diff -r 1f64d7a4d82d -r a7ccde9d9eed app/soc/logic/site/id_user.py
--- a/app/soc/logic/site/id_user.py	Fri Sep 12 10:56:21 2008 +0000
+++ b/app/soc/logic/site/id_user.py	Fri Sep 12 16:33:04 2008 +0000
@@ -23,6 +23,7 @@
 
 
 import re
+import logging
 
 from google.appengine.api import users
 from google.appengine.ext import db
@@ -216,6 +217,49 @@
   raise out_of_band.ErrorResponse(
       'There is no user with a "link name" of "%s".' % link_name, status=404)
 
+def checkLinkNameForId(link_name=None, id=None):
+  """Returns True if link name is correct and can be stored.
+
+  Args:
+    link_name: link name used in URLs to identify user
+    id: a Google Account object; optional, current logged-in user will
+      be used (or False will be returned if no user is logged in)
+  """
+  id = getIdIfMissing(id)
+    
+  if not id:
+    # id not supplied and no Google Account logged in, so link name cannot
+    # belong to an unspecified User
+    return False
+
+  user = getUserFromId(id)
+  link_name_exist = doesLinkNameExist(link_name)
+  
+  # New user with not existing link_name
+  if not user and not link_name_exist:
+    return True
+  # New user with existing link_name
+  elif not user and link_name_exist:
+    return False
+  
+  # Existing user with link_name that exists and doesn't belond to him     
+  if user and link_name_exist and (user.link_name != link_name):
+    return False
+    
+  return True
+
+
+def doesLinkNameExist(link_name=None):
+  """Returns True if link name exists in the Datastore.
+
+  Args:
+    link_name: link name used in URLs to identify user
+  """
+  if getUserFromLinkName(link_name):
+    return True
+  else:
+    return False
+
 
 def doesLinkNameBelongToId(link_name, id=None):
   """Returns True if supplied link name belongs to supplied Google Account.