Question

我正在尝试使用组合键选择用户及其模块。我可以使用SQL查询在MySQL上进行选择，但是我是实体框架的新手，因此很难弄清楚。感谢您能获得的任何帮助。

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>

#include <net/if.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/ioctl.h>

#include <linux/can.h>
#include <linux/can/raw.h>

  int
main(int argc, char *argv[])
{
  int s;
  int nbytes;
  struct sockaddr_can addr;
  struct can_frame frame;
  struct ifreq ifr;

  const char *ifname = "can0";

  if((s = socket(PF_CAN, SOCK_RAW, CAN_RAW)) < 0) {
    perror("Error while opening socket");
    return -1;
  }

  strcpy(ifr.ifr_name, ifname);
  ioctl(s, SIOCGIFINDEX, &ifr);

  addr.can_family  = AF_CAN;
  addr.can_ifindex = ifr.ifr_ifindex;

  printf("%s at index %d\n", ifname, ifr.ifr_ifindex);

  if(bind(s, (struct sockaddr *)&addr, sizeof(addr)) < 0) {
    perror("Error in socket bind");
    return -2;
  }

  frame.can_id  = 0x304;
  frame.can_dlc = 2;
  frame.data[0] = 0x11;
  frame.data[1] = 0x22;

  int sleep_ms = atoi(argv[1]) * 1000;

  for (;;) {
    nbytes = write(s, &frame, sizeof(struct can_frame));
    if (nbytes == -1) {
      perror("write");
      return 1;
    }
    usleep(sleep_ms);
  }
  return 0;
}

这是我的数据库上下文类。

public class module
    {
        public int moduleid { get; set; }

        public string modulename { get; set; }

        [ForeignKey("module_moduleid")]
        public ICollection<session> sessions { get; set; }

        public ICollection<user_has_module> usermodule { get; set; }

    }


    public class user_has_module
    {
        public String userid { get; set; }

        public int moduleid { get; set; }

        public user user { get; set; }

        public module module { get; set; }

    }

 public class user
    {
        [Key]
        public string userid { get; set; }

        public string userpw { get; set; }

        public string fullname { get; set; }

        public ICollection<user_has_module> usermodule { get; set; }

    }

这是我要在实体框架核心中执行的查询。

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

        }
        public DbSet<user> user { get; set; }

        public DbSet<module> module { get; set; }

        public DbSet<user_has_module> user_has_module { get; set; }



        protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            modelBuilder.Entity<user_has_module>()
                .HasKey(x => new { x.userid, x.moduleid });

            modelBuilder.Entity<user_has_module>()
                .HasOne(x => x.user)
                .WithMany(y => y.usermodule)
                .HasForeignKey(y => y.userid);

            modelBuilder.Entity<user_has_module>()
                .HasOne(x => x.module)
                .WithMany(y => y.usermodule)
                .HasForeignKey(y => y.moduleid);

        }
    }

Answer 1

要使用组合键从三个表中检索数据，您可能会受到以下示例的启发：

var query = from o in db.Orders
from p in db.Products
join d in db.OrderDetails
    on new {o.OrderID, p.ProductID} equals new {d.OrderID, d.ProductID} into details
    from d in details
    select new {o.OrderID, p.ProductID, d.UnitPrice};

希望有帮助！

使用组合键从两个表中选择数据

1 个答案: