#Django_ORM #filters
Select_related, prefetch_related

class ModelA(models.Model):
pass

class ModelB(models.Model):
a = ForeignKey(ModelA)

ModelB.objects.select_related('a').all() # Forward ForeignKey relationship
ModelA.objects.prefetch_related('modelb_set').all() # Reverse ForeignKey relationship

#Django_ORM #filters

Find Nth record from the query by using slice operator except using first() and last()

e.g user = User.objects.order_by('-last_login')[2] // Second Highest record w.r.t 'last_login'

Sql query will look like this:
SELECT "auth_user"."id",
"auth_user"."password",
"auth_user"."last_login",
"auth_user"."is_superuser",
"auth_user"."username",
"auth_user"."first_name",
"auth_user"."last_name",
"auth_user"."email",
"auth_user"."is_staff",
"auth_user"."is_active",
"auth_user"."date_joined"
FROM "auth_user"
ORDER BY "auth_user"."last_login" DESC
LIMIT 1
OFFSET 2

#Django_ORM #Aggregation

#Django_ORM
Make model save only one object
`class Origin(models.Model):
name = models.CharField(max_length=100)

def save(self, *args, **kwargs):
if self.class.objects.count():
self.pk = self.class.objects.first().pk
super().save(*args, **kwargs)`

#Django_ORM #Models
Signals vs Overriding .save
—If your fields depend on a model you control, override .save
—If your fields depend on a model from a 3rd party app, which you do no control, use signals.

#Django_ORM #ordering
User.objects.all().order_by('date_joined', '-last_login')

#Django_ORM #ordering
Order queryset by case insensitive manner
User.objects.all().order_by(Lower('username'))

#Django_ORM #Database
Convert existing databases to Django models
python manage.py inspectdb
or
python manage.py inspectdb > models.py

#Django_ORM #Database
Renaming Database table name in model class

#Django_ORM #Database
Rename column name in Database Table:
` a = models.CharField(max_length=40,db_column='column1')`