Return a custom error for duplicate elements, and ignore it when loading into mongo

common/lib/xmodule/xmodule/exceptions.py

 class InvalidDefinitionError(Exception):
     pass
 
+
 class NotFoundError(Exception):
     pass

common/lib/xmodule/xmodule/modulestore/exceptions.py

@@ -14,5 +14,10 @@ class InsufficientSpecificationError(Exception):
 class InvalidLocationError(Exception):
     pass
 
+
 class NoPathToItem(Exception):
     pass
+
+
+class DuplicateItemError(Exception):
+    pass

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

 import pymongo
 
-from bson.objectid import ObjectId
 from bson.son import SON
 from fs.osfs import OSFS
 from itertools import repeat
@@ -14,14 +13,13 @@ from mitxmako.shortcuts import render_to_string
 
 from . import ModuleStore, Location
 from .exceptions import (ItemNotFoundError, InsufficientSpecificationError,
-                         NoPathToItem)
+                         NoPathToItem, DuplicateItemError)
 
 # TODO (cpennington): This code currently operates under the assumption that
 # there is only one revision for each item. Once we start versioning inside the CMS,
 # that assumption will have to change
 
 
-
 class CachingDescriptorSystem(MakoDescriptorSystem):
     """
     A system that has a cache of module json that it will use to load modules
@@ -215,15 +213,22 @@ class MongoModuleStore(ModuleStore):
 
         return self._load_items(list(items), depth)
 
+    # TODO (cpennington): This needs to be replaced by clone_item as soon as we allow
+    # creation of items from the cms
     def create_item(self, location):
         """
-        Create an empty item at the specified location with the supplied editor
+        Create an empty item at the specified location.
+
+        If that location already exists, raises a DuplicateItemError
 
         location: Something that can be passed to Location
         """
+        try:
             self.collection.insert({
                 '_id': Location(location).dict(),
             })
+        except pymongo.errors.DuplicateKeyError:
+            raise DuplicateItemError(location)
 
     def update_item(self, location, data):
         """
@@ -286,8 +291,6 @@ class MongoModuleStore(ModuleStore):
                                     {'_id': True})
         return [i['_id'] for i in items]
 
-
-
     def path_to_location(self, location, course_name=None):
         '''
         Try to find a course_id/chapter/section[/position] path to this location.
@@ -361,7 +364,6 @@ class MongoModuleStore(ModuleStore):
         if path is None:
             raise(NoPathToItem(location))
 
-
         n = len(path)
         course_id = CourseDescriptor.location_to_id(path[0])
         chapter = path[1].name if n > 1 else None

common/lib/xmodule/xmodule/modulestore/xml_importer.py

 import logging
 
 from .xml import XMLModuleStore
+from .exceptions import DuplicateItemError
 
 log = logging.getLogger(__name__)
 
@@ -27,7 +28,7 @@ def import_from_xml(store, data_dir, course_dirs=None, eager=True,
         # This should in the future create new revisions of the items on import
         try:
             store.create_item(module.location)
-        except:
+        except DuplicateItemError:
             log.exception('Item already exists at %s' % module.location.url())
             pass
         if 'data' in module.definition: