EFCore：根据相关实体中某字段的最大值查询相关实体

Question

我需要根据相关实体中字段中的最大值查询相关实体，然后显示该项目的结果。

例如模型定义为：

public class Student
{
    public int StudentID {get; set;}
    public string Name {get; set;}
    public ICollection<ReportCard> ReportCards {get; set;}
}

public class ReportCard
{
    public int ReportCardID {get; set;}
    public int ProjectID { get; set; }
    public Project Project { get; set; }
    public int Result {get; set;}
    public int Comment {get; set;}
    public DateTime PublishDate {get; set;}
}

在剃须刀控制器中：

public class LatestResultsModel : PageModel
{
    ...
    public IList<Student> Students {get; set;}
    public async Task<IActionResult> OnGetAsync()
    {
        Students = await _context.Student
                                 .Include(student => student.ReportCard)
                                 .ToListAsync();
    }
}

在剃刀视图中：

@foreach (Student student in Model.Students)
{
    <p>@student.Name</p>
    <p>@student.ReportCard.Max(i => i.PublishDate).Result.ToString()</p>
}

Max语句后无法查询其他字段。 我已经尝试了一些方法来实现过滤相关数据的结果。

Filtered Includes are not supported.

是否有某种类型的联接可以实现此结果？

它也不处理学生没有 ReportCard 的情况。 InvalidOperationException：可为空的对象必须有一个值。

Answer 1

After the Max statement I cannot query the other fields. I've tried a few things to achieve this outcome of filtering the related data.

是的！你不能！因为 Max 语句 select 只有你在 Max.

中提到的字段

It also doesn't handle the situation when a Student has no ReportCard. InvalidOperationException: Nullable object must have a value.

按照以下步骤克服这两个问题：

@foreach (Student student in Model.Students)
{
    <p>@student.Name</p>

    if(student.ReportCards.Count > 0)
    {
      <p>@student.ReportCards.OrderByDescending(rc => rc.PublishDate).FirstOrDefault().Result.ToString()</p>

      <p>@student.ReportCards.OrderByDescending(rc => rc.PublishDate).FirstOrDefault().PublishDate.ToString()</p>
    }
    else
    {
      <p>Student has no report card!</p>
    }

}