PyNotes

#Django_ORM #Database Renaming Database table name in model class

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

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

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

#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 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 #Aggregation

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

#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 UNION q1.union(q2)

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

#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 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

#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)

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

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

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