Added aggregation methods to user guide

docs/userguide.rst

@@ -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]
+