GroupBy 与计数

Question

我返回了列表 testList，其中包含 5 个具有正确值的项目。

var testList = _context.LectureReportStudent
    .Include(x => x.LectureReport)
    .Include(x => x.LectureReport.Lecture)
    .Include(x => x.Student)
    .Where(x => stuIds.Contains(x.LectureReport.LectureId))
    .Select(x => new StudentModel
    {
        Name = x.Student.FirstName + "" + x.Student.LastName,
        Position = x.Student.JobTitle,
        JobId = x.LectureReport.LectureId,
        StudentId = x.StudentlId,
        LectureId = x.LectureReportId,
        Date = x.Date
    };

我进一步GroupBy这个名单上StudentId。

var result = testList.GroupBy(x => x.StudentId)

每个项目都有以下型号

Date, DaysOnLecture, LectureCount, LectureId, LectureReportId, Name, StudentId, Position

我正在尝试获取 Count of DaysOnLecture、LectureCount 属性。

LectureCount 表示没有。学生参加讲座的次数（在我的模型中定义）。

DatsOnLecture 学生参加本次讲座的天数（在我的模型中定义）。

我对逻辑感到震惊。有什么想法吗？

Answer 1

var counts = testList.GroupBy(x => x.StudentId)
                     .Select(g =>
    new
    {
        StudentId = g.Key,
        TotalDays = g.Sum(gg => gg.DaysOnLecture),
        TotalLecture = g.Sum(gg => gg.LectureCount)
    });

如果您想要每个 LectureId 的总数，那么您必须按 LectureId 对每个学生的数据进行进一步分组

Answer 2

注意：目前，这更多是为了澄清您的问题，而不是实际答案。

我已经开始使用 Fiddle 来尝试澄清您要完成的工作：https://dotnetfiddle.net/SB02lQ 这不是答案；这是为了澄清您要完成的工作。

using System;
using System.Linq;
using System.Collections.Generic;

public static class Program
{
    public static void Main()
    {
        var LectureReportStudent = GetLectureReportStudent();

        var testList = LectureReportStudent

        .Select(x => new StudentModel {

            Name = x.Student.FirstName + " " + x.Student.LastName,
            Position = x.Student.JobTitle,
            JobId = x.LectureReport.LectureId,
            StudentId = x.StudentlId,
            LectureId = x.LectureReportId,
            Date = x.Date

        }).GroupBy(x => new {

            StudentId = x.StudentId,
            LectureId = x.LectureId

        }).Select(x => new StudentModel() {

            StudentId = x.Key.StudentId, 
            LectureId = x.Key.LectureId,
            DaysOnLecture = x.Count(),
            LectureCount = -1 // how do we sum all the DaysOnLecture??

        }).ToList();

        foreach(var t in testList)
        {
            Console.WriteLine("StudentId: " + t.StudentId);
            Console.WriteLine("LectureId: " + t.LectureId);
            Console.WriteLine("DaysOnLecture: " + t.DaysOnLecture);
            Console.WriteLine("LectureCount: " + t.LectureCount);

            Console.WriteLine();
        }
    }

    public static void Dump(this IEnumerable<StudentModel> query, 
        string title)
    {
        Console.WriteLine(title);

        foreach(var i in query)
        {
            Console.WriteLine();

            Console.Write(i.Name + ", ");           
            Console.Write(i.Position + ", ");
            Console.Write(i.JobId + ", ");
            Console.Write(i.StudentId + ", ");
            Console.Write(i.LectureId + ", ");
            Console.Write(i.Date + "");
        }

        Console.WriteLine();
    }

    public static List<LectureReportStudent> GetLectureReportStudent()
    {
        var list = new List<LectureReportStudent>();

        var lectureId = 0;
        var studentId = 0;
        for(var i = 0; i < 24; ++i)
        {
            if(i % 12 == 0)
            {
                ++studentId;
                lectureId = 1;
            }

            if(i % 3 == 0)
                ++lectureId;

            var lrs = new LectureReportStudent()
            {
                LectureReport = new LectureReport() { 
                    LectureId = lectureId 
                },
                Student = new Student() { 
                    FirstName = "foo", 
                    LastName = "bar", 
                    JobTitle = "Job" },
                StudentlId = studentId,
                LectureReportId = lectureId,
                Date = DateTime.Now
            };
            list.Add(lrs);

            // Console.WriteLine(studentId + ":" + lectureId);
        }

        return list;
    }   

    public class LectureReportStudent
    {
        public LectureReport LectureReport { get; set; }
        public Student Student { get; set; }
        public int StudentlId { get; set; }
        public int LectureReportId { get; set; }
        public DateTime Date { get; set; }
    }

    public class LectureReport
    {
        public int LectureId { get; set; }
        public Lecture Lecture { get; set; }
    }

    public class Lecture {}

    public class Student 
    {
        public int stuIds { get; set; }
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public string JobTitle { get; set; }        
    }

    public class StudentModel 
    {
        public string Name  { get; set; }
        public string Position { get; set; }
        public int JobId { get; set; }
        public int StudentId { get; set; }
        public int LectureId { get; set; }
        public DateTime Date  { get; set; }
        public int DaysOnLecture  { get; set; }
        public int LectureCount  { get; set; }
        public int LectureReportId  { get; set; }
    }
}

虽然这不是一个答案，但它可能会帮助您找到答案。