Index internal ID fields

Signed-off-by: Bartosz Zurkowski <zurkowski.bartosz@gmail.com>

orca/graph/drivers/arangodb.py

@@ -55,14 +55,6 @@ def _graph(self):
             self.__graph = self._use_graph('graph')
         return self.__graph
 
-    @property
-    def _nodes(self):
-        return self._graph.vertex_collection('nodes')
-
-    @property
-    def _links(self):
-        return self._graph.edge_collection('links')
-
     def setup(self):
         sys_db = self._use_database('_system')
         if not sys_db.has_database(self._database_name):
@@ -81,6 +73,13 @@ def setup(self):
                 from_vertex_collections=['nodes'],
                 to_vertex_collections=['nodes'])
 
+        nodes_col = graph.vertex_collection('nodes')
+        nodes_col.add_hash_index(fields=['id'], unique=False)
+
+        links_col = graph.edge_collection('links')
+        links_col.add_hash_index(fields=['id'], unique=False)
+
+
     def get_nodes(self, **query):
         query_pattern = ('FOR node in nodes %(filters)s RETURN node')
         filters = self._build_filters(query, handle='node')