Nhibernate - 如何使用QueryOver获得有序的不同结果？

Question

public class City
{
    virtual public long Id { get; set; }
    virtual public string Name { get; set; }
}

城市表包含重复的名称,我想删除重复项.我也想通过Id订购结果.

首先我想到了以下查询.

select distinct Name from City order by Id;

But this breaks with 'ORDER BY items must appear in the select list if SELECT DISTINCT is specified.' exception. After seeing http://weblogs.sqlteam.com/jeffs/archive/2007/12/13/select-distinct-order-by-error.aspx I think I should do:

select Name from City group by Name order by min(Id)

So my question is how can I do this query with QueryOver?

Answer 1

这在ICriteria中是可能的:

var list =
    session.CreateCriteria<City>()
        .SetProjection(Projections.Group("Name"))
        .AddOrder(Order.Asc(Projections.Min("Id")))
        .List<string>();

但是目前在QueryOver中不可能,因为缺少.OrderBy(IProjection)重载.一旦添加了缺失的重载,它应该看起来像:

var list =
    s.QueryOver<City>()
        .Select(Projections.Group<City>(p => p.Name))
        .OrderBy(Projections.Min<City>(c => c.Id)).Asc
        .List<string>();

请注意,Projections重载现在就在那里,因此您可以在ICriteria中编写以下(类型安全)查询:

var list =
    session.CreateCriteria<City>()
        .SetProjection(Projections.Group<City>(c => c.Name))
        .AddOrder(Order.Asc(Projections.Min<City>(c => c.Id)))
        .List<string>();