Sharepoint LookUp field issue

Lookup fields one site to number of sites

list.ParentWeb.AllowUnsafeUpdates = true;
   if (list.Fields.ContainsField(fieldName))
   {                    
    SPFieldLookup field = (SPFieldLookup)list.Fields[fieldName];
    //check if the field link to the list is broken. 
    if (field.LookupList != targetList.ID.ToString())
    {
     //copy the schema from the field
     string newFieldXml = field.SchemaXml;
     string oldsourceid = field.LookupList;
     //put the ID of the current list as the source for the lookup
     field.SchemaXml = newFieldXml.Replace(oldsourceid, "{" + targetList.ID.ToString() + "}");
    }
   }
list.ParentWeb.AllowUnsafeUpdates = false;