Develop Reference

Django Rest Framework: Get field name from model definition

Within the Django Rest framework documentation it is suggested to declare the "field" list explicitly to avoid providing the data of new columns just by adding them to the model which may contain sensitive information.
The field list is an array of strings, containing the field ids. To avoid declaring field ids, which actually do not exist in the model (e.g. typos or changed models) I tried to declare the list using object references - but always end up with "DeferredAttribute: object has no attribute ".
I have read something that meta information is not available in objects and that you could solve that by defininig your own Meta class using Object._meta.get_fields() and store it in the class, but I thought there might be a simpler/more elegant way (and I do now know, how, in detail ;-)).
Example:
class Samples(models.Model):
# Meta data, primarily used in AdminSite.
class Meta:
verbose_name = _('Samples')
verbose_name_plural = _('Samples')
samples_boolfield = models.BooleanField
samples_textfield = models.CharField(max_length=2000, blank=True)
views.py:
class SamplesView(viewsets.ModelViewSet):
serializer_class = SamplesSerializer
queryset = Samples.objects.all()
serializers.py:
Version 1, which does not show any errors in pyCharm or makemigrations, but calling the API reults in "TypeError at /api/samples/: argument of type 'DeferredAttribute' is not iterable":
class SamplesSerializer(serializers.ModelSerializer):
class Meta:
model = Samples
fields = (
'id',
Samples.samples_boolfield,
Samples.samples_textfield,
)
Version 2, which does not show any errors in pyCharm, but makemigrations fails with "DeferredAttribute: object has no attribute name":
class SamplesSerializer(serializers.ModelSerializer):
class Meta:
model = Samples
fields = (
'id',
Samples.samples_boolfield.__name__,
Samples.samples_textfield.__name__,
)
Version 3, which does not show any errors in pyCharm, but makemigrations fails with "DeferredAttribute: object has no attribute get_attname":
class SamplesSerializer(serializers.ModelSerializer):
class Meta:
model = Samples
fields = (
'id',
Samples.samples_boolfield.get_attname(),
Samples.samples_textfield.get_attname(),
)
Is there a way to declare the field list using object references (so that it fails e.g. in pyCharm/during compilation)?
Thank you for your feedback.
Regards,
HerrB92

In Django, how can I easily inherit a field and create a new class with this field preconfigured parameters?

I am currently using UUID in my PostgreSQL database, therefore I am also using PrimaryKeyRelatedField() with some parameters in order to avoid problems when encoding to JSON the UUID field.
My serializer field looks like:
id = serializers.PrimaryKeyRelatedField(read_only=True,
allow_null=False,
pk_field=serializers.UUIDField(format='hex_verbose'))
And in every serializer that uses UUID I am having to use that.
My question is, how can I create a new class based on PrimaryKeyRelatedField so that I don't have to write all those parameters (read_only, allow_null...) ?
I am looking for something like:
id = BaseUUIDField()
Thanks

You can make an abstract class using the id which is a uuid field. Then inheret that model in your derived models.
import uuid
from django.db import models
//Abstract Model
class AbstractModel(models.Model):
id = models.UUIDField(primary_key=True,default=uuid.uuid4, editable=False)
class Meta:
Abstract =True
//Derived Model
class YourDerivedModel(Abstract.Model):
//fields here
Hope this helps your query

Expose multiple similar database fields as enumerable collection

I have a Django (1.8) Model for an underlying database table that has multiple columns that are logically a fixed-size array. For example:
from django.db import models
class Widget(models.Model):
# ...
description_1 = models.CharField(max_length=255)
description_2 = models.CharField(max_length=255)
description_3 = models.CharField(max_length=255)
# ...
I would like to be able to access these columns as if they were a collection on the model instance, e.g.:
instance = Widget.objects.get(...)
for description in instance.descriptions:
# do something with each description
My primary motivation is that I am exposing this model via Django Rest Framework (DRF), and would like the API clients to be able to easily enumerate the descriptions associated with the model. As it stands, the clients have to reference each logical 'index' manually, which makes the code repetitive.
My DRF serializer code is currently like this:
class WidgetSerializer(serializers.HyperlinkedModelSerializer):
class Meta:
model = Widget
There are a fixed number of descriptions for each Widget, and their ordering is important.
Is there a clean way to expose these fields as a collection on the Model object?

It really was as easy as adding a method to the Model class that returns the fields as a sequence, and then (for API clients), manually specifying that new method as a field to serialize.
So the Model definition becomes:
from django.db import models
class Widget(models.Model):
description_1 = models.CharField(max_length=255)
description_2 = models.CharField(max_length=255)
description_3 = models.CharField(max_length=255)
def descriptions(self):
return self.description_1, self.description_2, self.description_3
And the DRF serializer is updated like:
class WidgetSerializer(serializers.HyperlinkedModelSerializer):
class Meta:
model = Widget
fields = ('url', 'descriptions',)
This causes the API to return a JSON array for descriptions and omit all of the individual description_x fields.

Multiple Django Admin Arguments with Extensions

Is there a way to use multiple Django extensions in the admin.site.register() inside admin.py? I'm using "simple-history" and "import-export" extensions, but I can only have one of them in the admin.site.register().
Example: I have a model named, "Cars", that is using the "simple-history" extension so I need admin.site.register(Cars, SimpleHistoryAdmin), as their documentation says it should. I want to use the import/export extension as well to the same "Cars" model, but the admin.site.register() doesn't take multiple arguments for me to add it.
models.py
class Cars(models.Model):
Year = models.CharField(max_length=30)
Make = models.CharField(max_length=30)
Model = models.CharField(max_length=30)
history = HistoricalRecords()
class Meta:
verbose_name_plural = "Car Table"
def __str__(self):
return self.Make
admin.py
class CarResource(resources.ModelResource):
class Meta:
model = Cars
fields = ('id','Year', 'Make', 'Model',)
class CarAdmin(ImportExportModelAdmin):
resource_class = CarResource
pass
#I want to use the import/export extension (code above), along with simple-history
admin.site.register(Cars, CarAdmin)
admin.site.register(Cars, SimpleHistoryAdmin)
I've tried using a proxy and inlines, but the proxy makes a new model which I don't want and when using inlines I get an error saying that it requires a foreign key, but I'm not trying to get the model objects from a different model. Naming them the same model doesn't work because the model is already registered. Any help is much appreciated!

In python, class can have more than one parent. Just inherit from 2 parents at once. But both ImportExportModelAdmin and SimpleHistoryAdmin are inheriting from ModelAdmin, that's not good. There is also ImportExportMixin, we can use it instead of ImportExportModelAdmin, so there will be only one reference to ModelAdmin.
class CarResource(resources.ModelResource):
class Meta:
model = Cars
fields = ('id','Year', 'Make', 'Model',)
class CarAdmin(ImportExportMixin, SimpleHistoryAdmin):
resource_class = CarResource
pass
#I want to use the import/export extension (code above), along with simple-history
admin.site.register(Cars, CarAdmin)

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(...)