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fix #937

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This commit is contained in:
Stefan Wojcik 2017-03-05 00:05:33 -05:00
parent 30e8b8186f
commit 19ef2be88b
1 changed files with 11 additions and 6 deletions
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17
docs/guide/querying.rst
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@ -340,14 +340,19 @@ Javascript code that is executed on the database server.
Counting results
----------------
Just as with limiting and skipping results, there is a method on
:class:`~mongoengine.queryset.QuerySet` objects --
:meth:`~mongoengine.queryset.QuerySet.count`, but there is also a more Pythonic
way of achieving this::
Just as with limiting and skipping results, there is a method on a
:class:`~mongoengine.queryset.QuerySet` object --
:meth:`~mongoengine.queryset.QuerySet.count`::
num_users = len(User.objects)
num_users = User.objects.count()
Even if len() is the Pythonic way of counting results, keep in mind that if you concerned about performance, :meth:`~mongoengine.queryset.QuerySet.count` is the way to go since it only execute a server side count query, while len() retrieves the results, places them in cache, and finally counts them. If we compare the performance of the two operations, len() is much slower than :meth:`~mongoengine.queryset.QuerySet.count`.
You could technically use ``len(User.objects)`` to get the same result, but it
would be significantly slower than :meth:`~mongoengine.queryset.QuerySet.count`.
When you execute a server-side count query, you let MongoDB do the heavy
lifting and you receive a single integer over the wire. Meanwhile, len()
retrieves all the results, places them in a local cache, and finally counts
them. If we compare the performance of the two operations, len() is much slower
than :meth:`~mongoengine.queryset.QuerySet.count`.
Further aggregation
-------------------