fix issue avidal#14

Author: michiya

django_pyodbc/aggregates.py

@@ -1,20 +1,37 @@
-from django.db.models.sql.aggregates import *
+from django.db.models.sql.aggregates import Aggregate
 
-class StdDev(Aggregate):
+class _Aggregate(Aggregate):
+
+    def __init__(self, lookup, **extra):
+        self.lookup = lookup
+        self.extra = extra
+
+    def _default_alias(self):
+        return '%s__%s' % (self.lookup, self.__class__.__name__.lower())
+    default_alias = property(_default_alias)
+
+    def add_to_query(self, query, alias, col, source, is_summary):
+        super(_Aggregate, self).__init__(col, source, is_summary, **self.extra)
+        query.aggregates[alias] = self
+
+class StdDev(_Aggregate):
+    name = 'StdDev'
     is_computed = True
 
     def __init__(self, col, sample=False, **extra):
         super(StdDev, self).__init__(col, **extra)
         self.sql_function = sample and 'STDEV' or 'STDEVP'
 
-class Variance(Aggregate):
+class Variance(_Aggregate):
+    name = 'Variance'
     is_computed = True
 
     def __init__(self, col, sample=False, **extra):
         super(Variance, self).__init__(col, **extra)
         self.sql_function = sample and 'VAR' or 'VARP'
 
-class Avg(Aggregate):
+class Avg(_Aggregate):
+    name = 'Avg'
     is_computed = True
     sql_function = 'AVG'
     sql_template = '%(function)s(Convert(FLOAT, %(field)s))'