在net core 3.1中使用兩個表進行聯合查詢

          // 內連線（lambda表示式又稱點標試）join方法的引數，第一個是要join的物件（下面是students），第二個是外部的（下面的teacher）key，第三個引數內部（下面是students）的key，第四個是結果（自己定義）
           var result = await _dbContext.teachers.Join(_dbContext.students, t => t.Id, s => s.TeacherId, (t, s) =>
                    　　　　　　new
 
 TeacherAndStudentDto
                    　　　　　　{
                        　　　　　　TeacherId = t.Id,
                        　　　　　　TeacherName = t.Name,
                        　　　　　　StudentId = s.Id,
                        　　　　　　StudentName = s.Name
                    　　　　　　}).ToListAsync();

         //
 
 內連線（linq表示式又稱查詢表示式）
           var result = await (from s in _dbContext.students
                                join t in _dbContext.teachers
                                on s.TeacherId equals t.Id
                                select new TeacherAndStudentDto
                                {
                                    TeacherId 
 
= t.Id,
                                    TeacherName = t.Name,
                                    StudentId = s.Id,
                                    StudentName = s.Name
                                }).ToListAsync(); 

       //左連線（已左表為基礎表（下面就是已students為基礎表），匹配條件，如果有就加上，沒有就已預設值添上）和sql中left join類似 
            var result = await (from s in _dbContext.students
                                join t in _dbContext.teachers
                                on s.TeacherId equals t.Id into newTeacher   
                                from nt in newTeacher.DefaultIfEmpty()    //nt的型別為Teacher
                                select new TeacherAndStudentDto
                                {
                                    TeacherId = nt.Id,
                                    TeacherName = nt.Name,
                                    StudentId = s.Id,
                                    StudentName = s.Name
                                }).ToListAsync();

最後我用groupjoin 先用下面的方法，可以查到資料

 //左連線  
            var students = await _dbContext.students.ToListAsync();
            var teachers = await _dbContext.teachers.ToListAsync();
            var result = students.GroupJoin(teachers, s => s.TeacherId, t => t.Id,   //呼叫的是 IEnumerable<TResult> GroupJoin<>這個方法，是已studens為基礎表,Teacher表附加到上面
                (s, t) => new TeacherAndStudentDto         // t為IEnumerable<Teacher>
                {
                    StudentId = s.Id,
                    StudentName = s.Name,
                    TeacherId = s.TeacherId,
                    TeacherName = t.FirstOrDefault(c => c.Id == s.TeacherId) == null ? null : t.FirstOrDefault(c => c.Id == s.TeacherId).Name
                }
            ).ToList();

然後我換一個方法

var result = await _dbContext.students.GroupJoin(_dbContext.teachers, s => s.TeacherId, t => t.Id, //呼叫IQueryable<TResult> GroupJoin<>這個方法
                (s, t) => new TeacherAndStudentDto
                {
                    StudentId = s.Id,
                    StudentName = s.Name,
                    TeacherId = t.FirstOrDefault(c => c.Id == s.TeacherId) == null ? 0 : t.FirstOrDefault(c => c.Id == s.TeacherId).Id,
                    TeacherName = t.FirstOrDefault(c => c.Id == s.TeacherId) == null ? null : t.FirstOrDefault(c => c.Id == s.TeacherId).Name

                }).ToListAsync();

然後就報錯（暫時沒有找到問題所在）

An unhandled exception occurred while processing the request.
InvalidOperationException: Processing of the LINQ expression 'DbSet<Student>
.GroupJoin(
outer: DbSet<Teacher>,
inner: s => s.TeacherId,
outerKeySelector: t => t.Id,
innerKeySelector: (s, t) => new TeacherAndStudentDto{
StudentName = s.Name,
TeacherName = t
.FirstOrDefault(c => c.Id == s.TeacherId).Name
}
)' by 'NavigationExpandingExpressionVisitor' failed. This may indicate either a bug or a limitation in EF Core. See https://go.microsoft.com/fwlink/?linkid=2101433 for more detailed information.
Microsoft.EntityFrameworkCore.Query.Internal.NavigationExpandingExpressionVisitor.VisitMethodCall(MethodCallExpression methodCallExpression)

一些相關的類

 public class TeacherAndStudentDto
    {
        public int TeacherId { get; set; }
        public string TeacherName { get; set; }
        public string StudentName { get; set; }
        public int StudentId { get; set; }
    }

 public class Teacher
    {
        [Key]
        public int Id { get; set; }

        public string Name { get; set; }

        /// <summary>
        /// 所教科目
        /// </summary>
        public string Subject { get; set; }

        public GenderEnum GenderEnum { get; set; }

        public DateTime Birthday { get; set; }

        public double Hegiht { get; set; }

        public double? Weight { get; set; }
    }

public class Student
    {
        [Key]
        public int Id { get; set; }

        public string Name { get; set; }

        public DateTime? Birthday { get; set; }

        /// <summary>
        /// 學號
        /// </summary>
        public string Numerber { get; set; }

        /// <summary>
        /// 分數
        /// </summary>
        public int? Score { get; set; }

        /// <summary>
        /// 年級
        /// </summary>
        public string Grade { get; set; }

        public GenderEnum? GenderEnum { get; set; }

        public double Hegiht { get; set; }

        public double? Weight { get; set; }

        public int TeacherId { get; set; }

    }
 public enum GenderEnum
    {
        [Description("女")]
        Female=0,

        [Description("男")]
        Male =1,
    }

public class EFCoreDbContext : DbContext
    {
        public EFCoreDbContext(DbContextOptions<EFCoreDbContext> options) : base(options)
        {
        }

        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        {
            //如果appseting.json中沒有配置資料庫連線，會讀取這裡的配置
            if (!optionsBuilder.IsConfigured)
            {
                optionsBuilder.UseSqlServer("Server=.;Data Source=(local);uid=sa;pwd=xxx;DataBase=xxx");
            }

            base.OnConfiguring(optionsBuilder);
        }
        protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            //Fluent API


        }
        public DbSet<Teacher>  teachers { get; set; }
        public DbSet<Student> students { get; set; }

    }

這個問題還要摸索一下，以後解決吧，又要學習go語言了