Question

我正在使用带有MySQL数据库和DbContext的Entity Framework。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只返回每个接收器的“最新”消息的查询。但是，当我这样做时，我还想加载相关的“发件人”，以便我可以访问我需要包含在我返回的数据传输对象中的一个发件人的属性（电子邮件字段）。 “MessageDTO”是我返回的数据传输对象，包括邮件的ID，邮件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，则以下查询会准确返回我需要的内容（即每个收件人的最新邮件）：

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，上述语句不会加载与Message关联的Sender，因此当我在DTO中重新包含发件人的电子邮件时，我会收到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null。

如何在查询中加载发件人，以便我可以在我的DTO中包含发件人的电子邮件？

修改

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`}

Answer 1

在分组之前，您不需要SqlQuery构造进行排序：

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
                           };

这样做也是如此，但它将整个语句转换为SQL，这有两个优点

sender并未为每条消息延迟加载

sender.email

sender不会崩溃，因为在SQL中没有空对象引用。整个表达式（sender.email）只返回null。

如何使用Entity Framework和MySQL（包括相关实体）获取每个组的最新记录

1 个答案: