diff --git a/storage/compat/common.py b/storage/compat/common.py
index 1177d86..298975f 100644
--- a/storage/compat/common.py
+++ b/storage/compat/common.py
@@ -2,7 +2,7 @@
 
 """Compatibility layer on scopes.storage: common functionality."""
 
-from sqlalchemy import Table, Column, BigInteger, Text
+from sqlalchemy import Table, Column, Index, BigInteger, Text
 from zope.sqlalchemy import mark_changed
 
 from scopes.storage import common
@@ -16,13 +16,13 @@ class Storage(common.Storage):
         super(Storage, self).__init__(schema)
         self.uidTable = self.getUidTable(self.schema)
 
-    def storeUid(self, ouid, nuid):
+    def storeUid(self, ouid, prefix, id):
         ouid = int(ouid)
         t = self.uidTable
-        stmt = t.update().values(standard=nuid).where(t.c.legacy == ouid)
+        stmt = t.update().values(prefix=prefix, id=id).where(t.c.legacy == ouid)
         n = self.session.execute(stmt).rowcount
         if n == 0:
-            stmt = t.insert().values(legacy=ouid, standard=nuid)
+            stmt = t.insert().values(legacy=ouid, prefix=prefix, id=id)
             self.session.execute(stmt)
         mark_changed(self.session)
 
@@ -35,8 +35,10 @@ class Storage(common.Storage):
 def createUidTable(storage):
     metadata = storage.metadata
     cols = [Column('legacy', BigInteger, primary_key=True),
-            Column('standard', Text, nullable=False, unique=True, index=True)]
-    table = Table('uid_mapping', metadata, *cols, extend_existing=True)
+            Column('prefix', Text, nullable=False),
+            Column('id', BigInteger, nullable=False)]
+    idxs = [Index('idx_uid_mapping_prefix_id', 'prefix', 'id', unique=True)]
+    table = Table('uid_mapping', metadata, *(cols+idxs), extend_existing=True)
     metadata.create_all(storage.engine)
     return table
     
diff --git a/storage/migration/tracking.py b/storage/migration/tracking.py
index b6c1af4..c069c9d 100644
--- a/storage/migration/tracking.py
+++ b/storage/migration/tracking.py
@@ -33,7 +33,9 @@ def migrate(loopsRoot, source, factory=tracking.Container,
     storage = Storage(schema=schema)
     container = storage.create(factory)
     ix = 0
+    prefix = factory.itemFactory.prefix
     for ix, (id, inTrack) in enumerate(items):
+        id = int(id)
         ts = datetime.fromtimestamp(inTrack.timeStamp)
         #print('*** in:', id, inTrack)
         head = [inTrack.metadata[k] for k in container.itemFactory.headFields]
@@ -43,11 +45,11 @@ def migrate(loopsRoot, source, factory=tracking.Container,
             if k[0] == '_' and k[1] != '_':
                 print('*** _field!', k, v, head)
                 data[k] = v
-        track = container.itemFactory(*head, trackId=int(id), 
+        track = container.itemFactory(*head, trackId=id, 
                                     timeStamp=ts, data=data)
         container.upsert(track)
         ouid = util.getUidForObject(inTrack)
-        storage.storeUid(ouid, track.uid)
+        storage.storeUid(ouid, prefix, id)
         if autoDelete:
             inTrack.__parent__.removeTrack(inTrack)
         if divmod(ix+1, step)[1] == 0: