Fix split_test xblock so that it can render in Studio

This change addresses STUD-1403 where split_test modules could not render in Studio.
It changes Studio to pass a new parameter 'runtime-type' of 'studio' when rendering
 an xblock. In split_test, this will then cause all of the children to be rendered,
 rather than just the one that matches the conditional.

cms/djangoapps/contentstore/views/item.py

@@ -231,9 +231,10 @@ def xblock_view_handler(request, package_id, view_name, tag=None, branch=None, v
             # with the new container view.
             is_read_only_view = is_container_view
             context = {
+                'runtime_type': 'studio',
                 'container_view': is_container_view,
                 'read_only': is_read_only_view,
-                'root_xblock': component
+                'root_xblock': component,
             }
 
             fragment = get_preview_fragment(request, component, context)

cms/djangoapps/contentstore/views/tests/test_item.py

@@ -157,6 +157,22 @@ class GetItem(ItemTest):
              '<span class="action-button-text">View</span>')
         )
 
+    def test_split_test(self):
+        """
+        Test that a split_test module renders all of its children in Studio.
+        """
+        root_locator = self._create_vertical()
+        resp = self.create_xblock(category='split_test', parent_locator=root_locator)
+        self.assertEqual(resp.status_code, 200)
+        split_test_locator = self.response_locator(resp)
+        resp = self.create_xblock(parent_locator=split_test_locator, category='html', boilerplate='announcement.yaml')
+        self.assertEqual(resp.status_code, 200)
+        resp = self.create_xblock(parent_locator=split_test_locator, category='html', boilerplate='zooming_image.yaml')
+        self.assertEqual(resp.status_code, 200)
+        html, __ = self._get_container_preview(split_test_locator)
+        self.assertIn('Announcement', html)
+        self.assertIn('Zooming', html)
+
 
 class DeleteItem(ItemTest):
     """Tests for '/xblock' DELETE url."""

common/lib/xmodule/xmodule/split_test_module.py

@@ -41,7 +41,7 @@ class SplitTestFields(object):
 
 
 @XBlock.needs('user_tags')  # pylint: disable=abstract-method
-@XBlock.needs('partitions')
+@XBlock.wants('partitions')
 class SplitTestModule(SplitTestFields, XModule):
     """
     Show the user the appropriate child.  Uses the ExperimentState
@@ -61,7 +61,10 @@ class SplitTestModule(SplitTestFields, XModule):
 
         super(SplitTestModule, self).__init__(*args, **kwargs)
 
-        self.child_descriptor = self.get_child_descriptors()[0]
+        self.child_descriptor = None
+        child_descriptors = self.get_child_descriptors()
+        if len(child_descriptors) >= 1:
+            self.child_descriptor = child_descriptors[0]
         if self.child_descriptor is not None:
             self.child = self.system.get_module(self.child_descriptor)
         else:
@@ -106,7 +109,9 @@ class SplitTestModule(SplitTestFields, XModule):
         """
         For grading--return just the chosen child.
         """
-        group_id = self.runtime.service(self, 'partitions').get_user_group_for_partition(self.user_partition_id)
+        group_id = self.get_group_id()
+        if group_id is None:
+            return []
 
         # group_id_to_child comes from json, so it has to have string keys
         str_group_id = str(group_id)
@@ -126,6 +131,15 @@ class SplitTestModule(SplitTestFields, XModule):
 
         return [child_descriptor]
 
+    def get_group_id(self):
+        """
+        Returns the group ID, or None if none is available.
+        """
+        partitions_service = self.runtime.service(self, 'partitions')
+        if not partitions_service:
+            return None
+        return partitions_service.get_user_group_for_partition(self.user_partition_id)
+
     def _staff_view(self, context):
         """
         Render the staff view for a split test module.
@@ -155,11 +169,37 @@ class SplitTestModule(SplitTestFields, XModule):
         fragment.initialize_js('ABTestSelector')
         return fragment
 
+    def studio_preview_view(self, context):
+        """
+        Renders the Studio preview by rendering each child so that they can all be seen and edited.
+        """
+        fragment = Fragment()
+        contents = []
+
+        for child in self.descriptor.get_children():
+            rendered_child = self.runtime.get_module(child).render('student_view', context)
+            fragment.add_frag_resources(rendered_child)
+
+            contents.append({
+                'id': child.id,
+                'content': rendered_child.content
+            })
+
+        fragment.add_content(self.system.render_template('vert_module.html', {
+            'items': contents
+        }))
+
+        return fragment
+
     def student_view(self, context):
         """
         Render the contents of the chosen condition for students, and all the
         conditions for staff.
         """
+        # When rendering a Studio preview, render all of the block's children
+        if context and context['runtime_type'] == 'studio':
+            return self.studio_preview_view(context)
+
         if self.child is None:
             # raise error instead?  In fact, could complain on descriptor load...
             return Fragment(content=u"<div>Nothing here.  Move along.</div>")
@@ -197,7 +237,7 @@ class SplitTestModule(SplitTestFields, XModule):
 
 
 @XBlock.needs('user_tags')  # pylint: disable=abstract-method
-@XBlock.needs('partitions')
+@XBlock.wants('partitions')
 class SplitTestDescriptor(SplitTestFields, SequenceDescriptor):
     # the editing interface can be the same as for sequences -- just a container
     module_class = SplitTestModule