#Django_ORM #filters
OR in filters:
1) queryset = User.objects.filter( first_name__startswith='R') | User.objects.filter(last_name__startswith='D')

2)qs = User.objects.filter(Q(first_name__startswith='R')|Q(last_name__startswith='D'))

In both cases SQL query will be the same

#Django_ORM #filters
AND in filters:
1) queryset = User.objects.filter( first_name__startswith='R') and User.objects.filter(last_name__startswith='D')
2) queryset_1 & queryset_2
3)filter(Q(<condition_1>) & Q(<condition_2>))

#Django_ORM #filters
NOT
1)queryset = User.objects.filter(~Q(id__lt=5))
2)exclude(condition)

#Django_ORM #filters
UNION
q1.union(q2)

#Django_ORM #filters
Subquery

Django allows using SQL subqueries. Let’s start with something simple, We have a UserParent model which has OnetoOne relation with auth user. We will find all the UserParent which have a UserParent.

from django.db.models import Subquery
users = User.objects.all()
UserParent.objects.filter(user_id__in=Subquery(users.values('id')))

#source
https://books.agiliq.com/projects/django-orm-cookbook/en/latest/subquery.html

#Django_ORM #filters
F
F is used to filter fields of the same model:
User.objects.filter(last_name=F("first_name"))

#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')`