NHibernate一对多关系聚合查询 [英] NHibernate aggregate query for one-to-many relation

问题描述

我还有下一个实体:

class Topic
{
    public virtual int Id {get; private set;} 
    public virtual ICollection<Vote> Votes {get; private set; }
}

class Vote
{
    public virtual Topic Topic {get; private set;}
    public virtual VoteType VotedTo {get; private set;} // enum VotedUp, VotedDown
}

我需要从数据库中加载下一个信息-所有主题(ID，实际上是名称，但在此演示案例中都没有关系)以及另外两个字段 CountOfVotedUp，CountOfVotedDown(聚合).因此，据我了解，在SQL世界中，我们需要联接，分组，大小写和计数.

I need to load from the db the next info - all topics (IDs, actually Names, but in this demo case it does not matter) and two more fields CountOfVotedUp, CountOfVotedDown (aggregates). So as I understand in SQL world we need joins, group by, case and count.

是否可以通过LINQ进行较少的db操作来获取此信息?我的意思是N + 1，其他选择，联系方式等等.

Is it posible to get this info with LINQ with less operations with db? I mean N+1, additional selects, connections etc.

我所尝试的全部-是使用NH的LINQ，但它的Query汇总仅在Topic.Id上进行，我无法计算任何Votes集合.

All that I tried - is to use NH's LINQ, but it's Query aggregates only on Topic.Id and I could not count any of Votes collection.

推荐答案

提供了一个用于存储结果的摘要类:

Provided you have a summary class to store your result :

public class  SummaryDTO
{
    public int TopicId { get; set; }
    public VoteType TypeOfVote { get; set; }
    public int VoteCount { get; set; }
}

然后

Vote voteAlias = null;
SummaryDTO result = null;

youNHSession.QueryOver<Topic>()
  .JoinAlias(x=> x.Votes, ()=>voteAlias)
  .SelectList(
    list=>list
      .SelectGroup(topic=>topic.Id).WithAlias(()=>result.TopicId)            
      .SelectGroup(()=>voteAlias.VotedTo).WithAlias(()=>result.TypeOfVote)
      .SelectCount(()=>voteAlias.VotedTo).WithAlias(()=>result.VoteCount ))
  .TransformUsing(Transformers.AliasToBean<SummaryDTO>())
  .List<SummaryDTO>();

我想这并不是您要找的东西，但是希望这能使您步入正轨.

I guess that's not exactly what you are looking for, but hope this will set you on a good track.

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