c# - LINQ left join ordered with empty right-items at the end

Question

我对 LINQ 和 Entity Framework 的了解很少，我边学边学......

我正在尝试编写一个查询，从名为 GroupView 的 View 获取信息并在名为 GroupSequence 的表上进行左连接...然后信息将被 <asp:Repeater> 使用.

结果集应包含来自 GroupView 的所有 项目连接项在开头(按 GroupSequence 表定义的顺序)和未连接项在末尾(按 Id 项的 GroupView 定义的顺序)。

即...

[GroupView]             |    [GroupSequence]
[Id]  [Name]   [Calc]   |    [Id]  [GroupId]  [UserId]  [Sequence]
1     Group 1  23       |    1     1          1         3
2     Group 2  34       |    2     2          1         2
3     Group 3  45       |    3     3          1         1
4     Group 4  56
5     Group 5  67

预期结果...

[Id]  [Name]   [Calc]
3     Group 3  45
2     Group 2  34
1     Group 1  23
4     Group 4  56
5     Group 5  67

如果我执行以下操作，尽管使用了 DefaultIfEmpty ，我得到的只是链接到序列的 3 个组。但是页面显示，即使它只有 3 行...

from @group in context.GroupViews
join seq in context.GroupSequences on @group.Id equals seq.GroupId into groupSeq
from item in groupSeq.DefaultIfEmpty()
where item.UserId == 1
orderby item.Sequence
select new { Id = @group.Id, Name = @group.Name, Calc = @group.Calc };

如果我执行以下操作，.DataBind在中继器上提示...

The entity or complex type 'DevModel.GroupSequence' cannot be constructed in a LINQ to Entities query

from @group in context.GroupViews
join seq in context.GroupSequences on @group.Id equals seq.GroupId into groupSeq
from item in groupSeq.DefaultIfEmpty(new GroupSequence { Id = @group.Id, UserId = 1, Sequence = @group.Id + 1000 })
where item.UserId == 1
orderby item.Sequence
select new { Id = @group.Id, Name = @group.Name, Calc = @group.Calc };

基于 this question and accepted answer我也尝试过使用 DTO像这样……

class GroupViewSequenceDTO
{
    public int Id { get; set; }
    public string Name { get; set; }
    public int? Calc { get; set; }
}

from @group in context.GroupViews
join seq in context.GroupSequences on @group.Id equals seq.GroupId into groupSeq
from item in groupSeq.DefaultIfEmpty(new GroupSequence { Id = @group.Id, UserId = 1, Sequence = @group.Id + 1000 })
where item.UserId == 1
orderby item.Sequence
select new GroupViewSequenceDTO { Id = @group.Id, Name = @group.Name, Calc = @group.Calc };

但是还是和之前一样报错(无法构造)

问题...

我如何编写此查询，以便转发器显示所有 5 行，前 3 行按顺序显示，最后 2 行添加？我做错了什么？

Answer 1

您需要将过滤器移动到左连接条件中，因为当 item 为 null 时它将为 false。

from @group in context.GroupViews
from seq in context.GroupSequences.Where(x => @group.Id == x.GroupId && x.UserId == 1).DefaultIfEmpty()
orderby seq.Sequence ?? int.MaxValue
select new GroupViewSequenceDTO 
{
   Id = @group.Id, 
   Name = @group.Name, 
   Calc = @group.Calc 
};