Hook up the component delete button to the backend. Don't break if the process of updating a parent unit after deleting is interrupted (leaving a pointer to a missing module)

common/lib/xmodule/xmodule/modulestore/__init__.py

@@ -332,6 +332,14 @@ class ModuleStore(object):
         """
         raise NotImplementedError
 
+    def delete_item(self, location):
+        """
+        Delete an item from this modulestore
+
+        location: Something that can be passed to Location
+        """
+        raise NotImplementedError
+
     def get_courses(self):
         '''
         Returns a list containing the top level XModuleDescriptors of the courses

common/lib/xmodule/xmodule/modulestore/mongo.py

@@ -309,6 +309,14 @@ class MongoModuleStore(ModuleStoreBase):
 
         self._update_single_item(location, {'metadata': metadata})
 
+    def delete_item(self, location):
+        """
+        Delete an item from this modulestore
+
+        location: Something that can be passed to Location
+        """
+        self.collection.remove({'_id': Location(location).dict()})
+
     def get_parent_locations(self, location):
         '''Find all locations that are the parents of this location.  Needed
         for path_to_location().

common/lib/xmodule/xmodule/x_module.py

@@ -13,6 +13,7 @@ from xmodule.modulestore import Location
 from xmodule.timeparse import parse_time
 
 from xmodule.contentstore.content import StaticContent, XASSET_SRCREF_PREFIX
+from xmodule.modulestore.exceptions import ItemNotFoundError
 
 log = logging.getLogger('mitx.' + __name__)
 
@@ -531,7 +532,11 @@ class XModuleDescriptor(Plugin, HTMLSnippet, ResourceTemplates):
         if self._child_instances is None:
             self._child_instances = []
             for child_loc in self.definition.get('children', []):
-                child = self.system.load_item(child_loc)
+                try:
+                    child = self.system.load_item(child_loc)
+                except ItemNotFoundError:
+                    log.exception('Unable to load item {loc}, skipping'.format(loc=child_loc))
+                    continue
                 # TODO (vshnayder): this should go away once we have
                 # proper inheritance support in mongo.  The xml
                 # datastore does all inheritance on course load.