EF Core 关系配置 多对多

一对多和一对一都只需要在表中增加外键列，但是在多对多关系中，我们必须引入一张中间表保存两张表之间的关联关系。

多对多：不需要声明中间表实体，也不需要声明外键。

实体：

 public class Teacher
    {
        public long Id { get; set; }
        public string Name { get; set; }
        public List<Student> Students { get; set; }=new List<Student>();
    }

public class Student
    {
        public long Id { get; set; }
        public string Name { get; set; }    
        public List<Teacher> Teachers { get; set; }
    }

　　表结构：

 配置类：

 public class TeacherConfig : IEntityTypeConfiguration<Teacher>
    {
        public void Configure(EntityTypeBuilder<Teacher> builder)
        {
            builder.ToTable("T_Teachers");
            builder.Property(s => s.Name).HasMaxLength(20);
            builder.HasMany<Student>(t => t.Students).WithMany(s => s.Teachers)
                .UsingEntity(m => m.ToTable("T_Students_Teachers"));
        }
    }

public class StudentConfig : IEntityTypeConfiguration<Student>
    {
        public void Configure(EntityTypeBuilder<Student> builder)
        {
            builder.ToTable("T_Students");
            builder.Property(s => s.Name).IsUnicode().HasMaxLength(20);
        }
    }