Question

表Users与另一个表有一些关系

这是代码段

var users = DbContext.Users
           .Include(x => x.ApplicationUserGroups)
                .ThenInclude(x => x.ApplicationGroup)
           .Include(x => x.FormSubmit)
                .ThenInclude(x => x.FormLevel)
           .Where(x => true);

所以现在，我需要来自FormSubmit的最新记录。所以我修改了查询，如下所示：

var users = DbContext.Users
           .Include(x => x.ApplicationUserGroups)
                .ThenInclude(x => x.ApplicationGroup)
           .Include(x => x.FormSubmit.OrderByDescending(i => i.CreationDate).Take(1))
                .ThenInclude(x => x.FormLevel)
           .Where(x => true);

但是我有一个例外：

"Message": "The Include property lambda expression 'x => {from FormSubmit i in x.FormSubmit orderby [i].CreationDate desc select [i] => Take(1)}' is invalid. The expression should represent a property access: 't => t.MyProperty'. To target navigations declared on derived types, specify an explicitly typed lambda parameter of the target type, E.g. '(Derived d) => d.MyProperty'. For more information on including related data, see http://go.microsoft.com/fwlink/?LinkID=746393."

是否知道如何从FormSubmit获取最新记录？

Answer 1

到目前为止，您无法直接实现它，因为EF Core中的紧急加载或延迟加载都不支持子集合中的排序或过滤。

由于您的要求不清楚，因此我正在为所有可能的情况提供可能的替代解决方案：

1）如果您想要某个用户的最新FormSubmit：

var users = DbContext.Users
           .Include(x => x.ApplicationUserGroups)
                .ThenInclude(x => x.ApplicationGroup)
           .Include(x => x.FormSubmit)
                .ThenInclude(x => x.FormLevel)
           .Where(x => true && x.UserId == userId).FirstOrDefault();

var latestFormSubmit = users.Select(u => u.FormSubmit).OrderByDescending(fs => fs.CreationDate).FirstOrDefault();

2）如果要在所有用户中最晚FormSubmit：

var users = DbContext.Users
           .Include(x => x.ApplicationUserGroups)
                .ThenInclude(x => x.ApplicationGroup)
           .Include(x => x.FormSubmit)
                .ThenInclude(x => x.FormLevel)
           .Where(x => true).ToList();

var latestFormSubmit = users.SelectMany(u => u.FormSubmit).OrderByDescending(fs => fs.CreationDate).FirstOrDefault();

    //Or

var latestFormSubmit = DbContext.FormSubmits.Include(x => x.FormLevel).OrderByDescending(fs => fs.CreationDate).FirstOrDefault();

注意：如果只需要最新的.Include(x => x.ApplicationUserGroups).ThenInclude(x => x.ApplicationGroup)，则可以从查询中省略FormSubmit。

Answer 2

导航属性（如您的User.FormSubmit属性）旨在容纳 all 个相关实体（如果未加载，则不包含任何实体）。

您根本不能那样使用Include()。 Include()仅用于检索 all 相关记录。

如果只需要一个相关记录，则需要单独进行。例如：

var users = DbContext.Users
           .Include(x => x.ApplicationUserGroups)
                .ThenInclude(x => x.ApplicationGroup)
           .Where(x => true);

foreach (var user in users) {
    var latest = DbContext.FormSubmits.Include(s => s.FormLevel).Where(s => s.UserId == user.UserId).OrderByDescending(i => i.CreationDate).FirstOrDefault()
    //Do other stuff...
}

以此方式仅将检索每个用户的最新帖子。如果您在第一个查询中执行.Include(x => x.FormSubmit).ThenInclude(x => x.FormLevel)，然后进行过滤，最终将为每个用户 all 个提交，这比您实际使用的数据要多。

获取最新的子记录-EF Core

2 个答案: