protected IColumnMetadata[] GetKeyColumns(NHibernate.Dialect.Schema.ITableMetadata tableMetaData)
 {
     string key = string.Format("PK_{0}.{1}.{2}", tableMetaData.Catalog, tableMetaData.Schema, tableMetaData.Name);
     if (currentContext[key] == null)
     {
         if (currentContext.TableExceptions.HasException(tableMetaData.Name, tableMetaData.Catalog, tableMetaData.Schema))
         {
             cfg.db2hbmconfTable exc = currentContext.TableExceptions.GetTableException(tableMetaData.Name, tableMetaData.Catalog, tableMetaData.Schema);
             if (exc.primarykey != null)
             {
                 if (exc.primarykey.keycolumn != null && exc.primarykey.keycolumn.Length > 0)
                 {
                     logger.Info(string.Format("table {0}: using configured primary key def", tableMetaData.Name));
                     try
                     {
                         IColumnMetadata[] res = exc.primarykey.keycolumn.Select(q => tableMetaData.GetColumnMetadata(q.name)).Where(q => q != null).ToArray();
                         currentContext[key] = res;
                         return res;
                     }
                     catch (Exception e)
                     {
                         logger.Error("Can't obtain metadata for configured primary key columns.", e);
                         throw e;
                     }
                 }
             }
         }
         //try to found the primary key from the schema
         try
         {
             var dt = currentContext.Dialect.GetCompleteColumnSchema(currentContext.Connection, tableMetaData.Schema, tableMetaData.Name);
             int isKey = dt.Columns.IndexOf(ISKEY);
             int colName = dt.Columns.IndexOf(COLNAME);
             List<IColumnMetadata> cols = new List<IColumnMetadata>();
             foreach (DataRow dr in dt.Rows)
             {
                 if ((bool)dr.ItemArray[isKey])
                 {
                     var cmeta = tableMetaData.GetColumnMetadata(dr.ItemArray[colName].ToString());
                     if( null != cmeta )
                         cols.Add(cmeta);
                 }
             }
             currentContext[key] = cols.ToArray();
             return currentContext[key] as IColumnMetadata[];
         }
         catch (Exception e)
         {
             logger.Error("Can't obtain primary key metadata from schema.If database does not support key schema information, please consider to use configuration in order to provide keys", e);
             throw e;
         }
     }
     else
     {
         return currentContext[key] as IColumnMetadata[];
     }
 }