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select_related

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Returns a QuerySet that will “follow” foreign-key relationships, selecting additional related-object data when it executes its query. This is a performance booster which results in a single more complex query but means later use of foreign-key relationships won’t require database queries.

You can refer to any ForeignKey or OneToOneField relation in the list of fields passed to select_related().

If you need to clear the list of related fields added by past calls of select_related on a QuerySet, you can pass None as a parameter:

without_relations = queryset.select_related(None)

Chaining select_related calls works in a similar way to other methods - that is that select_related('foo', 'bar') is equivalent to select_related('foo').select_related('bar').

Plain Lookup

# Hits the database.
e = Entry.objects.get(id=5)

# Hits the database again to get the related Blog object.
b = e.blog

select_related lookup

# Hits the database.
e = Entry.objects.select_related("blog").get(id=5)

# Doesn't hit the database, because e.blog has been prepopulated
# in the previous query.
b = e.blog

prefetch_related

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• Returns a QuerySet that will automatically retrieve, in a single batch, related objects for each of the specified lookups.

• prefetch_related, on the other hand, does a separate lookup for each relationship, and does the ‘joining’ in Python.

• The additional queries in prefetch_related() are executed after the QuerySet has begun to be evaluated and the primary query has been executed.

Prefetch() objects

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class Prefetch(lookup, queryset=None, to_attr=None)

The Prefetch() object can be used to control the operation of prefetch_related().

nested prefetches example

Prefetch(
    "cross_references",
    queryset=Annotation.objects.annotate(
        law_abbreviation=F("translation__abbreviation"),
        document_abbreviation=F("document_translation__abbreviation"),
    )
    .prefetch_related("creator__profile")
    .all(),
),

N+1 Queries performance issue

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