I have to models
class Parent(object):
text_field = models.TextField()
boolean_field = models.BooleanField()
class Child(Parent):
another_text_field = models.TextField()
With the following ModelAdmin
class ChildAdmin(admin.ModelAdmin):
pass
admin.site.register(Child, ChildAdmin)
I currently see all fields in the admin page, i.e. text_field, boolean_field, and another_text_field.
Question: How can I get a parent select field and exclude text_field and boolean_field (for latter I guess I can use exclude).
Current Solution: I add a Form and use its clean method to set the parent field. text_field and boolean_field can be excluded by addind it to the ModelAdmin's excluded variable.
simply overwrite fields
class Child(Parent):
another_text_field = models.TextField()
text_field = None
boolean_field = None
if you want to use inheritance in django models use abstract models please.
I am not sure if it is really necessary to use model inheritance. if not, you may consider using OneToOneField without model inheritance.
Example using OneToOneField:
models.py
class Parent(models.Model):
text_field = models.TextField()
boolean_field = models.BooleanField()
class Child(models.Model):
parent = models.OneToOneField(Parent,
on_delete=models.CASCADE,
primary_key=True)
child_field = models.TextField()
admin.py
#admin.register(Parent)
class ParentAdmin(admin.ModelAdmin):
pass
doing so you can see a drop down menu for picking Parent instance at child admin page. but meanwhile, you lose one 'benefit' of using inheritance, which is the availability of Parent field in Child
as mentioned in the docs,
All of the fields of Place will also be available in Restaurant,
although the data will reside in a different database table.
but there is a easy fix for that, just use something like Child.objects.filter(parent__text_field="something"). Query performance should be the same (I guess) since implementation in db are basically the same for these two approaches (both use separated table) (please correct if I am wrong)
Apart from from this and admin display behavior, I am not sure how these two approaches (your approach and this answer) are differed.
I simplify my code structure, which contains two models:
# created by third part app, not Django one
# but we share same DB, so i have access to this one
class A(models.Model):
title = models.TextField()
# other fields ...
class Meta:
manage = False
class B(models.Model):
model_a = models.OneToOneField(A, related_name='+')
# other fields, to extend model A functionality
Is this a good way to extend third part app model A with my additional fields and methods? Now i have problem to sync this models true one-to-one field. Since I don't have access to trigger model A creation.
In ideal world i should have CarA and CarB. And CarB = CarA relation should be created if CarB exists.
I base this idea on Django 1.5 user extension. Is this clear enough? Or should i do something else?
You could use a property to create the B instance on access if it doesn't exist yet, ie,
class A(models.Model):
title = models.TextField()
# other fields ...
class Meta:
manage = False
#property
def b(self):
if not hasattr(self, "__bcache"):
self.__bcache, created = B.objects.get_or_create(model_a = self)
return self.__bcache
It seems like you're new to both Python and Django so let's explain quickly...
First, the "#property" part: it's a decorator that turns the following function into a computed attribute - IOW you use it as an attribute (myA.b.whatever), and under the hood it turns it into a method call (myA.b().whatever). It's not strictly required here, we would have used an explicit getter (the same method named get_a()) but it's cleaner that way.
Then our method implementation: obviously we don't want to hit the database each time someone looks up A.b, so
first we check if an attribute named __bcache ("b" "cache") is set on the current instance.
if not, we call B.objects.get_or_create(a_model=self) which will either retrieve the existing B instance for this A instance or create one if none exists yet and we store this B instance as self.__bcache so next call will retrieve it directly from __bcache instead of hitting the database.
and finally we return self.__bcache that is now garanteed to exists and point to the related B instance.
How can i override the model manager of a many-to-many field that i have considering the following:
class TermsManager(models.Manager):
def all(self):
return super(TermsManager, self).all().filter(condition_here)
class Term(models.Model):
objects = TermsManager()
name = models.CharField(max_length=255)
class Object(models.Model):
title = models.CharField(max_length=255)
terms = models.ManyToManyField(Term, blank=True)
class Channel(Object):
class Meta:
proxy = True
I also have a class which inherits from TermManager called ChannelTermManager.
How can i override the "terms" field of the Channel model so that
mychannel.terms calls the ChannelTermManager instead of TermManager?
First of all, you shouldn't be overriding all(). If you want to change the default queryset, override get_query_set like so:
class TermsManager(models.Manager):
def get_query_set(self):
return super(TermsManager, self).get_query_set().filter(condition_here)
This is because all() is often omitted when other queryset functions are chained on, and you want your queryset to behave the same whether all() is explicitly called or not.
But even so, what you're doing is still problematic. As explained in the documentation for managers, filtering the default related queryset will affect all sorts of automatic things behind the scenes (such as when dumping data to create backups/fixtures, etc.). You almost definitely do not want this. And you really don't want your related object managers doing this either (by setting use_for_related_fields = True), because you'll be masking what's actually stored in the database, rather than simply detecting out of date data and creating alerts or whatever to clean it up. use_for_related_fields is intended for creating managers that augment the normal capabilities of the vanilla manager, not to filter.
I had a similar situation to yours however, and I handled it like so:
class FilteredTermsManager(models.Manager):
def get_query_set(self):
return super(TermsManager, self).get_query_set().filter(condition_here)
class Term(models.Model):
allTerms = models.Manger() # Establish this as the default/automatic manager
objects = FilteredTermsManager()
name = models.CharField(max_length=255)
This way, I could do all my initial querying on the model through my filtered queryset and it looks like "regular Django", but all relational and behind the scenes queries would work on the unfiltered database. And I could always access the true full set of objects by manually doing Term.allTerms.all().
As for using different managers for different related objects, there's nothing you can really do there. But why not just add Channel specific objects to your custom manager, and simply not call them from methods that operate on get Term querysets from Object?
Where should I overwrite method add() for ManyToMany related fields.
Seems like it is not manager 'objects' of my model. Because when we are adding new relation for ManyToMany fields we are not writing Model.objects.add().
So what I need it overwrite method add() of instance. How can I do it?
Edit:
So i know that there is ManyRelatedManager. One thing remain how can i overwrite it?
Sorry... not overwrite, but assign it in my Model by default.
http://docs.djangoproject.com/en/1.2/topics/db/managers/#custom-managers
You can create any number of managers for a Model.
You can subclass a ManyRelatedManager and assign it to the Model.
This example may be what you're looking for
# Then hook it into the Book model explicitly.
class Book(models.Model):
title = models.CharField(max_length=100)
author = models.CharField(max_length=50)
objects = models.Manager() # The default manager.
dahl_objects = DahlBookManager() # The Dahl-specific manager.
The objects manage is the default. Do not change this.
The dahl_objects is a customized manager. You can have any number of these.
Is there explicit support for Single Table Inheritance in Django? Last I heard, the feature was still under development and debate.
Are there libraries/hacks I can use in the meantime to capture the basic behavior? I have a hierarchy that mixes different objects. The canonical example of a corporation structure with an Employee class, subclasses for types of employees, and a manager_id (parent_id) would be a good approximation of the problem I am solving.
In my case, I would like to represent the idea that an employee can manage other employees while being managed by a different employee. There are not separate classes for Manager and Worker, which makes this hard to spread across tables. Sub-classes would represent types of employees-programmers, accountants, sales, etc and would be independent of who supervises who (OK, I guess it's no longer a typical corporation in some respect).
Summary
Django's proxy models provide the basis for Single Table Inheritance.
However, some effort is required to make it work.
Skip to the end for a re-usable example.
Background
Martin Fowler describes Single Table Inheritance (STI) as follows:
Single Table Inheritance maps all fields of all classes of an inheritance structure into a single table.
This is precisely what Django's proxy model inheritance does.
Note, that, according to this blog post from 2010, proxy models have been around since Django 1.1.
A "normal" Django model is a concrete model, i.e. it has a dedicated table in the database.
There are two types of Django model that do not have dedicated database tables, viz. abstract models and proxy models:
Abstract models act as superclasses for concrete models. An abstract model can define fields, but it does not have a database table. The fields are only added to the database tables for its concrete subclasses.
Proxy models act as subclasses for concrete models. A proxy model cannot define new fields. Instead, it operates on the database table associated with its concrete superclass. In other words, a Django concrete model and its proxies all share a single table.
Django's proxy models provide the basis for Single Table Inheritance, viz. they allow different models to share a single table, and they allow us to define proxy-specific behavior on the Python side. However, Django's default object-relational mapping (ORM) does not provide all the behavior that would be expected, so a little customization is required. How much, that depends on your needs.
Let's build a minimal example, step by step, based on the simple data-model in the figure below:
Step 1: basic "proxy model inheritance"
Here's the content of models.py for a basic proxy inheritance implementation:
from django.db import models
class Party(models.Model):
name = models.CharField(max_length=20)
person_attribute = models.CharField(max_length=20)
organization_attribute = models.CharField(max_length=20)
class Person(Party):
class Meta:
proxy = True
class Organization(Party):
class Meta:
proxy = True
Person and Organization are two types of parties.
Only the Party model has a database table, so all the fields are defined on this model, including any fields that are specific either to Person or to Organization.
Because Party, Person, and Organization all use the Party database table, we can define a single ForeignKey field to Party, and assign instances of any of the three models to that field, as implied by the inheritance relation in the figure. Note, that, without inheritance, we would need a separate ForeignKey field for each model.
For example, suppose we define an Address model as follows:
class Address(models.Model):
party = models.ForeignKey(to=Party, on_delete=models.CASCADE)
We can then initialize an Address object using e.g. Address(party=person_instance) or Address(party=organization_instance).
So far, so good.
However, if we try to get a list of objects corresponding to a proxy model, using e.g. Person.objects.all(), we get a list of all Party objects instead, i.e. both Person objects and Organization objects. This is because the proxy models still use the model manager from the superclass (i.e. Party).
Step 2: add proxy model managers
To make sure that Person.objects.all() only returns Person objects, we need to assign a separate model manager that filters the Party queryset. To enable this filtering, we need a field that indicates which proxy model should be used for the object.
To be clear: creating a Person object implies adding a row to the Party table. The same goes for Organization. To distinguish between the two, we need a column to indicate if a row represents a Person or an Organization. For convenience and clarity, we add a field (i.e. column) called proxy_name, and use that to store the name of the proxy class.
So, enter the ProxyManager model manager and the proxy_name field:
from django.db import models
class ProxyManager(models.Manager):
def get_queryset(self):
return super().get_queryset().filter(proxy_name=self.model.__name__)
class Party(models.Model):
proxy_name = models.CharField(max_length=20)
name = models.CharField(max_length=20)
person_attribute = models.CharField(max_length=20)
organization_attribute = models.CharField(max_length=20)
def save(self, *args, **kwargs):
self.proxy_name = type(self).__name__
super().save(*args, **kwargs)
class Person(Party):
class Meta:
proxy = True
objects = ProxyManager()
class Organization(Party):
class Meta:
proxy = True
objects = ProxyManager()
Now the queryset returned by Person.objects.all() will only contain Person objects (and the same for Organization).
However, this does not work in the case of a ForeignKey relation to Party, as in Address.party above, because that will always return a Party instance, regardless of the value of the proxy_name field (also see docs). For example, suppose we create an address = Address(party=person_instance), then address.party will return a Party instance, instead of a Person instance.
Step 3: extend the Party constructor
One way to deal with the related-field issue is to extend the Party.__new__ method, so it returns an instance of the class specified in the 'proxy_name' field. The end result looks like this:
class Party(models.Model):
PROXY_FIELD_NAME = 'proxy_name'
proxy_name = models.CharField(max_length=20)
name = models.CharField(max_length=20)
person_attribute = models.CharField(max_length=20)
organization_attribute = models.CharField(max_length=20)
def save(self, *args, **kwargs):
""" automatically store the proxy class name in the database """
self.proxy_name = type(self).__name__
super().save(*args, **kwargs)
def __new__(cls, *args, **kwargs):
party_class = cls
try:
# get proxy name, either from kwargs or from args
proxy_name = kwargs.get(cls.PROXY_FIELD_NAME)
if proxy_name is None:
proxy_name_field_index = cls._meta.fields.index(
cls._meta.get_field(cls.PROXY_FIELD_NAME))
proxy_name = args[proxy_name_field_index]
# get proxy class, by name, from current module
party_class = getattr(sys.modules[__name__], proxy_name)
finally:
return super().__new__(party_class)
Now address.party will actually return a Person instance if the proxy_name field is Person.
As a last step, we can make the whole thing re-usable:
Step 4: make it re-usable
To make our rudimentary Single-Table Inheritance implementation re-usable, we can use Django's abstract inheritance:
inheritance/models.py:
import sys
from django.db import models
class ProxySuper(models.Model):
class Meta:
abstract = True
proxy_name = models.CharField(max_length=20)
def save(self, *args, **kwargs):
""" automatically store the proxy class name in the database """
self.proxy_name = type(self).__name__
super().save(*args, **kwargs)
def __new__(cls, *args, **kwargs):
""" create an instance corresponding to the proxy_name """
proxy_class = cls
try:
field_name = ProxySuper._meta.get_fields()[0].name
proxy_name = kwargs.get(field_name)
if proxy_name is None:
proxy_name_field_index = cls._meta.fields.index(
cls._meta.get_field(field_name))
proxy_name = args[proxy_name_field_index]
proxy_class = getattr(sys.modules[cls.__module__], proxy_name)
finally:
return super().__new__(proxy_class)
class ProxyManager(models.Manager):
def get_queryset(self):
""" only include objects in queryset matching current proxy class """
return super().get_queryset().filter(proxy_name=self.model.__name__)
Then we can implement our inheritance structure as follows:
parties/models.py:
from django.db import models
from inheritance.models import ProxySuper, ProxyManager
class Party(ProxySuper):
name = models.CharField(max_length=20)
person_attribute = models.CharField(max_length=20)
organization_attribute = models.CharField(max_length=20)
class Person(Party):
class Meta:
proxy = True
objects = ProxyManager()
class Organization(Party):
class Meta:
proxy = True
objects = ProxyManager()
class Placement(models.Model):
party = models.ForeignKey(to=Party, on_delete=models.CASCADE)
More work may be required, depending on your needs, but I believe this covers some of the basics.
I think the OP is asking about Single-Table Inheritance as defined here:
Relational databases don't support inheritance, so when mapping from objects to databases we have to consider how to represent our nice inheritance structures in relational tables. When mapping to a relational database, we try to minimize the joins that can quickly mount up when processing an inheritance structure in multiple tables. Single Table Inheritance maps all fields of all classes of an inheritance structure into a single table.
That is, a single database table for a whole hierarchy of entity classes. Django does not support that kind of inheritance.
There are currently two forms of inheritance in Django - MTI (model table inheritance) and ABC (abstract base classes).
I wrote a tutorial on what's going on under the hood.
You can also reference the official docs on model inheritance.
See my attempt:
http://djangosnippets.org/snippets/2408/
An emulation of "table per hierarchy" a.k.a. "single table inheritance" in Django. The base class must hold all the fields. It's subclasses are not allowed to contain any additional fields and optimally they should be proxies.
Not exactly "single table inheritance", but close enough for many situations.
this might be of use: https://github.com/craigds/django-typed-models
It looks to be somewhat of an implementation of Single Table Inheritance but it has the limitation that subclasses can't have any extra fields.
here is a recent discussion on the django developer mailing list about STI:
https://groups.google.com/forum/#!msg/django-developers/-UOM8HNUnxg/6k34kopzerEJ
I think you can do something akin to this.
I have to implement a solution for this problem myself, and here was how I solved it:
class Citrus(models.Model):
how_acidic = models.PositiveIntegerField(max_value=100)
skin_color = models.CharField()
type = models.CharField()
class TangeloManager(models.Manager):
def get_query_set(self):
return super(TangeloManager, self).get_query_set().filter(type='Tangelo')
class Tangelo(models.Model):
how_acidic = models.PositiveIntegerField(max_value=100)
skin_color = models.CharField()
type = models.CharField()
objects = TangeloManager()
class Meta:
# 'appname' below is going to vary with the name of your app
db_table = u'appname_citrus'
This may have some locking issues... I'm not really sure how django handles that off the top of my head. Also, I didn't really test the above code, it's strictly for entertainment purposes, to hopefully put you on the right track.