Question

我必须使用NHibernate

来实现这样的东西

DetachedCriteria maxDateQuery = DetachedCriteria.forClass(FtpStatus.class);
ProjectionList proj = Projections.projectionList();
proj.add(Projections.max("datetime"));
proj.add(Projections.groupProperty("connectionid"));
maxDateQuery.setProjection(proj);

Criteria crit = s.createCriteria(FtpStatus.class);
crit.add(Subqueries.propertiesEq(new String[] {"datetime", "connectionid"}, maxDateQuery));

List<FtpStatus> dtlList = crit.list();

（此代码段来自Hibernate criteria query using Max() projection on key field and group by foreign primary key）

我的问题是Nhibernate没有实现此行中使用的“Subqueries.propertiesEq”：

crit.add(Subqueries.propertiesEq(new String[] {"datetime", "connectionid"}, maxDateQuery));

你能建议一个解决方法吗？

Answer 1

这里的答案是（谈论 NHibernate ）以使用EXISTS代替IN。查询看起来像这样

// the below outer query, has essential feature
// ALIAS name "outer"
// and this is used here, in the "subquery"
var maxDateQuery = DetachedCriteria.For<MyEntity>("myEntity")
    .SetProjection(
        Projections.ProjectionList()
        .Add(Projections.GroupProperty("MyId"))
        .Add(Projections.Max("MyDate"))
        )
        .Add(Restrictions.EqProperty(
        Projections.Max("MyDate"),
        Projections.Property("outer.MyDate")) // compare inner MAX with outer current
        )
        .Add(Restrictions.EqProperty("MyId", "outer.MyId")) // inner and outer must fit
    ;

// here ... the "outer" is essential, because it is used in the subquery
var list = session.CreateCriteria<Contact>("outer")
    .Add(Subqueries.Exists(maxDateQuery))
    ... // e.g. paging
    .List<MyEntity>();

这将创建如下查询：

FROM [MyEntityTable] outer
WHERE exists 
(
  SELECT  myEntity.MyId
    , max(myEntity.MyDate) 
   FROM [MyEntityTable] myEntity
   WHERE    myEntity.MyId         = outer.MyId   // matching ID
   GROUP BY myEntity.MyId
   HAVING   max(myEntity.MyDate)  = outer.MyDate // matching max and current date
)

可以在此处找到QueryOver语法中相同类型的查询（甚至更复杂）：

Query on HasMany reference

NHibernate标准查询使用Max（）投影

1 个答案: