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How to respond with child object based on its parent id?

I have a parent class and a child class, like so:
class Animal(models.Model):
age = models.IntegerField()
class Bird(Animal)
wingSpan = models.DecimalField()
Here's the view to get birds:
class BirdViewSet(viewsets.ModelViewSet):
queryset = Bird.objects.all()
serializer_class = BirdSerializer
And here is the serializer:
class BirdSerializer(serializers.HyperlinkedModelSerializer):
class Meta:
model = Bird
fields = ['age', 'wingSpan']
In the database, jango appropriately created an animal_prt_id field in the birds table, so that this view/serializer pair knows where to find the age, which is mapped to the parent class, Animal.
How to make a view/serializer pair that does the opposite, meaning, it receives the id of an Animal and responds with the complete Bird (or with any other Animal subclass I might have)?

There is an useful package called django-model-utils that provides an InheritanceManager logic.
TLDR: After setting up the package, your code will sort of look like this
bird = Animal.objects.filter(pk=animal_id).select_subclasses("bird")

One table can not be foreign key to others except one table

For example
class Room(models.Model):
visitor = models.ForeignKey(Visitor)
number = models.PositiveIntegerField()
capacity = models.ForeignKey(Capacity, on_delete=models.PROTECT)
floor = models.ForeignKey(Floor, on_delete=models.PROTECT)
price = models.PositiveIntegerField()
is_premium = models.BooleanField(default=False)
is_vip = models.BooleanField(default=False)
expiry_date = models.DateTimeField()
class Meta:
unique_together = ('')
def __str__(self):
return '№{0}'.format(self.number)
class Reserved(models.Model):
room = models.ForeignKey(Room)
begin_date = models.DateTimeField()
def __str__(self):
return 'Reserved Room {0}'.format(self.room)
class Busy(models.Model):
room = models.ForeignKey(Room)
Table Room can not be connected to Tables Reserved and Busy at the same time. Room should be reserved or busy. Is there way put validation for this?
I tried to use unique_together but if for fields of table
Thanks

There is no way to enforce this at DB level nor a simple way to do it in Django level. With your structure you should add some validation before creating (or modifying) both Busy and Reserved. Something like:
class Busy(models.Model):
room = models.ForeignKey(Room)
def __save__(self, *args, **kwargs):
if Reserved.object.filter(room=self.room).exists():
raise RuntimeError('Trying to make a reserved room busy.')
super(Busy, self).__save__(*args, **kwargs)
If you are creating Busy and Reserved objects concurrently it's subject to race condition. I suggest to move room state into Room model itself and add some helper functions (something like in room_manager.py beside models.py) to change its state and make sure related models are created/modified in a consistent manner.

The only way to ensure at the database level that you have one single "status" at a given time for a room is to have your relationship the other way round - with Room have a foreign key on whatever represents the status. To make this work you'll need to use either some form of model inheritance or django's "generic" relationship (which can be handy sometimes but are really not SQL-friendly).
Here's an example using the very simplest form of "model inheritance" (which is not actually inheritance at all):
class Status(models.Model):
BUSY = 1
RESERVED = 2
TYPES = (
(BUSY,"busy"),
(RESERVED,"reserved")
)
type = models.CharField("Status type", max_length=10, choices=TYPES)
# only used for reservations
begin_date = models.DateTimeField(null=True, blank=True)
def save(self, *args, **kw):
# TODO : this should belong to `full_clean()`,
# cf the FineManual model's validation
if self.type == self.RESERVED and not self.begin_date:
raise ValueError("RESERVED statuses need a begin_date")
super(Status, self).save(*args, **kw)
class Room(models.Model):
status = models.ForeignKey(Status)
Note that this allows for a same status to be used for multiple rooms at the same time, which might be a problem too. Using a OneToOneField field instead might help on the Django side but will still be treated as a foreign key at the database level.

Correctly using subclasses/proxies in django that have self referential foreign keys?

I have two classes, with a super class. In essence the two classes are concrete classes on a tree. One is a leaf, one is a branch. They share properties defined in the super class.
None of the below classes are finished. I've tried both making the superclass abstract, and the subclasses proxies. Hopefully the code below explains what I'm trying to achieve.
This is the 'super class'
class Owner(models.Model):
name = models.CharField(max_length=200)
def __unicode__(self):
return self.name
class Meta:
abstract=True
This is the 'leaf'
class User(Owner):
pass
This is the 'branch'.
class Group(Owner):
head = models.ForeignKey(User)
members = models.ManyToManyField(Owner,through='Membership')
This shows how a user can belong to a group by a membership.
class Membership(models.Model):
date_joined = models.DateField()
user = models.ForeignKey(Owner)
group = models.ForeignKey(Group)
My restrictions are that each user can belong to many groups (via the linker Membership). Each group can be a member of a single group.
This fails because I'm referencing Owner in both the membership as the user, and in the group members. I feel like this is the sort of thing I could solve with generics in Java, but thats not going to help me here.
I've also seen ContentTypes used for this sort of thing, but they seem too complicated for what I'm trying to do. Am I wrong? I can't figure out how to apply that paradigm to my example. I also found this question but I'm still not certain on how it should be implemented.

You can't have foreign key fields pointing to an abstract class (there is no table in the DB for an abstract class).
You'll probably need to implement self-referential foreign key for each Group to belong to zero or one group. Something like this:
class Base(models.Model):
name = models.CharField(max_length=200)
def __unicode__(self):
return self.name
class Meta:
abstract=True
class User(Base):
groups = models.ManyToManyField('Group', through='Membership', related_name='members')
class Group(Base):
head = models.ForeignKey(User)
parent = models.ForeignKey('self', blank=True, null=True, related_name='children')
def descendants(self, **kwargs):
qs = self.children_set.filter(**kwargs)
for group in self.children_set.all():
qs = qs | group.descendants(**kwargs)
return qs
class Membership(models.Model):
date_joined = models.DateField()
user = models.ForeignKey(User)
group = models.ForeignKey(Group)
The Base class above does nothing other than dictate that each inherited class has a name field in their respective DB table and __unicode__ method- nothing more. You could simply copy and paste the name field and __unicode__ method into User and Group and it would be functionally identical. Having a common abstract parent doesn't create any DB relationships and you can't use it to query the DB.
I can't really see any reason for using proxy classes in this situation.

django abstract models versus regular inheritance

Besides the syntax, what's the difference between using a django abstract model and using plain Python inheritance with django models? Pros and cons?
UPDATE: I think my question was misunderstood and I received responses for the difference between an abstract model and a class that inherits from django.db.models.Model. I actually want to know the difference between a model class that inherits from a django abstract class (Meta: abstract = True) and a plain Python class that inherits from say, 'object' (and not models.Model).
Here is an example:
class User(object):
first_name = models.CharField(..
def get_username(self):
return self.username
class User(models.Model):
first_name = models.CharField(...
def get_username(self):
return self.username
class Meta:
abstract = True
class Employee(User):
title = models.CharField(...

I actually want to know the difference between a model class that
inherits from a django abstract class (Meta: abstract = True) and a
plain Python class that inherits from say, 'object' (and not
models.Model).
Django will only generate tables for subclasses of models.Model, so the former...
class User(models.Model):
first_name = models.CharField(max_length=255)
def get_username(self):
return self.username
class Meta:
abstract = True
class Employee(User):
title = models.CharField(max_length=255)
...will cause a single table to be generated, along the lines of...
CREATE TABLE myapp_employee
(
id INT NOT NULL AUTO_INCREMENT,
first_name VARCHAR(255) NOT NULL,
title VARCHAR(255) NOT NULL,
PRIMARY KEY (id)
);
...whereas the latter...
class User(object):
first_name = models.CharField(max_length=255)
def get_username(self):
return self.username
class Employee(User):
title = models.CharField(max_length=255)
...won't cause any tables to be generated.
You could use multiple inheritance to do something like this...
class User(object):
first_name = models.CharField(max_length=255)
def get_username(self):
return self.username
class Employee(User, models.Model):
title = models.CharField(max_length=255)
...which would create a table, but it will ignore the fields defined in the User class, so you'll end up with a table like this...
CREATE TABLE myapp_employee
(
id INT NOT NULL AUTO_INCREMENT,
title VARCHAR(255) NOT NULL,
PRIMARY KEY (id)
);

An abstract model creates a table with the entire set of columns for each subchild, whereas using "plain" Python inheritance creates a set of linked tables (aka "multi-table inheritance"). Consider the case in which you have two models:
class Vehicle(models.Model):
num_wheels = models.PositiveIntegerField()
class Car(Vehicle):
make = models.CharField(…)
year = models.PositiveIntegerField()
If Vehicle is an abstract model, you'll have a single table:
app_car:
| id | num_wheels | make | year
However, if you use plain Python inheritance, you'll have two tables:
app_vehicle:
| id | num_wheels
app_car:
| id | vehicle_id | make | model
Where vehicle_id is a link to a row in app_vehicle that would also have the number of wheels for the car.
Now, Django will put this together nicely in object form so you can access num_wheels as an attribute on Car, but the underlying representation in the database will be different.
Update
To address your updated question, the difference between inheriting from a Django abstract class and inheriting from Python's object is that the former is treated as a database object (so tables for it are synced to the database) and it has the behavior of a Model. Inheriting from a plain Python object gives the class (and its subclasses) none of those qualities.

The main difference is how the databases tables for the models are created.
If you use inheritance without abstract = True Django will create a separate table for both the parent and the child model which hold the fields defined in each model.
If you use abstract = True for the base class Django will only create a table for the classes that inherit from the base class - no matter if the fields are defined in the base class or the inheriting class.
Pros and cons depend on the architecture of your application.
Given the following example models:
class Publishable(models.Model):
title = models.CharField(...)
date = models.DateField(....)
class Meta:
# abstract = True
class BlogEntry(Publishable):
text = models.TextField()
class Image(Publishable):
image = models.ImageField(...)
If the Publishable class is not abstract Django will create a table for publishables with the columns title and date and separate tables for BlogEntry and Image. The advantage of this solution would be that you are able to query across all publishables for fields defined in the base model, no matter if they are blog entries or images. But therefore Django will have to do joins if you e.g. do queries for images...
If making Publishable abstract = True Django will not create a table for Publishable, but only for blog entries and images, containing all fields (also the inherited ones). This would be handy because no joins would be needed to an operation such as get.
Also see Django's documentation on model inheritance.

Just wanted to add something which I haven't seen in other answers.
Unlike with python classes, field name hiding is not permited with model inheritance.
For example, I have experimented issues with an use case as follows:
I had a model inheriting from django's auth PermissionMixin:
class PermissionsMixin(models.Model):
"""
A mixin class that adds the fields and methods necessary to support
Django's Group and Permission model using the ModelBackend.
"""
is_superuser = models.BooleanField(_('superuser status'), default=False,
help_text=_('Designates that this user has all permissions without '
'explicitly assigning them.'))
groups = models.ManyToManyField(Group, verbose_name=_('groups'),
blank=True, help_text=_('The groups this user belongs to. A user will '
'get all permissions granted to each of '
'his/her group.'))
user_permissions = models.ManyToManyField(Permission,
verbose_name=_('user permissions'), blank=True,
help_text='Specific permissions for this user.')
class Meta:
abstract = True
# ...
Then I had my mixin which among other things I wanted it to override the related_name of the groups field. So it was more or less like this:
class WithManagedGroupMixin(object):
groups = models.ManyToManyField(Group, verbose_name=_('groups'),
related_name="%(app_label)s_%(class)s",
blank=True, help_text=_('The groups this user belongs to. A user will '
'get all permissions granted to each of '
'his/her group.'))
I was using this 2 mixins as follows:
class Member(PermissionMixin, WithManagedGroupMixin):
pass
So yeah, I expected this to work but it didn't.
But the issue was more serious because the error I was getting wasn't pointing to the models at all, I had no idea of what was going wrong.
While trying to solve this I randomly decided to change my mixin and convert it to an abstract model mixin. The error changed to this:
django.core.exceptions.FieldError: Local field 'groups' in class 'Member' clashes with field of similar name from base class 'PermissionMixin'
As you can see, this error does explain what is going on.
This was a huge difference, in my opinion :)

The main difference is when you inherit the User class. One version will behave like a simple class, and the other will behave like a Django modeel.
If you inherit the base "object" version, your Employee class will just be a standard class, and first_name won't become part of a database table. You can't create a form or use any other Django features with it.
If you inherit the models.Model version, your Employee class will have all the methods of a Django Model, and it will inherit the first_name field as a database field that can be used in a form.
According to the documentation, an Abstract Model "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."

I will prefer the abstract class in most of the cases because it does not create a separate table and the ORM does not need to create joins in the database. And using abstract class is pretty simple in Django
class Vehicle(models.Model):
title = models.CharField(...)
Name = models.CharField(....)
class Meta:
abstract = True
class Car(Vehicle):
color = models.CharField()
class Bike(Vehicle):
feul_average = models.IntegerField(...)

Object has no attribute _state

I'm developing Django application, and I have following error
'Sheep' object has no attribute _state
My models are constructed like this
class Animal(models.Model):
aul = models.ForeignKey(Aul)
weight = models.IntegerField()
quality = models.IntegerField()
age = models.IntegerField()
def __init__(self,aul):
self.aul=aul
self.weight=3
self.quality=10
self.age=0
def __str__(self):
return self.age
class Sheep(Animal):
wool = models.IntegerField()
def __init__(self,aul):
Animal.__init__(self,aul)
What I must do?

firstly, you must be very careful overriding __init__ to have non-optional arguments. remember it will be called every time you get an object from a queryset!
this is the correct code you want:
class Animal(models.Model):
#class Meta: #uncomment this for an abstract class
# abstract = True
aul = models.ForeignKey(Aul)
weight = models.IntegerField(default=3)
quality = models.IntegerField(default=10)
age = models.IntegerField(default=0)
def __unicode__(self):
return self.age
class Sheep(Animal):
wool = models.IntegerField()
I highly suggest setting the abstract option on Animal if you will only ever be using subclasses of this object. This ensures a table is not created for animal and only for Sheep (etc..). if abstract is not set, then an Animal table will be created and the Sheep class will be given it's own table and an automatic 'animal' field which will be a foreign key to the Animal model.

Django docs recommend against you to use __init__ method in models:
You may be tempted to customize the model by overriding the __init__ method. If you do so, however, take care not to change the calling signature as any change may prevent the model instance from being saved. Rather than overriding __init__, try using one of these approaches:
Add a classmethod on the model class
Add a method on a custom manager (usually preferred)