Linq To Entities - 如何过滤子实体 [英] Linq To Entities - how to filter on child entities

问题描述

我有实体 Group 和 User.
Group 实体具有 Users 属性，它是一个用户列表.
用户有一个名为 IsEnabled 的属性.

I have entities Group and User.
the Group entity has Users property which is a list of Users.
User has a property named IsEnabled.

我想编写一个 linq 查询，它返回一个 Group 的列表，它只包含 IsEnabled 为 true 的 User .

I want to write a linq query that returns a list of Groups, which only consists of Users whose IsEnabled is true.

例如，对于像下面这样的数据
AllGroups
A组
用户 1 (IsEnabled = true)
用户 2 (IsEnabled = true)
用户 3(IsEnabled = false)

so for example, for data like below
AllGroups
Group A
User 1 (IsEnabled = true)
User 2 (IsEnabled = true)
User 3 (IsEnabled = false)

B组
用户 4 (IsEnabled = true)
用户 5 (IsEnabled = false)
用户 6(IsEnabled = false)

Group B
User 4 (IsEnabled = true)
User 5 (IsEnabled = false)
User 6 (IsEnabled = false)

我想要
过滤组
A组
用户 1 (IsEnabled = true)
用户 2(IsEnabled = true)

I want to get
FilteredGroups
Group A
User 1 (IsEnabled = true)
User 2 (IsEnabled = true)

B组
用户 4(IsEnabled = true)

Group B
User 4 (IsEnabled = true)

我尝试了以下查询，但 Visual Studio 告诉我
[无法分配属性或索引器用户"——它是只读的]

I tried the following query, but Visual Studio tells me that
[Property or indexer 'Users' cannot be assigned to -- it is read only]

FilteredGroups = AllGroups.Select(g => new Group()
                    {
                        ID = g.ID,
                        Name = g.Name,
                        ...
                        Users = g.Users.Where(u => u.IsInactive == false)
                    });

感谢您的帮助！

推荐答案

我设法通过颠倒查询来做到这一点:

I managed to do this by turning the query upside down:

var users = (from user in Users.Include("Group")
             where user.IsEnabled
             select user).ToList().AsQueryable()

from (user in users
      select user.Group).Distinct()

通过使用 ToList()，您可以强制往返数据库，这是必需的，否则会妨碍延迟执行.第二个查询仅对检索到的数据重新排序.

By using the ToList() you force a roundtrip to the database which is required because otherwise the deferred execution comes in the way. The second query only re-orders the retrieved data.

注意:之后您可能无法更新您的实体！

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