将Linq-to-SQL中的计算属性映射到实现SQL语句

I have created some extra functionality on my Linq-to-SQL classes to make things easier as I develop my applications. For example I have defined a property that retrieves active contracts from a list of contracts. However if I try to use this property in a lambda expression or in general in a query it either throws an exception that there is no SQL statement matching that property or it generates one query per item (= a lot of roundtrips to the server).I have created some extra functionality on my L