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c# - 试图从数据库表中读取，但输出不正确

问题描述

这是我的代码：

曲目.cshtml

@page
@using Project.Models
@model Project.Pages.TracksModel
@addTagHelper *, Microsoft.AspNetCore.Mvc.TagHelpers

<div class="row">
    <h1 class="display-2">Tracks</h1>
    <table class="table">
        <thead class="thead-inverse">
            <tr>
                <th>Artist name</th>
                <th>Album name</th>
                <th>Track ID</th>
                <th>Track name</th>
            </tr>
        </thead>

        <tbody>
            @foreach (Track track in Model.Tracks)
            {
                <tr>
                    <td>@Html.DisplayFor(modelArtists => track.Artist.Name)</td>
                    <td>@Html.DisplayFor(modelAlbums => track.Album.Title)</td>
                    <td>@track.TrackId</td>
                    <td>@track.Name</td>
                    <td><a asp-page="/Tracks/Details" asp-route-id="@track.TrackId">Details</a></td>
                    <td><a asp-page="/Tracks/Edit" asp-route-id="@track.TrackId">Edit</a></td>
                    <td><a asp-page="/Tracks/Delete" asp-route-id="@track.TrackId">Delete</a></td>
                </tr>
            }
        </tbody>
    </table>
</div>
<div class="row">
    <p>Enter a name & TrackID (over 3510) for a new track:&nbsp;</p>
    <form method="POST">
        <div><input asp-for="Track.Name" /></div>
        <div><input asp-for="Track.TrackId" /></div>
        <input type="submit" />
    </form>
</div>
<div>
    <a asp-page="/Index">Home</a>
</div>

曲目.cshtml.cs

using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.RazorPages;
using Microsoft.EntityFrameworkCore;
using Project.Models;
using System;
using System.Collections.Generic;
using System.Linq;

namespace Project.Pages
{
    public class TracksModel : PageModel
    {
        private Chinook db;

        public TracksModel(Chinook injectedContext)
        {
            db = injectedContext;
        }
        public IEnumerable<Track> Tracks { get; set; }
        public void OnGetAsync()
        {
            ViewData["Title"] = "Chinook Web Site - Tracks";
            Tracks = db.Tracks.Include(a => a.Album)
            .Include(a => a.Artist);
        }
        [BindProperty]
        public Track Track { get; set; }

        public IActionResult OnPost()
        {
            if (ModelState.IsValid)
            {
                db.Tracks.Add(Track);
                db.SaveChanges();
                return RedirectToPage("/tracks");
            }
            return Page();
        }

        public IActionResult DeleteTrack(int TrackId)
        {
            var track = db.Tracks.Find(TrackId);

            if (track == null) return Page();

            db.Tracks.Remove(track); db.SaveChanges(); return RedirectToPage("/tracks");
        }
    }
}

GitHub 仓库

出现问题，因为页面似乎多次重复艺术家姓名，以至于显示的所有艺术家姓名都以字母 A 开头。我应该说我设置数据库的方式ArtistId是外国的表的键Album，whileAlbumId是表的外键Tracks。

.Where(a => a.AlbumId == Tracks.AlbumId)也许我需要像and这样的子句.Where(a => a.ArtistId == Album.ArtistId)？我试图写这样的东西，但我一定是做错了，因为它完全阻止了曲目的显示。请问有什么帮助吗？TIA

编辑：我注意到虽然我的专辑标题是从Album表中加入的，但我的艺术家姓名不是，即使方法本质上是相同的。我突然想到，虽然Album表直接连接到Track表（通过AlbumId外键），但Artist表不是。它只是通过Albumtable` 间接与“Track”表相关。

Artist---- Album----Track

Edit2：有人告诉我Where在这种情况下不需要，我应该使用“ThenInclude”，所以我目前有

   public void OnGetAsync()
    {
        ViewData["Title"] = "Chinook Web Site - Tracks";
        Tracks = db.Tracks.Include(a => a.Album).ThenInclude(b => b.Artist).ToList();
    }

尽管如此，它仍然会导致完全相同的错误。

标签： c#asp.netentity-frameworkasp.net-corerazor-pages

解决方案

关键点在Context.Chinook.cs

protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            ....

            modelBuilder.Entity<Track>()
            .ToTable("tracks")
            .HasOne(mtype => mtype.Artist)
            .WithMany(trk => trk.Tracks)
            .HasForeignKey(mtype => mtype.AlbumId)
            .HasForeignKey(mtype => mtype.GenreId)
            .HasForeignKey(mtype => mtype.MediaTypeId)
            .OnDelete(DeleteBehavior.NoAction);
        }

如果HasForeignKey链与上述关系（1到n，n到1，1到1）无关，则必须分开。以下是我修改的代码。

上下文.Chinook.cs

protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            modelBuilder.Entity<Album>()
            .ToTable("albums")
                    .HasOne(al => al.Artist)
                .WithMany(ar => ar.Albums)
                .HasForeignKey(al => al.ArtistId);

            modelBuilder.Entity<Artist>().ToTable("artists");

            modelBuilder.Entity<Media_type>()
            .ToTable("media_types")
            .HasKey(c => c.MediaTypeId);

            modelBuilder.Entity<Genre>().ToTable("genres");

            modelBuilder.Entity<Track>()
            .ToTable("tracks")
            .HasOne(trk => trk.Album)
            .WithMany(trk => trk.Tracks)
            .HasForeignKey(trk => trk.AlbumId)
             .OnDelete(DeleteBehavior.NoAction);

            modelBuilder.Entity<Track>()
            .ToTable("tracks")
            .HasOne(trk => trk.Genre)
            .WithMany(trk => trk.Tracks)
            .HasForeignKey(trk => trk.GenreId);

            modelBuilder.Entity<Track>()
            .ToTable("tracks")
            .HasOne(trk => trk.Media_type)
            .WithMany(trk => trk.Tracks)
           .HasForeignKey(mtype => mtype.MediaTypeId);
        }

实体.艺术家

public class Artist
{
     public int ArtistId { get; set; }
     public string Name { get; set; }

     public ICollection<Album> Albums { get; set; }
}

实体.流派

public class Genre
{
    public int GenreId { get; set; }
    public string Name { get; set; }

    public ICollection<Track> Tracks { get; set; }
}

Entity.Media_type

public class Media_type
{
    public  int MediaTypeId { get; set; }
    public  string Name { get; set; }

    public ICollection<Track> Tracks { get; set; }
}

实体.Track

    public class Track
    {
        public int TrackId { get; set; }
        public string Name { get; set; }
        public int? AlbumId { get; set; }
        public int? MediaTypeId { get; set; }
        public int? GenreId { get; set; }
        public string Composer { get; set; }
        public int? Milliseconds { get; set; }
        public int? Bytes { get; set; }
        public int? UnitPrice { get; set; }

        public Album Album { get; set; }
        public Genre Genre { get; set; }
        public Media_type Media_type { get; set;}
    }

WebApp.tracks.cshtml.cs

   public void OnGetAsync()
   {
        ViewData["Title"] = "Chinook Web Site - Tracks";

        Tracks = db.Tracks
        .Include(a => a.Album)
        .ThenInclude(a => a.Artist);
   }

WebApp.tracks.cshtml

   <tbody>
                @foreach (Track track in Model.Tracks)
                {
                    <tr>
                        <td>@Html.DisplayFor(modelArtists => track.Album.Artist.Name)</td>
                        <td>@Html.DisplayFor(modelAlbums => track.Album.Title)</td>
                        <td>@track.TrackId</td>
                        <td>@track.Name</td>
                        <td><a asp-page="/Tracks/Details" asp-route-id="@track.TrackId">Details</a></td>
                        <td><a asp-page="/Tracks/Edit" asp-route-id="@track.TrackId">Edit</a></td>
                        <td><a asp-page="/Tracks/Delete" asp-route-id="@track.TrackId">Delete</a></td>
                    </tr>
                }
</tbody>

参考： 实体框架核心关系

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