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

问题描述

我有实体组和用户。

组实体具有用户属性，它是用户列表。

用户具有名为 IsEnabled 。

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，返回一个组列表 s，它只包含用户 s，其 IsEnabled 为true。

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

所以例如下面的数据
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)

我想要得到
FilteredGroups

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)

我尝试过以下查询，但Visual Studio告诉我，
[属性或索引器'用户'不能被分配到 - 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

我设法通过将查询上下颠倒：

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（），强制往返数据库是必需的，因为否则延迟执行就会出现。

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：以后您可能无法udpate您的实体！

Note: You might not be able to udpate your entities afterwards!

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