Fewer queries on stats/scales + Fix

Scales:
- Fix #chunks when used with std_chunk=True (there was one too many at
  the beginning)
- Scale.end gives the end of the last chunk (instead of its start)
  So scale.begin -> scale.end gives the full range of the scale.

`kfet_day` now returns an aware datetime.

ScaleMixin:
- new method `get_by_chunks` which use only one query and ranks
  elements according to the scale. Elements are returned by a generator
  for each scale chunk (and all chunks are returned as a generator too).

ArticlesStatSales and AccountStatOperations use this new method to
avoid issuing #scale_chunks queries.

ArticleStat:
- fixed on Chrome

kfet/statistic.py

@@ -4,6 +4,7 @@ from datetime import date, datetime, time, timedelta
 
 from dateutil.relativedelta import relativedelta
 from dateutil.parser import parse as dateutil_parse
+import pytz
 
 from django.utils import timezone
 from django.db.models import Sum
@@ -13,7 +14,8 @@ KFET_WAKES_UP_AT = time(7, 0)
 
 def kfet_day(year, month, day, start_at=KFET_WAKES_UP_AT):
     """datetime wrapper with time offset."""
-    return datetime.combine(date(year, month, day), start_at)
+    naive = datetime.combine(date(year, month, day), start_at)
+    return pytz.timezone('Europe/Paris').localize(naive, is_dst=None)
 
 
 def to_kfet_day(dt, start_at=KFET_WAKES_UP_AT):
@@ -32,16 +34,21 @@ class Scale(object):
         self.std_chunk = std_chunk
         if last:
             end = timezone.now()
+        if std_chunk:
+            if begin is not None:
+                begin = self.get_chunk_start(begin)
+            if end is not None:
+                end = self.do_step(self.get_chunk_start(end))
 
         if begin is not None and n_steps != 0:
-            self.begin = self.get_from(begin)
+            self.begin = begin
             self.end = self.do_step(self.begin, n_steps=n_steps)
         elif end is not None and n_steps != 0:
-            self.end = self.get_from(end)
+            self.end = end
             self.begin = self.do_step(self.end, n_steps=-n_steps)
         elif begin is not None and end is not None:
-            self.begin = self.get_from(begin)
-            self.end = self.get_from(end)
+            self.begin = begin
+            self.end = end
         else:
             raise Exception('Two of these args must be specified: '
                             'n_steps, begin, end; '
@@ -71,7 +78,7 @@ class Scale(object):
     def get_datetimes(self):
         datetimes = [self.begin]
         tmp = self.begin
-        while tmp <= self.end:
+        while tmp < self.end:
             tmp = self.do_step(tmp)
             datetimes.append(tmp)
         return datetimes
@@ -232,3 +239,87 @@ class ScaleMixin(object):
             qs.filter(**{begin_f: begin, end_f: end})
             for begin, end in scale
         ]
+
+    def get_by_chunks(self, qs, scale, field_callback=None, field_db='at'):
+        """Objects of queryset ranked according to a given scale.
+
+        Returns a generator whose each item, corresponding to a scale chunk,
+        is a generator of objects from qs for this chunk.
+
+        Args:
+            qs: Queryset of source objects, must be ordered *first* on the
+                same field returned by `field_callback`.
+            scale: Used to rank objects.
+            field_callback: Callable which gives value from an object used
+                to compare against limits of the scale chunks.
+                Default to: lambda obj: getattr(obj, field_db)
+            field_db: Used to filter against `scale` limits.
+                Default to 'at'.
+
+        Examples:
+            If queryset `qs` use `values()`, `field_callback` must be set and
+                could be: `lambda d: d['at']`
+            If `field_db` use foreign attributes (eg with `__`), it should be
+                something like: `lambda obj: obj.group.at`.
+
+        """
+        if field_callback is None:
+            def field_callback(obj):
+                return getattr(obj, field_db)
+
+        begin_f = '{}__gte'.format(field_db)
+        end_f = '{}__lte'.format(field_db)
+
+        qs = (
+            qs
+            .filter(**{begin_f: scale.begin, end_f: scale.end})
+        )
+
+        obj_iter = iter(qs)
+
+        last_obj = None
+
+        def _objects_until(obj_iter, field_callback, end):
+            """Generator of objects until `end`.
+
+            Ends if objects source is empty or when an object not verifying
+            field_callback(obj) <= end is met.
+
+            If this object exists, it is stored in `last_obj` which is found
+            from outer scope.
+            Also, if this same variable is non-empty when the function is
+            called, it first yields its content.
+
+            Args:
+                obj_iter: Source used to get objects.
+                field_callback: Returned value, when it is called on an object
+                    will be used to test ordering against `end`.
+                end
+
+            """
+            nonlocal last_obj
+
+            if last_obj is not None:
+                yield last_obj
+                last_obj = None
+
+            for obj in obj_iter:
+                if field_callback(obj) <= end:
+                    yield obj
+                else:
+                    last_obj = obj
+                    return
+
+        for begin, end in scale:
+            # forward last seen object, if it exists, to the right chunk,
+            # and fill with empty generators for intermediate chunks of scale
+            if last_obj is not None:
+                if field_callback(last_obj) > end:
+                    yield iter(())
+                    continue
+
+            # yields generator for this chunk
+            # this set last_obj to None if obj_iter reach its end, otherwise
+            # it's set to the first met object from obj_iter which doesn't
+            # belong to this chunk
+            yield _objects_until(obj_iter, field_callback, end)