Refactor RecursiveDefaultDict default behavior. Fixes #10

Don't use collections.defaultdict with a default_factory function
b/c that has problems with pickle.loads which triggers #10. Instead
refactor __getitem__ and create the default values for missing keys
by hand.

README.rst

@@ -29,29 +29,32 @@ Changelog
 =========
 
 * master
-    * Merge chartit_tests/ with demoproject/
+    * Merge ``chartit_tests/`` with ``demoproject/``
     * Load test DB with real data to use during testing
     * Add more tests
     * Update the path to demoproject.settings when building docs. Fixes
       a problem which caused some API docs to be empty
     * Fix ValueError: not enough values to unpack (expected 2, got 0)
       with PivotChart when the QuerySet returns empty data
-    * Dropped requirement on `simplejson`
+    * Dropped requirement on ``simplejson``
     * Properly handle unicode data in Pivot charts. Fixes
       `#5 <https://github.com/chartit/django-chartit/issues/5>`_
     * Demo project updated with Chart and PivotChart examples of
       rendering DateField values on the X axis
-    * Allow charting of extra() or annotate() fields. Fixes
+    * Allow charting of ``extra()`` or ``annotate()`` fields. Fixes
       `#8 <https://github.com/chartit/django-chartit/issues/8>`_ and
       `#12 <https://github.com/chartit/django-chartit/issues/12>`_
+    * Refactor ``RecursiveDefaultDict`` to allow chart objects to be
+      serialized to/from cache. Fixes
+      `#10 <https://github.com/chartit/django-chartit/issues/10>`_
 
 * 0.2.5 (August 3, 2016)
     * Workaround Python 3 vs. Python 2 list sort issue which breaks
       charts with multiple data sources displayed on the same axis!
     * Make demoproject/ compatible with Django 1.10
 
 * 0.2.4 (August 2, 2016)
-    * Fix for `get_all_field_names()` and `get_field_by_name()` removal
+    * Fix for ``get_all_field_names()`` and ``get_field_by_name()`` removal
       in Django 1.10. Fixes
       `#39 <https://github.com/chartit/django-chartit/issues/39>`_
     * Updated for django.db.sql.query.Query.aggregates removal

chartit/utils.py

@@ -1,6 +1,3 @@
-from collections import defaultdict
-
-
 def _convert_to_rdd(obj):
     """Accepts a dict or a list of dicts and converts it to a
     RecursiveDefaultDict."""
@@ -18,18 +15,26 @@ def _convert_to_rdd(obj):
         return obj
 
 
-class RecursiveDefaultDict(defaultdict):
-    """The name says it all.
+class RecursiveDefaultDict(dict):
+    """
+        Behaves exactly the same as a collections.defaultdict
+        but works with pickle.loads. Fixes #10.
     """
     def __init__(self, data=None):
-        self.default_factory = type(self)
         if data is not None:
             self.data = _convert_to_rdd(data)
             self.update(self.data)
             del self.data
 
     def __getitem__(self, key):
-        return super(RecursiveDefaultDict, self).__getitem__(key)
+        # create a default object if this key
+        # isn't in the dictionary
+        if key not in self.keys():
+            item = self.__class__()
+            self[key] = item
+            return item
+        else:
+            return super(RecursiveDefaultDict, self).__getitem__(key)
 
     def __setitem__(self, key, item):
         if not isinstance(item, RecursiveDefaultDict):