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我正在使用带有 MySQL 数据库和 DbContext 的实体框架。我有一个实体“消息”，它有一个相关的实体“发件人”。（“消息”也有一个相关的实体“接收者”）。我正在尝试编写一个查询，该查询将只返回每个接收者的“最新”消息。但是当我这样做时，我还想加载关联的“发件人”，以便我可以访问我需要包含在我要返回的数据传输对象中的发件人属性之一（电子邮件字段）。“MessageDTO”是我返回的数据传输对象，其中包括消息的 ID、消息的内容和发件人的电子邮件。

如果我从 DTO 中排除发件人的电子邮件，则以下查询将准确返回我需要的内容（即每个接收者的最新消息）：

var refGroupQuery = (from m in dbContext.Messages.SqlQuery("select * from messages order by created_at desc")
     group m by m.receiver_id into refGroup
     select new MessageDTO { id = refGroup.FirstOrDefault().id, content = refGroup.FirstOrDefault().content});

但是，上面的语句不会加载与消息关联的发件人，因此当我在 DTO 中重新包含发件人的电子邮件时，我得到一个 NullReferenceException，如下所示：

var refGroupQuery = (from m in dbContext.Messages.SqlQuery("select * from messages order by created_at desc")
     group m by m.receiver_id into refGroup
     select new MessageDTO { id = refGroup.FirstOrDefault().id, content = refGroup.FirstOrDefault().content, sender_email = refGroup.FirstOrDefault().sender.email});

refGroup.FirstOrDefault().sender.email 抛出 NullReferenceException 因为 sender 为空。

如何在查询中加载发件人，以便在我的 DTO 中包含发件人的电子邮件？

编辑：

根据要求，我包含了由 Gert Arnold 建议的方法生成的 SQL：

{SELECT
1 AS `C1`, 
`Apply1`.`id`, 
`Apply1`.`sender_id`, 
`Apply1`.`RECEIVER_ID1` AS `receiver_id`, 
`Apply1`.`created_at`, 
`Apply1`.`read_status`, 
`Extent3`.`email`
FROM (SELECT
`Distinct1`.`receiver_id`, 
(SELECT
`Project2`.`id`
FROM (SELECT
`Extent2`.`id`, 
`Extent2`.`sender_id`, 
`Extent2`.`receiver_id`, 
`Extent2`.`created_at`, 
`Extent2`.`read_status`
FROM `messages` AS `Extent2`
 WHERE (`Extent1`.`receiver_id` = `Extent2`.`receiver_id`) OR ((`Extent1`.`receiver_id` IS  NULL) AND (`Extent2`.`receiver_id` IS  NULL))) AS `Project2` LIMIT 1) AS `id`, 
(SELECT
`Project2`.`sender_id`
FROM (SELECT
`Extent2`.`id`, 
`Extent2`.`sender_id`, 
`Extent2`.`receiver_id`, 
`Extent2`.`content`, 
`Extent2`.`created_at`, 
`Extent2`.`read_status`
FROM `messages` AS `Extent2`
 WHERE (`Extent1`.`receiver_id` = `Extent2`.`receiver_id`) OR ((`Extent1`.`receiver_id` IS  NULL) AND (`Extent2`.`receiver_id` IS  NULL))) AS `Project2` LIMIT 1) AS `sender_id`, 
(SELECT
`Project2`.`receiver_id`
FROM (SELECT
`Extent2`.`id`, 
`Extent2`.`sender_id`, 
`Extent2`.`receiver_id`, 
`Extent2`.`content`, 
`Extent2`.`created_at`,
`Extent2`.`read_status`
FROM `messages` AS `Extent2`
 WHERE (`Extent1`.`receiver_id` = `Extent2`.`receiver_id`) OR ((`Extent1`.`receiver_id` IS  NULL) AND (`Extent2`.`receiver_id` IS  NULL))) AS `Project2` LIMIT 1) AS `RECEIVER_ID1`, 
(SELECT
`Project2`.`receivable_type`
FROM (SELECT
`Extent2`.`id`, 
`Extent2`.`sender_id`, 
`Extent2`.`receiver_id`, 
`Extent2`.`content`, 
`Extent2`.`created_at`,  
`Extent2`.`read_status`
 WHERE (`Extent1`.`receiver_id` = `Extent2`.`receiver_id`) OR ((`Extent1`.`receiver_id` IS  NULL) AND (`Extent2`.`receiver_id` IS  NULL))) AS `Project2` LIMIT 1) AS `content`, 
(SELECT
`Project2`.`created_at`
FROM (SELECT
`Extent2`.`id`, 
`Extent2`.`sender_id`, 
`Extent2`.`receiver_id`,  
`Extent2`.`content`, 
`Extent2`.`created_at`, 
`Extent2`.`read_status`
FROM `messages` AS `Extent2`
 WHERE (`Extent1`.`receiver_id` = `Extent2`.`receiver_id`) OR ((`Extent1`.`receiver_id` IS  NULL) AND (`Extent2`.`receiver_id` IS  NULL))) AS `Project2` LIMIT 1) AS `created_at`, 
(SELECT
`Project2`.`updated_at`
FROM (SELECT
`Extent2`.`id`, 
`Extent2`.`sender_id`, 
`Extent2`.`receiver_id`, 
`Extent2`.`content`, 
`Extent2`.`created_at`, 
`Extent2`.`read_status`
FROM `messages` AS `Extent2`
 WHERE (`Extent1`.`receiver_id` = `Extent2`.`receiver_id`) OR ((`Extent1`.`receiver_id` IS  NULL) AND (`Extent2`.`receiver_id` IS  NULL))) AS `Project2` LIMIT 1) AS `read_status`
FROM (SELECT DISTINCT 
`Extent1`.`receiver_id`
FROM `messages` AS `Extent1`) AS `Distinct1`) AS `Apply1` LEFT OUTER JOIN `users` AS `Extent3` ON `Apply1`.`sender_id` = `Extent3`.`id`}
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1 回答 1

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您不需要SqlQuery构造在分组之前进行排序：

var refGroupQuery = from m in dbContext.Messages
     group m by m.receiver_id into refGroup
     let firstItem = refGroup.OrderByDescending(x => x.created_at)
                             .FirstOrDefault()
     select new MessageDTO { 
                              id = firstItem.id, 
                              content = firstItem.content,
                              sender_email = firstItem.sender.email
                           };

这样做是一样的，但是它将整个语句翻译成 SQL，它有两个优点

  • sender不会为每条消息延迟加载
  • sender.email为空时不会崩溃sender，因为在 SQL 中没有空对象引用。整个表达式 ( sender.email) 只返回 null。
于 2013-10-04T06:06:59.953 回答