ASP.NET Core MVC 中的多对多关系

Question

我正在使用多对多关系为我的网站帖子创建标签。我按照

Microsoft 文档

设法使用实体框架创建 PostTags 数据库表。 我想在创建帖子时使用 Controller 和 ViewModel 在数据库中插入标签数据，但不知道如何继续。

GitHub 存储库

Models

：

public class PostModel
{
    [Key]
    public int Id { get; set; }
    public string? Title { get; set; }

    public List<TagModel> Tags { get; set; }
    public List<PostTagModel> PostTags { get; set; }
}


public class TagModel
{
    [Key]
    public int Id { get; set; }
    public string? Title { get; set; }

    public List<PostModel> Posts { get; set; }
    public List<PostTagModel> PostTags { get; set; }
}


public class PostTagModel
{
    public int PostId { get; set; }
    public int TagId { get; set; }

    public PostModel Post { get; set; }
    public TagModel Tag { get; set; }
}

DatabaseContext.cs

：

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

    }

    public DbSet<PostModel> Posts { get; set; }
    public DbSet<TagModel> Tags { get; set; }


    public DbSet<PostTagModel> PostTags { get; set; }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<PostModel>()
            .HasMany(e => e.Tags)
            .WithMany(e => e.Posts)
            .UsingEntity<PostTagModel>();
    }
}

ViewModel

：

public class CreatePostViewModel
{
    public string? Title { get; set; }
    public int? TagId { get; set; }

    public List<TagModel> TagList { get; set; }
}

Controller

：

[HttpGet]
public IActionResult Create()
{
    return View();
}

[HttpPost]
[ValidateAntiForgeryToken]
public IActionResult Create(CreatePostViewModel postVM)
{
    if (ModelState.IsValid)
    {
        var post = new PostModel
        {
            Title = postVM.Title,
            Tags = postVM.TagId  // CS0029 : cannot implicitly convert type 'int?' to 'System.Collections.GenericList<Website.Model.TagModel>
        };

        _postInterface.Add(post);

        return RedirectToAction("Index");
    }
    else
    {
        // Error
    }
    return View(postVM);
}

View

：

@using SimpleWebsite.ViewModels.PostViewModel

@model CreatePostViewModel

<!-- Code above -->


<!-- Tag -->
<div class="col-md-12 mb-3">
    <label asp-for="TagId" class="control-label"></label>

    <div class="form-check-inline">
        @foreach (var item in Model.TagList)
        {
            <input type="checkbox" class="btn-check" id="@item.Id" value="@item.Id">
            <label asp-for="TagId" class="btn btn-outline-primary" for="@item.Id">@item.Title</label>
        }
    </div>

    <div class="invalid-feedback">
        Please select a valid category.
    </div>
</div>


<!-- Code Below -->

Answer 1

目前您的 CreatePostViewModel 中只能有一个标签，因为它存储为 int TagId。将其更改为列表以存储更多标签：

public class CreatePostViewModel
{
    public string? Title { get; set; }
    public List<int> SelectedTagIds { get; set; } = new List<int>();  // To store multiple tag IDs

    public List<TagModel> TagList { get; set; }
}

您需要相应地更新控制器：

@model CreatePostViewModel

<!-- Other fields -->

<!-- Tag Selection -->
<div class="col-md-12 mb-3">
    <label asp-for="SelectedTagIds" class="control-label"></label>
    <div class="form-check-inline">
        @foreach (var tag in Model.TagList)
        {
            <input type="checkbox" class="btn-check" name="SelectedTagIds" id="[email protected]" value="@tag.Id">
            <label class="btn btn-outline-primary" for="[email protected]">@tag.Title</label>
        }
    </div>
</div>

<!-- Submit button and other code -->

还有景色：

@model CreatePostViewModel

<!-- Other fields -->

<!-- Tag Selection -->
<div class="col-md-12 mb-3">
    <label asp-for="SelectedTagIds" class="control-label"></label>
    <div class="form-check-inline">
        @foreach (var tag in Model.TagList)
        {
            <input type="checkbox" class="btn-check" name="SelectedTagIds" id="[email protected]" value="@tag.Id">
            <label class="btn btn-outline-primary" for="[email protected]">@tag.Title</label>
        }
    </div>
</div>

<!-- Submit button and other code -->

使用Entity Framework时，更新数据库的方法如下：

• 打开命令行
• 导航到您的项目所在的目录
• 运行：

dotnet ef migrations add UpdatePostTagRelationship (or whatever you want to call it)

• 完成后，运行：

dotnet ef database update

您可以打开笔记本电脑的命令行或在您正在使用的IDE的命令行中执行。