Question

给出以下（简化）类：

public class Master
{
    public int Id { get; set; }
    public string UniqueName { get; set; }
    public virtual List<Detail> Details { get; set; }
}

public class Detail
{
    public int Id { get; set; }
    public string UniqueCode { get; set; }
}

如何为主服务器提供UniqueName，为Lini to Entities的详细信息选择UniqueCode？

请注意，从Detail到Master没有导航属性。

如果有导航属性，我想我可以这样做：

Detail detail = (from m in ctx.Master.Include(p=>p.Details)
                 join d in ctx.Details
                 on m.Id = d.MasterId
                 where m.UniqueName == "Something" && d.UniqueCode == "Else"
                 select d).SingleOrDefault();

如果没有从Detail到Master的导航属性，我看不到如何在没有首先获取Master的情况下继续进行，包括该Master的所有Detail实例，然后迭代Detail实例。如果给定的Master有许多Detail实例，那可能效率很低。

Answer 1

尝试

var detail = ctx.Master.Where(m => m.UniqueName == "Something")
    .Select(m => m.Details.Where(d => d.UniqueCode == "Else").FirstOrDefault())
    .SingleOrDefault();

Answer 2

我不完全确定你将linq用于对象，或者linq用于实体。如果只涉及您的课程，请尝试此查询：

    var mst = new List<Master> { 
        new Master { Id = 1, UniqueName = "Something", Details = new List<Detail> { 
            new Detail { Id = 10, UniqueCode = "Else" }, new Detail { Id = 11, UniqueCode = "det1" }} 
        } ,
        new Master { Id = 2, UniqueName = "mast2", Details = new List<Detail> { new Detail { Id = 20, UniqueCode = "det2" } } } 
    };

    var detail = (from d in
                        ((from m in mst
                            where m.UniqueName == "Something"
                            select m.Details).SingleOrDefault())
                where d.UniqueCode == "Else"
                select d).SingleOrDefault();

如果你有数据表，那么Detail表可能有一个MasterID（int）列，所以我会尝试以下查询：

    var detail = (from d in ctx.Detail
                    where d.MasterID == (
                        (from m in ctx.Master
                         where m.UniqueName == "Something"
                         select m.Id).SingleOrDefault())
                    && d.UniqueCode == "Else"
                    select d).SingleOrDefault();

在实体框架中没有导航属性的情况下从Detail导航到Master

2 个答案: