如何使用带有NHibernate的条件Api将子查询写为别名

Question

hey guys, say I have Entities and mappings like this:

public class Episode
{
    Guid Id {get;set;}
    String Title {get;set;}
    List<Group> Groups {get;set;}
}

public class Group
{
    Guid Id {get;set;}
    DateTime PubDate {get;set;}
}

public class EpisodeMap : ClassMap<Episode>
{
    public EpisodeMap()
    {
        //other mappings..
        Map.HasMany(ep => ep.Groups);
    }
}

So, basically, an Episode has many Groups. Each Group has a PubDate, and so an Episode has many PubDates.

I'm trying to write a query using the NHibernate Criteria API which lets me query Episodes and Order them by PubDate given that I have a Group Id.

Essentially, how do I write the equivalent Criteria API query for this SQL query:

Select 
    e.*, 
    (Select top 1 ReleaseDate From EpisodeGroups where EpisodeFk = e.Id and GroupFk = @GroupId) as myPubDate 
From Episodes e 
Order By myPubDate

Please help! cheers guys

Answer 1

public DetachedCriteria BuildCriteria(int episodeId, int groupId)
{

    var groupCriteria = DetachedCriteria.For<Groups>()
        .Add(Restrictions.Eq("this.Id", groupId))
        .Add(Restrictions.Eq("Group.Id", groupId)) 
        .AddOrder(Order.Asc("Group.PubDate"));

    return DetachedCriteria.For<EpisodeGroups>()
               .Add(Restrictions.Eq("this.Id", episodeId))
               .Add(Subqueries.PropertyIn("this.Groups", groupCriteria)
               .SetMaxResult(1);
}

Then you can do something like this...

var episodes = _repository.ExectueCriteria<EpisodeGroups>(BuildCriteria(episodeId, groupId))

Just as a special consideration, the reason why you would want to eagerly load the Group entity is in case you would like to use LINQ to compare the PUBDATE later on in your business logic as opposed to using a strict detached criteria.

We have found that by eagerly loading the attributes of an entity will reduce the total number of calls to our database.

No guarantees this code works... but it should at least get you started, good luck :)