实体框架 .NET Core 2.0 查询在执行子查询时挂起

Question

使用常规版本的 EF 6.x，我可以通过以下查询非常轻松地执行子查询：

var q = from u in db.User
        let actions = from ua in db.UserActions
                      where ua.UserId == u.Id && ua.Approved
                      select ua
        where u.Active
        select new { User = u, Actions = actions.ToList() }

var result = await q.ToListAsync()

当我尝试在我的 ASP.NET Core 2.0 EntityFramework 驱动的应用程序中执行相同的代码时，我的应用程序只是冻结并且永远不会返回结果。

我尝试了一些变体，但一旦子查询作为初始查询的一部分执行，它们都会挂起：

1. 子查询（与上面相同，但有一些变化）
2. 左外连接

另一个子查询变体（也在执行中挂起）：

var q = from u in db.User
        let actions = (from ua in db.UserActions
                      where ua.UserId == u.Id && ua.Approved
                      select ua).ToList()
        where u.Active
        select new { User = u, Actions = actions }

又一个子查询变体（也在执行中挂起）：

var q = from u in db.User
        let actions = db.UserActions.Where(ua => ua.UserId == u.Id && ua.Approved)
        where u.Active
        select new { User = u, Actions = actions.ToList() }

左外连接的另一种变体（也在执行时挂起）：

var forUserId = "test";
var q = from u in db.User
        join ua in db.UserActions on u.Id equals ua.UserId into actions
        from ua in action.DefaultIfEmpty()
        where u.Active
        select new { User = u, Actions = ua}

以下代码有效，但显然子查询执行被延迟，一旦我们尝试访问Action 属性，它就会启动另一个查询：

var q = from u in db.User
        let actions = from ua in db.UserActions
                      where ua.UserId == u.Id && ua.Approved
                      select ua
        where u.Active
        select new { User = u, Actions = actions }

var result = await q.ToListAsync()
foreach (var r in result)
{
     foreach(var a in r.Actions)
     {
          // extra database requests is executed for each result row
          // .. process action.
     }
}

如何执行子查询并获取实体列表，其中每个实体分配子实体列表，使用子查询接收？

Answer 1

我不确定。但是，你可以试试'.ToList()'。

 var q = (from u in db.User
            join ua in db.UserActions on u.Id equals ua.UserId into actions
            from ua in action.DefaultIfEmpty()
            where u.Active
            select new {u,ua}).ToList();