Spaces or Tabs

Generally, spaces are the preferred indentation means but if you find some Python scripts already using the tabs, you should go on doing indentation with tabs. Otherwise, you should change the indentation of all the expressions in your script with spaces.

Note that Python 3 doesn't allow mixing tabs and spaces for indentation. That's why you should choose one of the two and stick with it!

Code Should have line length of 79 characters
Comments should have 72 characters of line length.

In Python scripts, top-level function and classes are separated by two blank lines. Method definitions inside classes should be separated by one blank line.

Know when to be inconsistent – sometimes the style guide just doesn’t apply. When in doubt, use your best judgment. Look at other examples and decide what looks best. And don’t hesitate to ask!

знайте, когда нужно быть непоследовательным - иногда руководство по стилю просто неприменимо. В случае сомнений используйте здравый смысл.
Посмотрите другие примеры и решите, что выглядит лучше всего. И не стесняйтесь спрашивать!
Andrei Boyanov SSE

If you want to change your last commit message that is not pushed yet:
git commit --amend -m "New and correct message"

#Filters
gt - greater than
gte- greater thank or equal
lt -less than
lte less than or equal
e.g Event.objects.filter(visits__gt=15)

#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