Bir model verileriniz hakkında kesin bilgi kaynağıdır. Sakladığınız verilerin önemli alanlarını ve davranışlarını içerir. Genellikle, her model tek bir veritabanı tablosuna eşlenir.
The basics:
Bu örnek model isim ve soy_isim bulunduran bir şahsı tanımlıyor:
from django.db import models
class Sahis(models.Model):
isim = models.CharField(max_length=30)
soy_isim = models.CharField(max_length=30)
isim ve soy_isim modelin bir alanınır. Her alan bir sınıf özniteğiyle belirtilir ve her öznikelik bir veritabanı sütunuyla eşleştirilir.
Yukarıdaki Sahis modeli şöyle bir veritabanı tablosu oluşturur:
CREATE TABLE myapp_sahis (
"id" serial NOT NULL PRIMARY KEY,
"isim" varchar(30) NOT NULL,
"soy_isim" varchar(30) NOT NULL
);
Bazı teknik notlar:
Modellerinizi tanımladıktan sonra, Django'ya bu modelleri kullanacağını söylemelisiniz. Bunu, ayarlar dosyanızı düzenleyerek ve models.py dosyasını içeren modülün adını eklemek için INSTALLED_APPS ayarını değiştirerek yapın.
Örneğin, uygulamanızın modelleri myapp.models modülündeyse (bir uygulama için manage.py startapp komut dosyası tarafından oluşturulan paket yapısı), INSTALLED_APPS okunmalı:
INSTALLED_APPS = [
#...
'myapp',
#...
]
INSTALLED_APPS ürününe yeni uygulamalar eklediğinizde, manage.py migrate çalıştırın, isteğe bağlı olarak manage.py makemigrations ile geçiş yapabilirsiniz.
The most important part of a model – and the only required part of a model – is the list of database fields it defines. Fields are specified by class attributes. Be careful not to choose field names that conflict with the models API like clean, save, or delete.
Example:
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()
Each field in your model should be an instance of the appropriate Field class. Django uses the field class types to determine a few things:
Django ships with dozens of built-in field types; you can find the complete list in the model field reference. You can easily write your own fields if Django’s built-in ones don’t do the trick; see Writing custom model fields.
Each field takes a certain set of field-specific arguments (documented in the model field reference). For example, CharField (and its subclasses) require a max_length argument which specifies the size of the VARCHAR database field used to store the data.
There’s also a set of common arguments available to all field types. All are optional. They’re fully explained in the reference, but here’s a quick summary of the most often-used ones:
null
If True, Django will store empty values as NULL in the database. Default is False.
blank
If True, the field is allowed to be blank. Default is False.
Note that this is different than null. null is purely database-related, whereas blank is validation-related. If a field has blank=True, form validation will allow entry of an empty value. If a field has blank=False, the field will be required.
choices
An iterable (e.g., a list or tuple) of 2-tuples to use as choices for this field. If this is given, the default form widget will be a select box instead of the standard text field and will limit choices to the choices given.
A choices list looks like this:
YEAR_IN_SCHOOL_CHOICES = (
('FR', 'Freshman'),
('SO', 'Sophomore'),
('JR', 'Junior'),
('SR', 'Senior'),
('GR', 'Graduate'),
)
The first element in each tuple is the value that will be stored in the database. The second element is displayed by the field’s form widget.
Given a model instance, the display value for a field with choices can be accessed using the get_FOO_display() method. For example:
from django.db import models
class Person(models.Model):
SHIRT_SIZES = (
('S', 'Small'),
('M', 'Medium'),
('L', 'Large'),
)
name = models.CharField(max_length=60)
shirt_size = models.CharField(max_length=1, choices=SHIRT_SIZES)
>>> p = Person(name="Fred Flintstone", shirt_size="L")
>>> p.save()
>>> p.shirt_size
'L'
>>> p.get_shirt_size_display()
'Large'
default
The default value for the field. This can be a value or a callable object. If callable it will be called every time a new object is created.
help_text
Extra “help” text to be displayed with the form widget. It’s useful for documentation even if your field isn’t used on a form.
primary_key
If True, this field is the primary key for the model.
If you don’t specify primary_key=True for any fields in your model, Django will automatically add an IntegerField to hold the primary key, so you don’t need to set primary_key=True on any of your fields unless you want to override the default primary-key behavior. For more, see Automatic primary key fields.
The primary key field is read-only. If you change the value of the primary key on an existing object and then save it, a new object will be created alongside the old one. For example:
from django.db import models
class Fruit(models.Model):
name = models.CharField(max_length=100, primary_key=True)
>>> fruit = Fruit.objects.create(name='Apple')
>>> fruit.name = 'Pear'
>>> fruit.save()
>>> Fruit.objects.values_list('name', flat=True)
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.
By default, Django gives each model the following field:
id = models.AutoField(primary_key=True)
This is an auto-incrementing primary key.
If you’d like to specify a custom primary key, just specify primary_key=True on one of your fields. If Django sees you’ve explicitly set Field.primary_key, it won’t add the automatic id column.
Each model requires exactly one field to have primary_key=True (either explicitly declared or automatically added).
Each field type, except for ForeignKey, ManyToManyField and OneToOneField, takes an optional first positional argument – a verbose name. If the verbose name isn’t given, Django will automatically create it using the field’s attribute name, converting underscores to spaces.
In this example, the verbose name is "person's first name":
first_name = models.CharField("person's first name", max_length=30)
In this example, the verbose name is "first name":
first_name = models.CharField(max_length=30)
ForeignKey, ManyToManyField and OneToOneField require the first argument to be a model class, so use the verbose_name keyword argument:
poll = models.ForeignKey(
Poll,
on_delete=models.CASCADE,
verbose_name="the related poll",
)
sites = models.ManyToManyField(Site, verbose_name="list of sites")
place = models.OneToOneField(
Place,
on_delete=models.CASCADE,
verbose_name="related place",
)
The convention is not to capitalize the first letter of the verbose_name. Django will automatically capitalize the first letter where it needs to.
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.
To define a many-to-one relationship, use django.db.models.ForeignKey. You use it just like any other Field type: by including it as a class attribute of your model.
ForeignKey requires a positional argument: the class to which the model is related.
For example, if a Car model has a Manufacturer – that is, a Manufacturer makes multiple cars but each Car only has one Manufacturer – use the following definitions:
from django.db import models
class Manufacturer(models.Model):
# ...
pass
class Car(models.Model):
manufacturer = models.ForeignKey(Manufacturer, on_delete=models.CASCADE)
# ...
You can also create recursive relationships (an object with a many-to-one relationship to itself) and relationships to models not yet defined; see the model field reference for details.
It’s suggested, but not required, that the name of a ForeignKey field (manufacturer in the example above) be the name of the model, lowercase. You can, of course, call the field whatever you want. For example:
class Car(models.Model):
company_that_makes_it = models.ForeignKey(
Manufacturer,
on_delete=models.CASCADE,
)
# ...
ForeignKey fields accept a number of extra arguments which are explained in the model field reference. These options help define how the relationship should work; all are optional.
For details on accessing backwards-related objects, see the Following relationships backward example.
For sample code, see the Many-to-one relationship model example.
To define a many-to-many relationship, use ManyToManyField. You use it just like any other Field type: by including it as a class attribute of your model.
ManyToManyField requires a positional argument: the class to which the model is related.
For example, if a Pizza has multiple Topping objects – that is, a Topping can be on multiple pizzas and each Pizza has multiple toppings – here’s how you’d represent that:
from django.db import models
class Topping(models.Model):
# ...
pass
class Pizza(models.Model):
# ...
toppings = models.ManyToManyField(Topping)
As with ForeignKey, you can also create recursive relationships (an object with a many-to-many relationship to itself) and relationships to models not yet defined.
It’s suggested, but not required, that the name of a ManyToManyField (toppings in the example above) be a plural describing the set of related model objects.
It doesn’t matter which model has the ManyToManyField, but you should only put it in one of the models – not both.
Generally, ManyToManyField instances should go in the object that’s going to be edited on a form. In the above example, toppings is in Pizza (rather than Topping having a pizzas ManyToManyField ) because it’s more natural to think about a pizza having toppings than a topping being on multiple pizzas. The way it’s set up above, the Pizza form would let users select the toppings.
See the Many-to-many relationship model example for a full example.
ManyToManyField fields also accept a number of extra arguments which are explained in the model field reference. These options help define how the relationship should work; all are optional.
When you’re only dealing with simple many-to-many relationships such as mixing and matching pizzas and toppings, a standard ManyToManyField is all you need. However, sometimes you may need to associate data with the relationship between two models.
For example, consider the case of an application tracking the musical groups which musicians belong to. There is a many-to-many relationship between a person and the groups of which they are a member, so you could use a ManyToManyField to represent this relationship. However, there is a lot of detail about the membership that you might want to collect, such as the date at which the person joined the group.
For these situations, Django allows you to specify the model that will be used to govern the many-to-many relationship. You can then put extra fields on the intermediate model. The intermediate model is associated with the ManyToManyField using the through argument to point to the model that will act as an intermediary. For our musician example, the code would look something like this:
from django.db import models
class Person(models.Model):
name = models.CharField(max_length=128)
def __str__(self):
return self.name
class Group(models.Model):
name = models.CharField(max_length=128)
members = models.ManyToManyField(Person, through='Membership')
def __str__(self):
return self.name
class Membership(models.Model):
person = models.ForeignKey(Person, on_delete=models.CASCADE)
group = models.ForeignKey(Group, on_delete=models.CASCADE)
date_joined = models.DateField()
invite_reason = models.CharField(max_length=64)
When you set up the intermediary model, you explicitly specify foreign keys to the models that are involved in the many-to-many relationship. This explicit declaration defines how the two models are related.
There are a few restrictions on the intermediate model:
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()
]>
>>> 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() 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() 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 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.'
To define a one-to-one relationship, use OneToOneField. You use it just like any other Field type: by including it as a class attribute of your model.
This is most useful on the primary key of an object when that object “extends” another object in some way.
OneToOneField requires a positional argument: the class to which the model is related.
For example, if you were building a database of “places”, you would build pretty standard stuff such as address, phone number, etc. in the database. Then, if you wanted to build a database of restaurants on top of the places, instead of repeating yourself and replicating those fields in the Restaurant model, you could make Restaurant have a OneToOneField to Place (because a restaurant “is a” place; in fact, to handle this you’d typically use inheritance, which involves an implicit one-to-one relation).
As with ForeignKey, a recursive relationship can be defined and references to as-yet undefined models can be made.
See the One-to-one relationship model example for a full example.
OneToOneField fields also accept an optional parent_link argument.
OneToOneField classes used to automatically become the primary key on a model. This is no longer true (although you can manually pass in the primary_key argument if you like). Thus, it’s now possible to have multiple fields of type OneToOneField on a single model.
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,
)
Django places only two restrictions on model field names:
class Example(models.Model):
pass = models.IntegerField() # 'pass' is a reserved word!
class Example(models.Model):
foo__bar = models.IntegerField() # 'foo__bar' has two underscores!
These limitations can be worked around, though, because your field name doesn’t necessarily have to match your database column name. See the db_column option.
SQL reserved words, such as join, where or select, are allowed as model field names, because Django escapes all database table names and column names in every underlying SQL query. It uses the quoting syntax of your particular database engine.
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 Writing custom model fields.
Give your model metadata by using an inner class Meta, like so:
from django.db import models
class Ox(models.Model):
horn_length = models.IntegerField()
class Meta:
ordering = ["horn_length"]
verbose_name_plural = "oxen"
Model metadata is “anything that’s not a field”, such as ordering options (ordering), database table name (db_table), or human-readable singular and plural names (verbose_name and verbose_name_plural). None are required, and adding class Meta to a model is completely optional.
A complete list of all possible Meta options can be found in the model option reference.
objects
The most important attribute of a model is the Manager. It’s the interface through which database query operations are provided to Django models and is used to retrieve the instances from the database. If no custom Manager is defined, the default name is objects. Managers are only accessible via model classes, not the model instances.
Define custom methods on a model to add custom “row-level” functionality to your objects. Whereas Manager methods are intended to do “table-wide” things, model methods should act on a particular model instance.
This is a valuable technique for keeping business logic in one place – the model.
For example, this model has a few custom methods:
from django.db import models
class Person(models.Model):
first_name = models.CharField(max_length=50)
last_name = models.CharField(max_length=50)
birth_date = models.DateField()
def baby_boomer_status(self):
"Returns the person's baby-boomer status."
import datetime
if self.birth_date < datetime.date(1945, 8, 1):
return "Pre-boomer"
elif self.birth_date < datetime.date(1965, 1, 1):
return "Baby boomer"
else:
return "Post-boomer"
@property
def full_name(self):
"Returns the person's full name."
return '%s %s' % (self.first_name, self.last_name)
The last method in this example is a property.
The model instance reference has a complete list of methods automatically given to each model. You can override most of these – see overriding predefined model methods, below – but there are a couple that you’ll almost always want to define:
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.
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.
There’s another set of model methods that encapsulate a bunch of database behavior that you’ll want to customize. In particular you’ll often want to change the way save() and delete() work.
You’re free to override these methods (and any other model method) to alter behavior.
A classic use-case for overriding the built-in methods is if you want something to happen whenever you save an object. For example (see save() for documentation of the parameters it accepts):
from django.db import models
class Blog(models.Model):
name = models.CharField(max_length=100)
tagline = models.TextField()
def save(self, *args, **kwargs):
do_something()
super().save(*args, **kwargs) # Call the "real" save() method.
do_something_else()
You can also prevent saving:
from django.db import models
class Blog(models.Model):
name = models.CharField(max_length=100)
tagline = models.TextField()
def save(self, *args, **kwargs):
if self.name == "Yoko Ono's blog":
return # Yoko shall never have her own blog!
else:
super().save(*args, **kwargs) # Call the "real" save() method.
It’s important to remember to call the superclass method – that’s that super().save(*args, **kwargs) business – to ensure that the object still gets saved into the database. If you forget to call the superclass method, the default behavior won’t happen and the database won’t get touched.
It’s also important that you pass through the arguments that can be passed to the model method – that’s what the *args, **kwargs bit does. Django will, from time to time, extend the capabilities of built-in model methods, adding new arguments. If you use *args, **kwargs in your method definitions, you are guaranteed that your code will automatically support those arguments when they are added.
Note that the delete() method for an object is not necessarily called when deleting objects in bulk using a QuerySet or as a result of a cascading delete. To ensure customized delete logic gets executed, you can use pre_delete and/or post_delete signals.
Unfortunately, there isn’t a workaround when creating or updating objects in bulk, since none of save(), pre_save, and post_save are called.
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.
Model inheritance in Django works almost identically to the way normal class inheritance works in Python, but the basics at the beginning of the page should still be followed. That means the base class should subclass 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.
Abstract base classes are useful when you want to put some common information into a number of other models. You write your base class and put abstract=True in the Meta class. This model will then not be used to create any database table. Instead, when it is used as a base class for other models, its fields will be added to those of the child class. 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).
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.
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.
When an abstract base class is created, Django makes any Meta inner class you declared in the base class available as an attribute. If a child class does not declare its own Meta class, it will inherit the parent’s Meta. If the child wants to extend the parent’s Meta class, it can subclass it. For example:
from django.db import models
class CommonInfo(models.Model):
# ...
class Meta:
abstract = True
ordering = ['name']
class Student(CommonInfo):
# ...
class Meta(CommonInfo.Meta):
db_table = 'student_info'
Django does make one adjustment to the Meta class of an abstract base class: before installing the Meta attribute, it sets abstract=False. This means that children of abstract base classes don’t automatically become abstract classes themselves. Of course, you can make an abstract base class that inherits from another abstract base class. You just need to remember to explicitly set abstract=True each time.
Some attributes won’t make sense to include in the Meta class of an abstract base class. For example, including db_table would mean that all the child classes (the ones that don’t specify their own Meta) would use the same database table, which is almost certainly not what you want.
If you are using related_name or related_query_name on a ForeignKey or ManyToManyField, you must always specify a unique reverse name and query name for the field. This would normally cause a problem in abstract base classes, since the fields on this class are included into each of the child classes, with exactly the same values for the attributes (including related_name and related_query_name) each time.
To work around this problem, when you are using related_name or related_query_name in an abstract base class (only), part of the value should contain '%(app_label)s' and '%(class)s'.
For example, given an app common/models.py:
from django.db import models
class Base(models.Model):
m2m = models.ManyToManyField(
OtherModel,
related_name="%(app_label)s_%(class)s_related",
related_query_name="%(app_label)s_%(class)ss",
)
class Meta:
abstract = True
class ChildA(Base):
pass
class ChildB(Base):
pass
Along with another app rare/models.py:
from common.models import Base
class ChildB(Base):
pass
The reverse name of the common.ChildA.m2m field will be common_childa_related and the reverse query name will be common_childas. The reverse name of the common.ChildB.m2m field will be common_childb_related and the reverse query name will be common_childbs. Finally, the reverse name of the rare.ChildB.m2m field will be rare_childb_related and the reverse query name will be rare_childbs. It’s up to you how you use the '%(class)s' and '%(app_label)s' portion to construct your related name or related query name but if you forget to use it, Django will raise errors when you perform system checks (or run migrate).
If you don’t specify a related_name attribute for a field in an abstract base class, the default reverse name will be the name of the child class followed by '_set', just as it normally would be if you’d declared the field directly on the child class. For example, in the above code, if the related_name attribute was omitted, the reverse name for the m2m field would be childa_set in the ChildA case and childb_set for the ChildB field.
The second type of model inheritance supported by Django is when each model in the hierarchy is a model all by itself. Each model corresponds to its own database table and can be queried and created individually. The inheritance relationship introduces links between the child model and each of its parents (via an automatically-created OneToOneField). For example:
from django.db import models
class Place(models.Model):
name = models.CharField(max_length=50)
address = models.CharField(max_length=80)
class Restaurant(Place):
serves_hot_dogs = models.BooleanField(default=False)
serves_pizza = models.BooleanField(default=False)
All of the fields of Place will also be available in Restaurant, although the data will reside in a different database table. So these are both possible:
>>> Place.objects.filter(name="Bob's Cafe")
>>> Restaurant.objects.filter(name="Bob's Cafe")
If you have a Place that is also a Restaurant, you can get from the Place object to the Restaurant object by using the lower-case version of the model name:
>>> p = Place.objects.get(id=12)
# If p is a Restaurant object, this will give the child class:
>>> p.restaurant
However, if p in the above example was not a Restaurant (it had been created directly as a Place object or was the parent of some other class), referring to p.restaurant would raise a Restaurant.DoesNotExist exception.
The automatically-created OneToOneField on Restaurant that links it to Place looks like this:
place_ptr = models.OneToOneField(
Place, on_delete=models.CASCADE,
parent_link=True,
)
You can override that field by declaring your own OneToOneField with parent_link=True on Restaurant.
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 = []
Because multi-table inheritance uses an implicit OneToOneField to link the child and the parent, it’s possible to move from the parent down to the child, as in the above example. However, this uses up the name that is the default related_name value for ForeignKey and ManyToManyField relations. If you are putting those types of relations on a subclass of the parent model, you must specify the related_name attribute on each such field. If you forget, Django will raise a validation error.
For example, using the above Place class again, let’s create another subclass with a ManyToManyField:
class Supplier(Place):
customers = models.ManyToManyField(Place)
This results in the error:
Reverse query name for 'Supplier.customers' clashes with reverse query
name for 'Supplier.place_ptr'.
HINT: Add or change a related_name argument to the definition for
'Supplier.customers' or 'Supplier.place_ptr'.
Adding related_name to the customers field as follows would resolve the error: models.ManyToManyField(Place, related_name='provider').
As mentioned, Django will automatically create a 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 and set parent_link=True to indicate that your field is the link back to the parent class.
When using multi-table inheritance, a new database table is created for each subclass of a model. This is usually the desired behavior, since the subclass needs a place to store any additional data fields that are not present on the base class. Sometimes, however, you only want to change the Python behavior of a model – perhaps to change the default manager, or add a new method.
This is what proxy model inheritance is for: creating a proxy for the original model. You can create, delete and update instances of the proxy model and all the data will be saved as if you were using the original (non-proxied) model. The difference is that you can change things like the default model ordering or the default manager in the proxy, without having to alter the original.
Proxy models are declared like normal models. You tell Django that it’s a proxy model by setting the proxy attribute of the Meta class to True.
For example, suppose you want to add a method to the Person model. You can do it like this:
from django.db import models
class Person(models.Model):
first_name = models.CharField(max_length=30)
last_name = models.CharField(max_length=30)
class MyPerson(Person):
class Meta:
proxy = True
def do_something(self):
# ...
pass
The MyPerson class operates on the same database table as its parent Person class. In particular, any new instances of Person will also be accessible through MyPerson, and vice-versa:
>>> p = Person.objects.create(first_name="foobar")
>>> MyPerson.objects.get(first_name="foobar")
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
Now normal Person queries will be unordered and OrderedPerson queries will be ordered by last_name.
Proxy models inherit Meta attributes in the same way as regular models.
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.
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. A proxy model may also inherit from any number of proxy models that share a common non-abstract parent class.
If you don’t specify any model managers on a proxy model, it inherits the managers from its model parents. If you define a manager on the proxy model, it will become the default, although any managers defined on the parent classes will still be available.
Continuing our example from above, you could change the default manager used when you query the Person model like this:
from django.db import models
class NewManager(models.Manager):
# ...
pass
class MyPerson(Person):
objects = NewManager()
class Meta:
proxy = True
If you wanted to add a new manager to the Proxy, without replacing the existing default, you can use the techniques described in the custom manager documentation: create a base class containing the new managers and inherit that after the primary base class:
# Create an abstract class for the new manager.
class ExtraManagers(models.Model):
secondary = NewManager()
class Meta:
abstract = True
class MyPerson(Person, ExtraManagers):
class Meta:
proxy = True
You probably won’t need to do this very often, but, when you do, it’s possible.
Proxy model inheritance might look fairly similar to creating an unmanaged model, using the managed attribute on a model’s Meta class.
With careful setting of Meta.db_table you could create an unmanaged model that shadows an existing model and adds Python methods to it. However, that would be very repetitive and fragile as you need to keep both copies synchronized if you make any changes.
On the other hand, proxy models are intended to behave exactly like the model they are proxying for. They are always in sync with the parent model since they directly inherit its fields and managers.
The general rules are:
Just as with Python’s subclassing, it’s possible for a Django model to inherit from multiple parent models. Keep in mind that normal Python name resolution rules apply. The first base class that a particular name (e.g. Meta) appears in will be the one that is used; for example, this means that if multiple parents contain a Meta class, only the first one is going to be used, and all others will be ignored.
Generally, you won’t need to inherit from multiple parents. The main use-case where this is useful is for “mix-in” classes: adding a particular extra field or method to every class that inherits the mix-in. Try to keep your inheritance hierarchies as simple and straightforward as possible so that you won’t have to struggle to work out where a particular piece of information is coming from.
Note that inheriting from multiple models that have a common id primary key field will raise an error. To properly use multiple inheritance, you can use an explicit AutoField in the base models:
class Article(models.Model):
article_id = models.AutoField(primary_key=True)
...
class Book(models.Model):
book_id = models.AutoField(primary_key=True)
...
class BookReview(Book, Article):
pass
Or use a common ancestor to hold the AutoField. This requires using an explicit OneToOneField from each parent model to the common ancestor to avoid a clash between the fields that are automatically generated and inherited by the child:
class Piece(models.Model):
pass
class Article(Piece):
article_piece = models.OneToOneField(Piece, on_delete=models.CASCADE, parent_link=True)
...
class Book(Piece):
book_piece = models.OneToOneField(Piece, on_delete=models.CASCADE, parent_link=True)
...
class BookReview(Book, Article):
pass
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.
Model managers are inherited from abstract base classes. Overriding an inherited field which is referenced by an inherited Manager may cause subtle bugs. See custom managers and model inheritance.
Some fields define extra attributes on the model, e.g. a ForeignKey defines an extra attribute with _id appended to the field name, as well as related_name and related_query_name on the foreign model.
These extra attributes cannot be overridden unless the field that defines it is changed or removed so that it no longer defines the extra attribute.
Overriding fields in a parent model leads to difficulties in areas such as initializing new instances (specifying which field is being initialized in Model.__init__) and serialization. These are features which normal Python class inheritance doesn’t have to deal with in quite the same way, so the difference between Django model inheritance and Python class inheritance isn’t arbitrary.
This restriction only applies to attributes which are Field instances. Normal Python attributes can be overridden if you wish. It also only applies to the name of the attribute as Python sees it: if you are manually specifying the database column name, you can have the same column name appearing in both a child and an ancestor model for multi-table inheritance (they are columns in two different database tables).
Django will raise a FieldError if you override any model field in any ancestor model.
The manage.py startapp command creates an application structure that includes a models.py file. If you have many models, organizing them in separate files may be useful.
To do so, create a models package. Remove models.py and create a myapp/models/ directory with an __init__.py file and the files to store your models. You must import the models in the __init__.py file.
For example, if you had organic.py and synthetic.py in the models directory:
myapp/models/__init__.py
from .organic import Person
from .synthetic import Robot
Explicitly importing each model rather than using from .models import * has the advantages of not cluttering the namespace, making code more readable, and keeping code analysis tools useful.
The Models Reference
Covers all the model related APIs including model fields, related objects, and QuerySet.