Added average to QuerySet

mongoengine/queryset.py

@@ -231,6 +231,24 @@ class QuerySet(object):
         """
         return self.exec_js(sum_func, field)
 
+    def average(self, field):
+        """Average over the values of the specified field.
+        """
+        average_func = """
+            function(averageField) {
+                var total = 0.0;
+                var num = 0;
+                db[collection].find(query).forEach(function(doc) {
+                    if (doc[averageField]) {
+                        total += doc[averageField];
+                        num += 1;
+                    }
+                });
+                return total / num;
+            }
+        """
+        return self.exec_js(average_func, field)
+
     def item_frequencies(self, list_field, normalize=False):
         """Returns a dictionary of all items present in a list field across
         the whole queried set of documents, and their corresponding frequency.

tests/queryset.py

@@ -220,6 +220,19 @@ class QuerySetTest(unittest.TestCase):
 
         BlogPost.drop_collection()
 
+    def test_average(self):
+        """Ensure that field can be averaged correctly.
+        """
+        ages = [23, 54, 12, 94, 27]
+        for i, age in enumerate(ages):
+            self.Person(name='test%s' % i, age=age).save()
+
+        avg = float(sum(ages)) / len(ages)
+        self.assertAlmostEqual(int(self.Person.objects.average('age')), avg)
+
+        self.Person(name='ageless person').save()
+        self.assertEqual(int(self.Person.objects.average('age')), avg)
+
     def test_sum(self):
         """Ensure that field can be summed over correctly.
         """