如何从实体框架中的可空关系中选择非空值 [英] How to select non nullables from nullable relation in Entity Framework

问题描述

我正在做一个基于实体框架和Scott Guthrie的动态Linq库的报告工具。

I am making a reporting tool that is based off Entity Framework and Scott Guthrie's Dynamic Linq library.

当尝试选择不可为空时，我遇到了一个错误当相关的记录不总是在相关表中的字段。例如，我有一个参与者表具有一个可空的外键到一个团队表。这是因为一些参与者将会在一个团队中，有些参与者将不会。这个障碍是，我想拉一个报告，显示参与者名单以及他们的团队信息，如果他们在一个团队。团队表中的其中一列不可空，因此当我尝试使用标准投影或动态列表选择它时：

I have run into a snag when trying to select a non nullable field from a related table when the related record isnt always there. For example, I have a Participant table that has a nullable foreign key to a Team table. This is because some participants will be on a team and some wont. The snag is that I want to pull a report that shows a list of participants along with some of their team information IF they are on a team. One of the columns on the team table isn't nullable so when I try to select it using either a standard projection or a dynamic one:

    var standardProjection = data.Select(i => new
    {
        i.FirstName,
        i.ParticipantTeam.CaptainPickupFlg <--- Non Nullable Boolean
    });

    var dynamicProjection = data.Select("new (FirstName, ParticipantTeam.CaptainPickupFlg)");

尝试枚举结果时收到错误：

I get an error when trying to enumerate the results:

转换为值类型Boolean失败，因为实例化值为null。结果类型的泛型参数或查询必须使用可空类型。

"The cast to value type 'Boolean' failed because the materialized value is null. Either the result type's generic parameter or the query must use a nullable type."

如何避免这个错误，只有参与者Team.CaptainPickupFlg在匿名类型中被实例化为可空的布尔？

How can I avoid this error and just have the ParticipantTeam.CaptainPickupFlg just materialized as a nullable bool in the anonymous type?

ScottGu的动态临床： http ：//weblogs.asp.net/scottgu/archive/2008/01/07/dynamic-linq-part-1-using-the-linq-dynamic-query-library.aspx

ScottGu's Dynamic Linq: http://weblogs.asp.net/scottgu/archive/2008/01/07/dynamic-linq-part-1-using-the-linq-dynamic-query-library.aspx

推荐答案

data.Where( i => i.ParticipantTeam != null ).Select( i=> new
    {
        i.FirstName,
        i.ParticipantTeam.CaptainPickupFlg
    });

如果您需要所有参与者，您有两个选择：

If you need all the participants, you have two options:

• 如果您可以负担返回匿名类型的可空字段：

• If you can afford the nullable field in returned anonymous type:

var standardProjection = data.Select(i => new
{
    i.FirstName,
    CaptainPickupFlg = (bool?)i.ParticipantTeam.CaptainPickupFlg
});

如果这不是您的选择，您应该决定该字段的默认值：

If that's not an option for you, you should decide on the default value for that field:

var standardProjection = data.Select(i => new
{
    i.FirstName,
    CaptainPickupFlg = ((bool?)i.ParticipantTeam.CaptainPickupFlg) ?? false
});

其默认值为 false 。

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