diff --git a/docs/ref/models/querysets.txt b/docs/ref/models/querysets.txt index 96fa5c9f26..269e2ce61c 100644 --- a/docs/ref/models/querysets.txt +++ b/docs/ref/models/querysets.txt @@ -31,6 +31,9 @@ You can evaluate a ``QuerySet`` in the following ways: for e in Entry.objects.all(): print(e.headline) + Note: Don't use this if all you want to do is determine if at least one + result exists. It's more efficient to use :meth:`~QuerySet.exists`. + * **Slicing.** As explained in :ref:`limiting-querysets`, a ``QuerySet`` can be sliced, using Python's array-slicing syntax. Slicing an unevaluated ``QuerySet`` usually returns another unevaluated ``QuerySet``, but Django @@ -75,7 +78,7 @@ You can evaluate a ``QuerySet`` in the following ways: Note: *Don't* use this if all you want to do is determine if at least one result exists, and don't need the actual objects. It's more efficient to - use :meth:`exists() ` (see below). + use :meth:`~QuerySet.exists` (see below). .. _pickling QuerySets: @@ -1268,7 +1271,7 @@ The :exc:`~django.core.exceptions.DoesNotExist` exception inherits from e = Entry.objects.get(id=3) b = Blog.objects.get(id=1) except ObjectDoesNotExist: - print("Either the entry or blog doesn't exist.") + print "Either the entry or blog doesn't exist." create ~~~~~~ @@ -1523,9 +1526,40 @@ exists Returns ``True`` if the :class:`.QuerySet` contains any results, and ``False`` if not. This tries to perform the query in the simplest and fastest way -possible, but it *does* execute nearly the same query. This means that calling -:meth:`.QuerySet.exists` is faster than ``bool(some_query_set)``, but not by -a large degree. If ``some_query_set`` has not yet been evaluated, but you know +possible, but it *does* execute nearly the same query as a normal +:class:`.QuerySet` query. + +:meth:`~.QuerySet.exists` is useful for searches relating to both +object membership in a :class:`.QuerySet` and to the existence of any objects in +a :class:`.QuerySet`, particularly in the context of a large :class:`.QuerySet`. + +The most efficient method of finding whether a model with a unique field +(e.g. ``primary_key``) is a member of a :class:`.QuerySet` is:: + + entry = Entry.objects.get(pk=123) + if some_query_set.filter(pk=entry.pk).exists(): + print "Entry contained in queryset" + +Which will be faster than the following which requires evaluating and iterating +through the entire queryset:: + + if entry in some_query_set: + print "Entry contained in QuerySet" + +And to find whether a queryset contains any items:: + + if some_query_set.exists(): + print "There is at least one object in some_query_set" + +Which will be faster than:: + + if some_query_set: + print "There is at least one object in some_query_set" + +... but not by a large degree (hence needing a large queryset for efficiency +gains). + +Additionally, if a ``some_query_set`` has not yet been evaluated, but you know that it will be at some point, then using ``some_query_set.exists()`` will do more overall work (one query for the existence check plus an extra one to later retrieve the results) than simply using ``bool(some_query_set)``, which