nHibernate - 在选择中按顺序进行子选择答案

【问题标题】：nHibernate - subselect in select with order bynHibernate - 在选择中按顺序进行子选择
【发布时间】：2014-04-03 12:50:16
【问题描述】：

我与许多消息进行了对话，我想按照 ConversationMessages 表中最后一条消息的创建日期对对话进行排序。

我将如何在 nHibernate 的 queryOver 中编写这个。我应该使用 subQuery 吗？

select distinct c.ConversationId,
  (select max(CreatedDate) 
    from tConversationMessage mess where mess.ConversationId = c.ConversationId
  ) as LastMessage 
from tConversation c order by LastMessage

【问题讨论】：

标签： c# sql nhibernate

【解决方案1】：

猜测您的 C# 命名，NHibernate 映射...这将是方式：

Conversation conversation = null;
ConversationMessage message = null;

// the subselect of a max
var subQuery = QueryOver.Of<ConversationMessage>(() => message)
    .Where(() => message.Conversation.ID == conversation.ID)
    .Select(Projections.Max("CreatedDate"));

// the alias of the Conversation to be injected into subquery
var query = session.QueryOver<Conversation>(() => conversation);

// a select and group by (distinct) clause
query.SelectList(l => l
    .SelectGroup(s => conversation.ID)
    .Select(Projections.SubQuery(subQuery))
    );

// Order by the max Created date (asc)
var list = query
    .OrderBy(Projections.SubQuery(subQuery))
        .Asc
    .List<object[]>()
    ;

对于每个对话 ID，我们将获得 ConversationMessage 的 max(CreateDate)

【讨论】：