add qcombine class, to combine multiple models in a single queryset

cms/qcombine.py

@@ -0,0 +1,76 @@
+import operator
+import itertools
+import heapq
+
+from django.db.models.query import QuerySet
+
+class QCombine:
+    """
+    This class helps to combine querysets of different models and lists of
+    object, and to iter over it.
+
+    Notes:
+    - when working on fields, we assume that they exists on all of them;
+    - for efficiency, there is no possibility to change order per field;
+      to do so, do it directly on the querysets
+    """
+    order_fields = None
+    lists = None
+
+    def __init__(self, *lists):
+        """
+        lists: list of querysets that are used to initialize the stuff.
+        """
+        self.lists = lists or []
+
+    def filter(self, **kwargs):
+        for qs in self.lists:
+            if issubclass(type(qs), QuerySet):
+                qs.filter(**kwargs)
+        return self
+
+    def exclude(self, **kwargs):
+        for qs in self.lists:
+            if issubclass(type(qs), QuerySet):
+                qs.exclude(**kwargs)
+        return self
+
+    def order_by(self, *fields, reverse = False):
+        """
+        Order using these fields. For compatibility, if there is
+        at least one fields whose name starts with '-', reverse
+        the order
+        """
+        for i, field in enumerate(fields):
+            if field[0] == '-':
+                reverse = True
+                fields[i] = field[1:]
+
+        self.order_reverse = reverse
+        self.order_fields = fields
+
+        for qs in self.lists:
+            if issubclass(type(qs), QuerySet):
+                qs.order_by(*fields)
+            else:
+                qs.sort(key = operator.attrgetter(fields),
+                        reverse = reverse)
+        return self
+
+    def __len__(self):
+        return sum([len(qs) for qs in self.lists])
+
+    def __iter__(self):
+        if self.order_fields:
+            return heapq.merge(
+                *self.lists,
+                key = operator.attrgetter(*self.order_fields),
+                reverse = self.order_reverse
+            )
+        return itertools.chain(self.lists)
+
+    def __getitem__(self, k):
+        if type(k) == slice:
+            return list(itertools.islice(iter(self), k.start, k.stop, k.step))
+        return list(itertools.islice(iter(self), k))
+