Make the id property available on both modules and descriptors, and make it a property, so that we aren't duplicating the location data

common/lib/xmodule/xmodule/poll_module.py

@@ -37,8 +37,6 @@ class PollFields(object):
     answers = List(help="Poll answers from xml", scope=Scope.content, default=[])
     question = String(help="Poll question", scope=Scope.content, default='')
 
-    id = String(help="ID attribute for this module", scope=Scope.settings)
-
 
 class PollModule(PollFields, XModule):
     """Poll Module"""
@@ -193,7 +191,6 @@ class PollDescriptor(PollFields, MakoModuleDescriptor, XmlDescriptor):
             tag_name=self._tag_name, text=self.question)
         xml_object = etree.fromstring(poll_str)
         xml_object.set('display_name', self.display_name)
-        xml_object.set('id', self.id)
 
         def add_child(xml_obj, answer):
             child_str = '<{tag_name} id="{id}">{text}</{tag_name}>'.format(

common/lib/xmodule/xmodule/x_module.py

@@ -123,15 +123,18 @@ class XModule(XModuleFields, HTMLSnippet, XBlock):
         model_data: A dictionary-like object that maps field names to values
             for those fields.
         '''
+        self._model_data = model_data
         self.system = system
         self.location = Location(location)
         self.descriptor = descriptor
-        self.id = self.location.url()
         self.url_name = self.location.name
         self.category = self.location.category
-        self._model_data = model_data
         self._loaded_children = None
 
+    @property
+    def id(self):
+        return self.location.url()
+
     @property
     def display_name_with_default(self):
         '''
@@ -393,6 +396,10 @@ class XModuleDescriptor(XModuleFields, HTMLSnippet, ResourceTemplates, XBlock):
 
         self._child_instances = None
 
+    @property
+    def id(self):
+        return self.location.url()
+
     @property
     def display_name_with_default(self):
         '''