data: add support for Media class to CellBase (fixes #8293)

You can produce media for a whole page with

  media = sum((cell.media for cell in cells), Media())

The `Media()` is ugly but sum is not able to find itself the base case
for the folding, it defaults to 0 which is not addable to Media.

combo/data/models.py

@@ -26,11 +26,13 @@ from django.core.cache import cache
 from django.core.exceptions import ObjectDoesNotExist
 from django.core import serializers
 from django.db import models
+from django.db.models.base import ModelBase
 from django.db.models import Max
 from django.forms import models as model_forms
 from django import template
 from django.utils.safestring import mark_safe
 from django.utils.translation import ugettext_lazy as _
+from django.forms.widgets import MediaDefiningClass
 
 from ckeditor.fields import RichTextField
 import cmsplugin_blurp.utils
@@ -210,8 +212,13 @@ class Page(models.Model):
                 page.save()
 
 
+class CellMeta(MediaDefiningClass, ModelBase):
+    pass
+
 
 class CellBase(models.Model):
+    __metaclass__ = CellMeta
+
     page = models.ForeignKey(Page)
     placeholder = models.CharField(max_length=20)
     order = models.PositiveIntegerField()

tests/test_cells.py

@@ -1,6 +1,7 @@
 import pytest
 
 from combo.data.models import Page, CellBase, TextCell, LinkCell
+from django.forms.widgets import Media
 
 pytestmark = pytest.mark.django_db
 
@@ -17,6 +18,20 @@ def test_cell_reference():
 
     assert CellBase.get_cell(cell.get_reference()) == cell
 
+
+def test_media():
+    class TextCelleWithMedia(TextCell):
+        class Media:
+            js = ['coincoin.js']
+
+        class Meta:
+            # to prevent error in Models metaclass as the current module is not
+            # in a registered applicatoin
+            app_label = 'data'
+    cells = [TextCelleWithMedia() for i in range(3)]
+    assert unicode(sum((cell.media for cell in cells), Media())) == u'<script type="text/javascript" src="/static/coincoin.js"></script>'
+
+
 def test_additional_label():
     page = Page()
     page.save()