如何在 LINQ 中实现客户端评估？

Question

我不明白 StackOverflow 的代码格式化功能。

我正在尝试进行客户端评估，但似乎没有进行。如果查看输出，它仍然具有 AsEnumerable 查询中的对象。不应该只是结果吗？我不明白为什么它仍然试图将其写入查询中。还有其他方法可以启用客户端评估吗？

var groups = Groups!.Where(g => g.Selected).Select(g => (Group)g.Object!).AsEnumerable();
SelectedGroupPosts.AddRange(from p in DB.Post
                            join u in DB.User
                               on p.UserId equals u.Id
                            join gm in DB.GroupMember
                                on u.Id equals gm.UserId
                            join g in DB.Group
                                on gm.GroupId equals g.Id
                            where groups.Contains(g)
                            select p);

System.InvalidOperationException
  HResult=0x80131509
  Message=The LINQ expression 'DbSet<Group>()
    .Join(
        inner: DbSet<GroupMember>(), 
        outerKeySelector: g1 => (object)g1.Id, 
        innerKeySelector: g2 => (object)g2.GroupId, 
        resultSelector: (g1, g2) => new TransparentIdentifier<Group, GroupMember>(
            Outer = g1, 
            Inner = g2
        ))
    .Join(
        inner: DbSet<User>(), 
        outerKeySelector: ti2 => (object)ti2.Inner.UserId, 
        innerKeySelector: u0 => (object)u0.Id, 
        resultSelector: (ti2, u0) => new TransparentIdentifier<TransparentIdentifier<Group, GroupMember>, User>(
            Outer = ti2, 
            Inner = u0
        ))
    .Where(ti3 => ti3.Inner.Email == __Email_0)
    .Where(ti3 => new ClickSelectorModel{ 
        Label = ti3.Outer.Outer.Name, 
        Object = ti3.Outer.Outer 
    }
    .Selected)' could not be translated. Either rewrite the query in a form that can be translated, or switch to client evaluation explicitly by inserting a call to 'AsEnumerable', 'AsAsyncEnumerable', 'ToList', or 'ToListAsync'. See https://go.microsoft.com/fwlink/?linkid=2101038 for more information.
  Source=Microsoft.EntityFrameworkCore

Answer 1

与其使用像

where ObjectCollection.Contains(objectFromQuery)

这样的条件，我相信使用像

where ObjectIdList.Contains(IdFromQuery)

这样的条件会更好。

更具体地说，如果您使用

var groupIdList = ... .Select(g => g.Id).ToList();

构建组 ID 列表，则后续 LINQ 查询可以删除

join g ...

并使用条件

where groupIdList.Contains(gm.GroupId)

。

这应该可以干净地翻译并在服务器端运行。你真的不想在客户端做这个。

尝试：

var groupIdList = Groups!.Where(g => g.Selected).Select(g => g.Id).ToList();
SelectedGroupPosts.AddRange(from p in DB.Post
                            join u in DB.User
                               on p.UserId equals u.Id
                            join gm in DB.GroupMember
                                on u.Id equals gm.UserId
                            where groupIdList.Contains(gm.GroupId)
                            select p);