如何獲取每個組使用實體框架和MySQL包括相關實體的最新記錄

Question

我正在使用實體框架與MySQL數據庫和DbContext。我有一個實體「消息」，有一個相關的實體「發件人」。 （「消息」也有一個相關的實體「接收者」）。我正在嘗試編寫一個查詢，它將只返回每個接收者的「最新」消息。但是當我這樣做時，我也想加載關聯的「發件人」，以便我可以訪問我要返回的數據傳輸對象中包含的發件人屬性（電子郵件字段）之一。 「MessageDTO」是我返回的數據傳輸對象，包括消息的標識，消息的內容和發件人的電子郵件。

如果我排除發件人的電子郵件從DTO，那麼下面的查詢返回正是我需要的（即針對每個接收最新消息）：

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}); 

然而，上述說法不加載與該消息相關聯的發送者，所以當我再包括髮送者的電子郵件中DTO我得到一個NullReferenceException，如下圖所示：

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拋出一個NullReferenceException因爲發件人爲空。 。

如何在查詢中加載發件人，以便我可以將發件人的電子郵件包含在我的DTO中？

編輯：

按照要求，我包括獲取由格特·阿諾德的建議方法生成的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

你不需要SqlQuery構建分組之前做排序：

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 
          }; 

這也是一樣的，但它將整個語句翻譯成SQL，它有兩個優點

• sender不延遲加載每個消息
• sender.email當sender爲空不會崩潰，因爲在SQL沒有空對象引用。整個表達式（sender.email）只是返回null。