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Added aggregation methods to user guide

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Harry Marr 2010-01-02 20:42:18 +00:00
parent e9254f471f
commit 62bda75112
1 changed files with 48 additions and 1 deletions
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docs/userguide.rst
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@ -295,7 +295,7 @@ Just as with traditional ORMs, you may limit the number of results returned, or
skip a number or results in you query.
:meth:`mongoengine.queryset.QuerySet.limit` and
:meth:`mongoengine.queryset.QuerySet.skip` and methods are available on
:meth:`mongoengine.queryset.QuerySet` objects, but the prefered syntax for
:class:`mongoengine.queryset.QuerySet` objects, but the prefered syntax for
achieving this is using array-slicing syntax::
# Only the first 5 people
@ -306,3 +306,50 @@ achieving this is using array-slicing syntax::
# 5 users, starting from the 10th user found
users = User.objects[10:15]
Aggregation
-----------
MongoDB provides some aggregation methods out of the box, but there are not as
many as you typically get with an RDBMS. MongoEngine provides a wrapper around
the built-in methods and provides some of its own, which are implemented as
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::
num_users = len(User.objects)
Further aggregation
^^^^^^^^^^^^^^^^^^^
You may sum over the values of a specific field on documents using
:meth:`mongoengine.queryset.QuerySet.sum`::
yearly_expense = Employee.objects.sum('salary')
.. note::
If the field isn't present on a document, that document will be ignored from
the sum.
To get the average (mean) of a field on a collection of documents, use
:meth:`mongoengine.queryset.QuerySet.average`::
mean_age = User.objects.average('age')
As MongoDB provides native lists, MongoEngine provides a helper method to get a
dictionary of the frequencies of items in lists across an entire collection --
:meth:`mongoengine.queryset.QuerySet.item_frequencies`. An example of its use
would be generating "tag-clouds"::
class Article(Document):
tag = ListField(StringField())
# After adding some tagged articles...
tag_freqs = Article.objects.item_frequencies('tag', normalize=True)
from operator import itemgetter
top_tags = sorted(tag_freqs.items(), key=itemgetter(1), reverse=True)[:10]