kyle-eshares

for book in Books.objects.all():
    do_stuff(book)

kyle-eshares

for book in Books.objects.all().iterator():
    do_stuff(book)

kyle-eshares

if book.author_id:
    do_stuff()

do_stuff_with_author_id(book.author_id)

kyle-eshares

if book.author:
    do_stuff()
#  Or
do_stuff_with_author_id(book.author.id)

kyle-eshares

>>> Author.objects.all()
SELECT "library_author"."id", "library_author"."name" FROM "library_author" LIMIT 21
Execution time: 0.001393s [Database: default]
<QuerySet [<Author: Author object>]>

kyle-eshares

>>> from django.db import connection
>>> connection.queries
[]
>>> Author.objects.all()
<QuerySet [<Author: Author object>]>
>>> connection.queries
[{u'time': u'0.002', u'sql': u'SELECT "library_author"."id", "library_author"."name" FROM "library_author" LIMIT 21'}]

kyle-eshares

class Author(models.Model):
    name = models.TextField()

class Book(models.Model):
    title = models.TextField()
    author = models.ForeignKey(
        Author, on_delete=models.PROTECT, related_name='books', null=True
    )

kyle-eshares

>>> Author.objects.count()
20
>>> Book.objects.count()
100
#  This block is 101 queries.
#  1 for the books and 1 for each author that lazy-loaded 
books = Book.objects.all()
for book in books:
   do_stuff(book.title, book.author.name)

kyle-eshares

#  This block is 1 query
#  The authors of all the books are pre-fetched in one query
book = Book.objects.selected_related('author').all()
for book in books:
   do_stuff(book.title, book.author)

#  This block is 1 query
#  The books of all the authors are pre-fetched in one query
authors = Author.objects.prefetch_related('books').all()
for author in authors:

kyle-eshares

#  Retrieve values as a dictionary
>>> Book.objects.values('title', 'author__name')
<QuerySet [{'author__name': u'Nikolai Gogol', 'title': u'The Overcoat'}, {'author__name': u'Leo Tolstoy', 'title': u'War and Peace'}]>

#  Retrieve values as a tuple
>>> Book.objects.values_list('title', 'author__name')
<QuerySet [(u'The Overcoat', u'Nikolai Gogol'),
(u'War and Peace', u'Leo Tolstoy')]>
>>> Book.objects.values_list('title')
<QuerySet [(u'The Overcoat',), (u'War and Peace',)]>