https://books.agiliq.com/projects/django-orm-cookbook/en/latest/

django_orm_cookbook_8_13_example.py

# Ref: https://books.agiliq.com/projects/django-orm-cookbook/en/latest/

# 8. How to filter a queryset with criteria based on comparing their field values
User.objects.filter(name_startswith='R')
User.objects.filter(last_name=F("first_name"))
User.objects.annotate(first=Substr("first_name", 1, 1), last=Substr("last_name", 1, 1)).filter(first=F("last"))


# 9. How to filter FileField without any file?
no_files_objects = MyModel.objects.filter(
    Q(file='')|Q(file=None)
)


# 10. How to perform join operations in django ORM?
>>> a1 = Article.objects.select_related('reporter') // Using select_related
>>> a1
<QuerySet [<Article: International News>, <Article: Local News>, <Article: Morning news>, <Article: Prime time>, <Article: Test Article>, <Article: Weather Report>]>
>>> print(a1.query)
SELECT "events_article"."id", "events_article"."headline", "events_article"."pub_date", "events_article"."reporter_id", "events_article"."slug", "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 "events_article" INNER JOIN "auth_user" ON ("events_article"."reporter_id" = "auth_user"."id") ORDER BY "events_article"."headline" ASC
>>> a2 = Article.objects.filter(reporter__username='John')
>>> a2
<QuerySet [<Article: International News>, <Article: Local News>, <Article: Prime time>, <Article: Test Article>, <Article: Weather Report>]>
>>> print(a2.query)
SELECT "events_article"."id", "events_article"."headline", "events_article"."pub_date", "events_article"."reporter_id", "events_article"."slug" FROM "events_article" INNER JOIN "auth_user" ON ("events_article"."reporter_id" = "auth_user"."id") WHERE "auth_user"."username" = John ORDER BY "events_article"."headline" ASC


# 11. How to find second largest record using Django ORM ?
>>> user = User.objects.order_by('-last_login')[1] // Second Highest record w.r.t 'last_login'
>>> user.first_name
'Raghu'
>>> user = User.objects.order_by('-last_login')[2] // Third Highest record w.r.t 'last_login'
>>> user.first_name
'Sohan'

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


# 12. Find rows which have duplicate field values
>>> duplicates = User.objects.values(
    'first_name'
    ).annotate(name_count=Count('first_name')).filter(name_count__gt=1)
>>> duplicates
<QuerySet [{'first_name': 'John', 'name_count': 3}]>

>>> records = User.objects.filter(first_name__in=[item['first_name'] for item in duplicates])
>>> print([item.id for item in records])
[2, 11, 13]


# 13. How to find distinct field values from queryset?
distinct = User.objects.values(
    'first_name'
).annotate(
    name_count=Count('first_name')
).filter(name_count=1)
records = User.objects.filter(first_name__in=[item['first_name'] for item in distinct])