Learn Python Coding on Telegram: “entry = Entry.objects.get(pk=1) entry.rating = 5…”

entry = Entry.objects.get(pk=1)
entry.rating = 5
entry.save()

• Update multiple objects in a single query.

Entry.objects.filter(pub_date__year=2022).update(rating=0)

• Remove an object from a ManyToManyField.

entry.tags.remove(tag1)

VII. Deleting Objects

• Delete a single object.

entry = Entry.objects.get(pk=1)
entry.delete()

• Delete multiple objects in a single query.

Entry.objects.filter(rating=0).delete()

VIII. Spanning Relationships

• Filter based on a related model's field.

Entry.objects.filter(author__name="John Doe")

• Use a field lookup on a related model.

Entry.objects.filter(author__name__startswith="John")

• Access related objects from the "one" side of a relationship.

author = Author.objects.get(name="John Doe")
authors_entries = author.entry_set.all()

IX. Aggregation
(Requires from django.db.models import ...)

• Calculate aggregate values over an entire queryset.

from django.db.models import Avg
avg_rating = Entry.objects.aggregate(Avg('rating'))

• Average (Avg).

Entry.objects.aggregate(average_rating=Avg('rating'))

• Sum (Sum).

Entry.objects.aggregate(total_ratings=Sum('rating'))

• Maximum (Max) and Minimum (Min).

Entry.objects.aggregate(max_r=Max('rating'), min_r=Min('rating'))

• Count (Count).

Entry.objects.aggregate(num_entries=Count('pk'))

• Count distinct values.

Entry.objects.aggregate(num_authors=Count('author', distinct=True))

X. Annotation

• Add a calculated field to each object in a queryset.

from django.db.models import Count
authors = Author.objects.annotate(entry_count=Count('entry'))

• Access an annotated value.

for author in authors:
    print(author.name, author.entry_count)

• Get results as a list of dictionaries.

Entry.objects.values('headline', 'author__name')

XI. F() Expressions

• Import F() expressions.

from django.db.models import F

• Update a field based on its own value.

Entry.objects.filter(pk=1).update(rating=F('rating') + 1)

• Filter by comparing two fields on the same model.

Entry.objects.filter(rating__gt=F('author__rating'))

XII. Q() Objects

• Import Q() objects for complex queries.

from django.db.models import Q

• Create an OR query.

Entry.objects.filter(Q(headline__startswith='A') | Q(headline__startswith='B'))

• Create a NOT query.

Entry.objects.filter(~Q(headline__icontains='boring'))

XIII. QuerySet Evaluation

• Get the count of objects efficiently.

count = Entry.objects.count()

• Check if any objects exist efficiently.

exists = Entry.objects.filter(rating=5).exists()

• Get the count (evaluates the full queryset).

count = len(Entry.objects.all())

• Check for existence (evaluates the full queryset).

if Entry.objects.filter(rating=5): # True if queryset is not empty
    ...

• Iteration (evaluates the queryset).

for entry in Entry.objects.all():
    print(entry.headline)

XIV. Optimization