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Django models

null=True and blank=True

null=True and blank=True are two different concepts in Django models.

• null=True specifies that the database column will accept NULL values.
• blank=True specifies that the field is allowed to be blank in forms.

For example, if you have a CharField in your model and you want to allow empty strings in the database, you should set null=True:

from django.db import models

class MyModel(models.Model):
    name = models.CharField(max_length=100, null=True)

If you want to allow empty strings in forms, you should set blank=True:

from django.db import models

class MyModel(models.Model):
    name = models.CharField(max_length=100, blank=True)

created and modified fields

Create a model which as a created and modified field. This is useful for tracking when a record was created and last updated.

from django.db import models

class SimpleTimeStampedModel(models.Model):
    created = models.DateTimeField(auto_now_add=True)
    modified = models.DateTimeField(auto_now=True)

    class Meta:
        abstract = True

Then, inherit from this model in your models.

from django.db import models

class MyModel(SimpleTimeStampedModel):
    name = models.CharField(max_length=100)

Now, whenever you create or update a MyModel instance, the created and modified fields will be automatically updated.

Using update_fields with save()

When you call save() on a model instance, you can pass an update_fields argument to specify which fields should be updated. This can be useful when you only want to update specific fields.
However, if you pass update_fields to save(), the modified field will not be updated. The CustomTimeStampedModel below fixes this issue.

from django.db import models

class CustomTimeStampedModel(models.Model):
    created = models.DateTimeField(auto_now_add=True)
    modified = models.DateTimeField(auto_now=True)

    class Meta:
        abstract = True

    def save(self, *args, **kwargs):
        update_fields = kwargs.get('update_fields', None)
        if update_fields:
            kwargs['update_fields'] = set(update_fields).union({'modified'})
        super().save(*args, **kwargs)

Now, when you call save() with update_fields, the modified field will be updated as well.

my_model = MyModel.objects.get(pk=1)
my_model.name = 'New Name'
my_model.save(update_fields=['name'])

slug field

Create a model which has a slug field. This is useful for creating SEO-friendly URLs. Using AutoSlugField from django-autoslug makes this easy.

from django.db import models
from autoslug import AutoSlugField

class Example(models.Model):
    name = models.CharField(max_length=100)
    slug = models.AutoSlugField(unique=True, always_update=False, populate_from="name")

model managers

Model managers are a feature of Django that allows you to customize the behavior of model instances. It is recommended to always set objects = models.Manager() above any custom model manager that has a new name.

from django.db import models

# First, define the Manager subclass.
class DahlBookManager(models.Manager):
    def get_queryset(self):
        return super().get_queryset().filter(author="Roald Dahl")


# 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.