （如：
还有提供给所有字段类型的常见参数，这些都是可选的。 详情可查看 reference 。这里总结一下经常使用的通用参数：
YEAR_IN_SCHOOL_CHOICES = ( ('FR', 'Freshman'), ('SO', 'Sophomore'), ('JR', 'Junior'), ('SR', 'Senior'), ('GR', 'Graduate'), )
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=2, choices=SHIRT_SIZES)
>>> p = Person(name="Fred Flintstone", shirt_size="L") >>> p.save() >>> p.shirt_size u'L' >>> p.get_shirt_size_display() u'Large'
这些都是这些字段的简短描述，详情可参阅 common model field option reference 。
id = models.AutoField(primary_key=True)
Verbose field names¶
Each field type, except for
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
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 = models.CharField(max_length=30)
poll = models.ForeignKey(Poll, verbose_name="the related poll") sites = models.ManyToManyField(Site, verbose_name="list of sites") place = models.OneToOneField(Place, verbose_name="related place")
The convention is not to capitalize the first letter of the
verbose_name. Django will automatically capitalize the first
letter where it needs to.
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.
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:
class Manufacturer(models.Model): # ... class Car(models.Model): manufacturer = models.ForeignKey(Manufacturer) # ...
It’s suggested, but not required, that the name of a
ForeignKey field (
manufacturer in the example
above) be the name of the model, lowercase. You can, of course, call the field
whatever you want. For example:
class Car(models.Model): company_that_makes_it = models.ForeignKey(Manufacturer) # ...
For details on accessing backwards-related objects, see the Following relationships backward example.
For sample code, see the Many-to-one relationship model example.
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:
class Topping(models.Model): # ... class Pizza(models.Model): # ... toppings = models.ManyToManyField(Topping)
ForeignKey, you can also create
recursive relationships (an object with a
many-to-many relationship to itself) and relationships to models not yet
defined; see the model field reference for details.
It’s suggested, but not required, that the name of a
toppings in the example above)
be a plural describing the set of related model objects.
It doesn’t matter which model has the
ManyToManyField, but you should only put it in one
of the models – not both.
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
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 the Many-to-many relationship model example for a full example.
Extra fields on many-to-many relationships¶
When you’re only dealing with simple many-to-many relationships such as
mixing and matching pizzas and toppings, a standard
ManyToManyField is all you need. However, sometimes
you may need to associate data with the relationship between two models.
For example, consider the case of an application tracking the musical groups
which musicians belong to. There is a many-to-many relationship between a person
and the groups of which they are a member, so you could use a
ManyToManyField to represent this relationship.
However, there is a lot of detail about the membership that you might want to
collect, such as the date at which the person joined the group.
For these situations, Django allows you to specify the model that will be used
to govern the many-to-many relationship. You can then put extra fields on the
intermediate model. The intermediate model is associated with the
ManyToManyField using the
through argument to point to the model
that will act as an intermediary. For our musician example, the code would look
something like this:
class Person(models.Model): name = models.CharField(max_length=128) def __unicode__(self): return self.name class Group(models.Model): name = models.CharField(max_length=128) members = models.ManyToManyField(Person, through='Membership') def __unicode__(self): return self.name class Membership(models.Model): person = models.ForeignKey(Person) group = models.ForeignKey(Group) 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 ManyToMany relation. 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 target model (this would be
Personin our example). If you have more than one foreign key, a validation error will be raised.
- Your intermediate model must contain one - and only one - foreign key
to the source model (this would be
Groupin our example). If you have more than one foreign key, a validation error will be raised.
- The only exception to this is a model which has a many-to-many relationship to itself, through an intermediary model. In this case, two foreign keys to the same model are permitted, but they will be treated as the two (different) sides of the many-to-many relation.
- When defining a many-to-many relationship from a model to
itself, using an intermediary model, you must use
symmetrical=False(see the model field reference).
Now that you have set up your
ManyToManyField to use
your intermediary model (
Membership, in this case), you’re ready to start
creating some many-to-many relationships. You do this by creating instances of
the intermediate model:
>>> ringo = Person.objects.create(name="Ringo Starr") >>> paul = Person.objects.create(name="Paul McCartney") >>> beatles = Group.objects.create(name="The Beatles") >>> m1 = Membership(person=ringo, group=beatles, ... date_joined=date(1962, 8, 16), ... invite_reason= "Needed a new drummer.") >>> m1.save() >>> beatles.members.all() [<Person: Ringo Starr>] >>> ringo.group_set.all() [<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() [<Person: Ringo Starr>, <Person: Paul McCartney>]
Unlike normal many-to-many fields, you can’t use
or assignment (i.e.,
beatles.members = [...]) to create relationships:
# THIS WILL NOT WORK >>> beatles.members.add(john) # NEITHER WILL THIS >>> beatles.members.create(name="George Harrison") # AND NEITHER WILL THIS >>> beatles.members = [john, paul, ringo, george]
Why? You can’t just create a relationship between a
Person and a
- you need to specify all the detail for the relationship required by the
Membership model. The simple
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
# Beatles have broken up >>> beatles.members.clear()
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') [<Group: The Beatles>]
As you are using an intermediate model, you can also query on its attributes:
# Find all the members of the Beatles that joined after 1 Jan 1961 >>> Person.objects.filter( ... group__name='The Beatles', ... membership__date_joined__gt=date(1961,1,1)) [<Person: Ringo Starr]
If you need to access a membership’s information you may do so by directly
>>> ringos_membership = Membership.objects.get(group=beatles, person=ringo) >>> ringos_membership.date_joined datetime.date(1962, 8, 16) >>> ringos_membership.invite_reason u'Needed a new drummer.'
Another way to access the same information is by querying the
many-to-many reverse relationship from a
>>> ringos_membership = ringo.membership_set.get(group=beatles) >>> ringos_membership.date_joined datetime.date(1962, 8, 16) >>> ringos_membership.invite_reason u'Needed a new drummer.'
To define a one-to-one relationship, use
OneToOneField. You use it just like any other
Field type: by including it as a class attribute of your model.
This is most useful on the primary key of an object when that object “extends” another object in some way.
OneToOneField requires a positional argument: the
class to which the model is related.
For example, if you were building a database of “places”, you would
build pretty standard stuff such as address, phone number, etc. in the
database. Then, if you wanted to build a database of restaurants on
top of the places, instead of repeating yourself and replicating those
fields in the
Restaurant model, you could make
Place (because a
restaurant “is a” place; in fact, to handle this you’d typically use
inheritance, which involves an implicit
See the One-to-one relationship model example for a full example.
OneToOneField classes used to automatically become
the primary key on a model. This is no longer true (although you can manually
pass in the
primary_key argument if you like).
Thus, it’s now possible to have multiple fields of type
OneToOneField on a single model.
Models across files¶
It’s perfectly OK to relate a model to one from another app. To do this, import the related model at the top of the file where your model is defined. Then, just refer to the other model class wherever needed. For example:
from geography.models import ZipCode class Restaurant(models.Model): # ... zip_code = models.ForeignKey(ZipCode)
Field name restrictions¶
Django places only two restrictions on model field names:
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!
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
SQL reserved words, such as
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.
Give your model metadata by using an inner
class Meta, like so:
class Ox(models.Model): horn_length = models.IntegerField() class Meta: ordering = ["horn_length"] verbose_name_plural = "oxen"
Model metadata is “anything that’s not a field”, such as ordering options
ordering), database table name (
human-readable singular and plural names (
verbose_name_plural). None are required, and adding
Meta to a model is completely optional.
A complete list of all possible
Meta options can be found in the model
Define custom methods on a model to add custom “row-level” functionality to your
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.contrib.localflavor.us.models import USStateField class Person(models.Model): first_name = models.CharField(max_length=50) last_name = models.CharField(max_length=50) birth_date = models.DateField() address = models.CharField(max_length=100) city = models.CharField(max_length=50) state = USStateField() # Yes, this is America-centric... 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" def is_midwestern(self): "Returns True if this person is from the Midwest." return self.state in ('IL', 'WI', 'MI', 'IN', 'OH', 'IA', 'MO') def _get_full_name(self): "Returns the person's full name." return '%s %s' % (self.first_name, self.last_name) full_name = property(_get_full_name)
The last method in this example is a property.
The model instance reference has a complete list of methods automatically given to each model. You can override most of these – see overriding predefined model methods, below – but there are a couple that you’ll almost always want to define:
A Python “magic method” that returns a unicode “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.
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¶
You’re free to override these methods (and any other model method) to alter behavior.
A classic use-case for overriding the built-in methods is if you want something
to happen whenever you save an object. For example (see
save() for documentation of the parameters it accepts):
class Blog(models.Model): name = models.CharField(max_length=100) tagline = models.TextField() def save(self, *args, **kwargs): do_something() super(Blog, self).save(*args, **kwargs) # Call the "real" save() method. do_something_else()
You can also prevent saving:
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(Blog, self).save(*args, **kwargs) # Call the "real" save() method.
It’s important to remember to call the superclass method – that’s
super(Blog, self).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
**kwargs in your method definitions, you are guaranteed that your
code will automatically support those arguments when they are added.
Overridden model methods are not called on bulk operations
Note that the
delete() method for an object is not
necessarily called when deleting objects in bulk using a
QuerySet. To ensure customized delete logic
gets executed, you can use
Model inheritance in Django works almost identically to the way normal class inheritance works in Python. 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.
- Often, you will just want to use the parent class to hold information that you don’t want to have to type out for each child model. This class isn’t going to ever be used in isolation, so Abstract base classes are what you’re after.
- If you’re subclassing an existing model (perhaps something from another application entirely) and want each model to have its own database table, Multi-table inheritance is the way to go.
- Finally, if you only want to modify the Python-level behavior of a model, without changing the models fields in any way, you can use Proxy models.
Abstract base classes¶
Abstract base classes are useful when you want to put some common
information into a number of other models. You write your base class
abstract=True in the Meta
class. This model will then not be used to create any database
table. Instead, when it is used as a base class for other models, its
fields will be added to those of the child class. It is an error to
have fields in the abstract base class with the same name as those in
the child (and Django will raise an exception).
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)
Student model will have three fields:
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.
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, whilst still only creating one database table per child model at the database level.
When an abstract base class is created, Django makes any Meta inner class you declared in the base class available as an attribute. If a child class does not declare its own Meta class, it will inherit the parent’s Meta. If the child wants to extend the parent’s Meta class, it can subclass it. For example:
class CommonInfo(models.Model): ... class Meta: abstract = True ordering = ['name'] class Student(CommonInfo): ... class Meta(CommonInfo.Meta): db_table = 'student_info'
Django does make one adjustment to the Meta class of an abstract base
class: before installing the Meta attribute, it sets
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
abstract=True each time.
Some attributes won’t make sense to include in the Meta class of an
abstract base class. For example, including
db_table would mean that all
the child classes (the ones that don’t specify their own Meta) would use
the same database table, which is almost certainly not what you want.
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
class Place(models.Model): name = models.CharField(max_length=50) address = models.CharField(max_length=80) class Restaurant(Place): serves_hot_dogs = models.BooleanField() serves_pizza = models.BooleanField()
All of the fields of
Place will also be available in
although the data will reside in a different database table. So these are both
>>> 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: ...>
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),
p.restaurant would raise a Restaurant.DoesNotExist exception.
Meta and multi-table inheritance¶
In the multi-table inheritance situation, it doesn’t make sense for a child class to inherit from its parent’s Meta class. All the Meta options have already been applied to the parent class and applying them again would normally only lead to contradictory behavior (this is in contrast with the abstract base class case, where the base class doesn’t exist in its own right).
So a child model does not have access to its parent’s Meta class. However, there are a few limited cases where the child
inherits behavior from the parent: if the child does not specify an
ordering attribute or a
get_latest_by attribute, it will inherit
these from its parent.
If the parent has an ordering and you don’t want the child to have any natural ordering, you can explicitly disable it:
class ChildModel(ParentModel): ... class Meta: # Remove parent's ordering effect ordering = 
Inheritance and reverse relations¶
Because multi-table inheritance uses an implicit
OneToOneField to link the child and
the parent, it’s possible to move from the parent down to the child,
as in the above example. However, this uses up the name that is the
related_name value for
ManyToManyField relations. If you
are putting those types of relations on a subclass of another model,
you must specify the
related_name attribute on each
such field. If you forget, Django will raise an error when you run
For example, using the above
Place class again, let’s create another
subclass with a
class Supplier(Place): # Must specify related_name on all relations. customers = models.ManyToManyField(Restaurant, related_name='provider')
When using multi-table inheritance, a new database table is created for each subclass of a model. This is usually the desired behavior, since the subclass needs a place to store any additional data fields that are not present on the base class. Sometimes, however, you only want to change the Python behavior of a model – perhaps to change the default manager, or add a new method.
This is what proxy model inheritance is for: creating a proxy for the original model. You can create, delete and update instances of the proxy model and all the data will be saved as if you were using the original (non-proxied) model. The difference is that you can change things like the default model ordering or the default manager in the proxy, without having to alter the original.
Proxy models are declared like normal models. You tell Django that it’s a
proxy model by setting the
proxy attribute of
Meta class to
For example, suppose you want to add a method to the
Person model described
above. You can do it like this:
class MyPerson(Person): class Meta: proxy = True def do_something(self): ...
MyPerson class operates on the same database table as its parent
Person class. In particular, any new instances of
Person will also be
MyPerson, and vice-versa:
>>> p = Person.objects.create(first_name="foobar") >>> MyPerson.objects.get(first_name="foobar") <MyPerson: foobar>
You could also use a proxy model to define a different default ordering on
a model. You might not always want to order the
Person model, but regularly
order by the
last_name attribute when you use the proxy. This is easy:
class OrderedPerson(Person): class Meta: ordering = ["last_name"] proxy = True
Person queries will be unordered
OrderedPerson queries will be ordered by
QuerySets still return the model that was requested¶
There is no way to have Django return, say, a
MyPerson object whenever you
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¶
A proxy model must inherit from exactly one non-abstract model class. You can’t inherit from multiple non-abstract models as the proxy model doesn’t provide any connection between the rows in the different database tables. A proxy model can inherit from any number of abstract model classes, providing they do not define any model fields.
Proxy models inherit any
Meta options that they don’t define from their
non-abstract model parent (the model they are proxying for).
Proxy model managers¶
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:
class NewManager(models.Manager): ... class MyPerson(Person): objects = NewManager() class Meta: proxy = True
If you wanted to add a new manager to the Proxy, without replacing the existing default, you can use the techniques described in the custom manager documentation: create a base class containing the new managers and inherit that after the primary base class:
# Create an abstract class for the new manager. class ExtraManagers(models.Model): secondary = NewManager() class Meta: abstract = True class MyPerson(Person, ExtraManagers): class Meta: proxy = True
You probably won’t need to do this very often, but, when you do, it’s possible.
Differences between proxy inheritance and unmanaged models¶
Proxy model inheritance might look fairly similar to creating an unmanaged
model, using the
managed attribute on a
Meta class. The two alternatives are not quite the same and it’s
worth considering which one you should use.
One difference is that you can (and, in fact, must unless you want an empty
model) specify model fields on models with
Meta.managed=False. You could,
with careful setting of
Meta.db_table create an unmanaged model that shadowed
an existing model and add 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.
The other difference that is more important for proxy models, is how model managers are handled. Proxy models are intended to behave exactly like the model they are proxying for. So they inherit the parent model’s managers, including the default manager. In the normal multi-table model inheritance case, children do not inherit managers from their parents as the custom managers aren’t always appropriate when extra fields are involved. The manager documentation has more details about this latter case.
When these two features were implemented, attempts were made to squash them into a single option. It turned out that interactions with inheritance, in general, and managers, in particular, made the API very complicated and potentially difficult to understand and use. It turned out that two options were needed in any case, so the current separation arose.
So, the general rules are:
- 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.
- 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.
Just as with Python’s subclassing, it’s possible for a Django model to inherit from multiple parent models. Keep in mind that normal Python name resolution rules apply. The first base class that a particular name (e.g. Meta) appears in will be the one that is used; for example, this means that if multiple parents contain a Meta class, only the first one is going to be used, and all others will be ignored.
Generally, you won’t need to inherit from multiple parents. The main use-case where this is useful is for “mix-in” classes: adding a particular extra field or method to every class that inherits the mix-in. Try to keep your inheritance hierarchies as simple and straightforward as possible so that you won’t have to struggle to work out where a particular piece of information is coming from.
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 is not permitted
for attributes that are
Field instances (at
least, not at the moment). If a base class has a field called
cannot create another model field called
author in any class that inherits
from that base class.
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
This restriction only applies to attributes which are
Field instances. Normal Python attributes
can be overridden if you wish. It also only applies to the name of the
attribute as Python sees it: if you are manually specifying the database
column name, you can have the same column name appearing in both a child and
an ancestor model for multi-table inheritance (they are columns in two
different database tables).
Django will raise a
FieldError if you override
any model field in any ancestor model.