流畅的 nHibernate 查询 => QueryOver、Join、Distinct

Question

我尝试将该查询更改为 QueryOver<> 以便能够在（生成的 sql）查询

中执行 Distinct 操作

var result = (from x in Session.Query<Events>()
              join o in Session.Query<Receivers>() on x.ID equals o.ID
              where x.Owner.ID == 1 //the user is the owner of that Event (not null)
                    ||
                    x.EVType.ID == 123 //(not null)
                    ||
                    x.Receivers.Count(y => y.User.ID == 1) > 0 //the user is one of the Event Receivers
              select x.StartDate)
              .Distinct();

我试过类似的东西

Events x = null;
List<Receivers> t = null;

var result = Session.QueryOver<Events>(() => x)
                    .JoinAlias(() => x.Receivers, () => t)
                    .Where(() => x.Owner.ID == 1
                                ||
                                x.EVType.ID == 123
                                ||
                                t.Count(y => y.User.ID == 1) > 0)
                   .TransformUsing(Transformers.DistinctRootEntity)
                   .Select(a => a.StartDate)
                   .List();

但后来我遇到了 Value can not be null. Parameter name: source 异常。有什么想法可以解决该查询吗？

编辑

感谢 xanatos 的回答，最终 SQL 查询是正确的（我使用了他的第二种方法）：

SELECT distinct this_.StartDate as y0_ 
FROM Events this_ 
WHERE 
(
    this_.UserID = ? 
    or
    this_.EventTypeID = ? 
    or
    exists (SELECT this_0_.ID as y0_ 
            FROM Receivers this_0_ 
            WHERE this_0_.UserID = ?)
)

Answer 1

希望这个回答可以帮助到其他人。此错误是由声明

引起的

List<Receivers> t = null;

后跟查询表达式

 t.Count(y => y.User.ID == 1) > 0

QueryOver 文档指出 "The variable can be declared anywhere (but should be empty/default at runtime)." 因为在这种情况下，占位符是一个列表，您必须将它初始化为一个空列表。

List<Receivers> t = new List<Receivers>();

否则，当您尝试在占位符对象上引用 Count 方法或任何其他方法时，源 (t) 将为空。

然而，这仍然会留下一个问题，如@fex 和@xanatos，其中从别名 List t 引用 Count() 是没有意义的，因为它不会转换为 SQL。相反，您应该创建一个子查询。查看他们的答案以获得更全面的答案。

Answer 2

"In QueryOver, aliases are assigned using an empty variable. The variable can be declared anywhere (but should be empty/default at runtime). The compiler can then check the syntax against the variable is used correctly, but at runtime the variable is not evaluated (it's just used as a placeholder for the alias)." http://nhibernate.info/blog/2009/12/17/queryover-in-nh-3-0.html

像您一样将 List<Receivers> t 设置为空集合（正如您在评论中提到的那样）意味着您检查的是本地空集合中的事件 ID - 根本没有意义。

您可以尝试使用子查询进行查询（应该可以，但我不确定，我没有测试就写了它，"by hand"）：

Receivers receiversSubQueryAlias = null;
var subquery = session.QueryOver<Events>()
                      .JoinQueryOver<Receivers>(x => x.Receivers, () => receiversSubqueryAlias, JoinType.Inner)
                      .Where(()=> receiversSubQueryAlias.UserId == 1)
                      .Select(x => x.Id)
                      .TransformUsing(Transformers.DistinctRootEntity);

Events eventsAlias = null;
var mainQueryResults = session.QueryOver<Events>(() => eventsAilas)
                       .Where(Restrictions.Disjunction()
                             .Add(() => eventAlias.OwnerId == 1)
                             .Add(() => eventAlias.EVType.Id == 123)
                             .Add(Subqueries.WhereProperty<Events>(() => eventAlias.Id).In(subquery))
                        ).Select(x => x.StartDate)
                        .TransformUsing(Transformers.DistinctRootEntity)
                        .List();

Answer 3

如@fex 所写，您不能简单地执行 new List<Receivers>。问题是您不能将 QueryOver 与 "LINQ"（t.Count(...) 部分）混合使用。 QueryOver "parser" 尝试执行 "locally" t.Count(...) 而不是在 SQL.

中执行

正如别人所写，TransformUsing(Transformers.DistinctRootEntity) 是客户端。如果你想做一个 DISTINCT 服务器端，你必须使用 Projections.Distinct .

您必须进行显式子查询。这里有两种查询变体。第一个更类似于 LINQ 查询，第二个不使用 Count 但使用 Exist（在 LINQ 中，您可以通过将 Count(...) > 0 更改为 Any(...)

请注意，当您使用 .Select() 时，您通常必须明确告诉 NHibernate .List<something>()

的类型

Events x = null;
Receivers t = null;

// Similar to LINQ, with COUNT

var subquery2 = QueryOver.Of<Receivers>(() => t)
    .Where(() => t.SOMETHING == x.SOMETHING) // The JOIN clause between Receivers and Events
    .ToRowCountQuery();

var result2 = Session.QueryOver<Events>(() => x)
    .Where(Restrictions.Disjunction()
        .Add(() => x.Owner.ID == 1)
        .Add(() => x.EVType.ID == 123)
        .Add(Subqueries.WhereValue(0).Lt(subquery2))
    )
    .Select(Projections.Distinct(Projections.Property(() => x.StartDate)))
    .List<DateTime>();

// With EXIST

var subquery = QueryOver.Of<Receivers>(() => t)
    .Where(() => t.SOMETHING == x.SOMETHING) // The JOIN clause between Receivers and Events
    .Select(t1 => t1.ID);

var result = Session.QueryOver<Events>(() => x)
    .Where(Restrictions.Disjunction()
        .Add(() => x.Owner.ID == 1)
        .Add(() => x.EVType.ID == 123)
        .Add(Subqueries.WhereExists(subquery))
    )
    .Select(Projections.Distinct(Projections.Property(() => x.StartDate)))
    .List<DateTime>();

请注意，您必须在子查询中设置 "manually" JOIN 条件。

流畅的 nHibernate 查询 => QueryOver、Join、Distinct

Fluent nHibernate Query => QueryOver, Join, Distinct

c#

fluent-nhibernate

编辑