Fixes #261 -- Adds better hints if widget is not serialisable to django cache

Closes #263

django_select2/__init__.py

@@ -9,4 +9,4 @@ The app includes Select2 driven Django Widgets and Form Fields.
 
 """
 
-__version__ = "5.8.0"
+__version__ = "5.8.1"

django_select2/forms.py

@@ -50,6 +50,7 @@ from __future__ import absolute_import, unicode_literals
 
 from functools import reduce
 from itertools import chain
+from pickle import PicklingError
 
 from django import forms
 from django.core import signing
@@ -221,11 +222,20 @@ class HeavySelect2Mixin(object):
         return "%s%s" % (settings.SELECT2_CACHE_PREFIX, id(self))
 
     def set_to_cache(self):
-        """Add widget object to Django's cache."""
-        cache.set(self._get_cache_key(), {
-            'widget': self,
-            'url': self.get_url(),
-        })
+        """
+        Add widget object to Django's cache.
+
+        You may need to overwrite this method, to pickle all information
+        that is required to serve your JSON response view.
+        """
+        try:
+            cache.set(self._get_cache_key(), {
+                'widget': self,
+                'url': self.get_url(),
+            })
+        except (PicklingError, AttributeError):
+            msg = "You need to overwrite \"set_to_cache\" or ensure that %s is serialisable."
+            raise NotImplementedError(msg % self.__class__.__name__)
 
     def render_options(self, *args):
         """Render only selected options."""

tests/test_forms.py

@@ -157,6 +157,16 @@ class TestHeavySelect2Mixin(TestSelect2Mixin):
         widget = self.widget_cls(data_view='heavy_data_1', attrs={'class': 'my-class'})
         assert isinstance(widget.get_url(), text_type)
 
+    def test_can_not_pickle(self):
+        widget = self.widget_cls(data_view='heavy_data_1', attrs={'class': 'my-class'})
+
+        class NoPickle(object):
+            pass
+
+        widget.no_pickle = NoPickle()
+        with pytest.raises(NotImplementedError):
+            widget.set_to_cache()
+
 
 class TestModelSelect2Mixin(TestHeavySelect2Mixin):
     form = forms.AlbumModelSelect2WidgetForm(initial={'primary_genre': 1})