Python: What is Django select_related and prefetch_related?

1from django.db import models
2
3class Author(models.Model):
4    name = models.CharField(max_length=100)
5
6class Book(models.Model):
7    title = models.CharField(max_length=200)
8    author = models.ForeignKey(Author, on_delete=models.CASCADE)
9    tags = models.ManyToManyField("Tag")
10
11class Tag(models.Model):
12    name = models.CharField(max_length=50)
13
14# Without optimization — N+1 queries!
15for book in Book.objects.all():
16    print(book.author.name)  # Extra query each time!
17
18# select_related — JOIN (ForeignKey, OneToOne)
19for book in Book.objects.select_related("author").all():
20    print(book.author.name)  # No extra query
21
22# prefetch_related — separate query (ManyToMany)
23for book in Book.objects.prefetch_related("tags").all():
24    print(book.tags.all())  # Tags prefetched
25
26# Chaining
27books = (
28    Book.objects
29    .select_related("author")
30    .prefetch_related("tags")
31    .filter(author__name="Alice")
32)
33
34# Prefetch with custom queryset
35from django.db.models import Prefetch
36
37books = Book.objects.prefetch_related(
38    Prefetch("tags", queryset=Tag.objects.filter(name__startswith="D"))
39)