[LINQ]此查詢包含在不同資料內容上定義的項目參考

在兩個Table ClassesDataContext做join查詢時會出現：
System.InvalidOperationException: 此查詢包含在不同資料內容上定義的項目參考。
System.InvalidOperationException: The query contains references to items defined on a different data context.

參考：
Use LINQ to join two tables in two different database?
LINQ: Outer Join against two tables in *different* databases
How to do a cross-database LINQ-to-SQL join (to avoid the 「the query contains references to items defined on a different data context.」 InvalidOperationException)