add method to get natural weeks order

emissions/models.py

@@ -231,6 +231,20 @@ class Schedule(models.Model, WeekdayMixin):
             return ugettext('2nd and 4th')
         return None
 
+    def week_sort_key(self):
+        order = [
+            0b1111, # Every week
+            0b0001, # First week
+            0b0101, # First and third week
+            0b0010, # Second week
+            0b1010, # Second and fourth week
+            0b0100, # Third week
+            0b1000, # Fourth week
+        ]
+        if self.weeks in order:
+            return order.index(self.weeks)
+        return -1
+
     def get_duration(self):
         if self.duration:
             return self.duration