.NET Core 将多个入口关联到一个用户

Question

我正在尝试将用户和类之间的一对多关系相关联。当我尝试创建帖子时，我也在尝试将其添加到用户模型中，但我似乎无法正确处理。

应该能够拥有多个项目入口的用户

public class AppUser : IdentityUser
{
public ICollection<UserProject> Projects { get; set; }
}

项目模型

public class UserProject
{
public int Id { get; set; }
public string Name { get; set; }
}

添加项目并将其与用户关联的操作

[HttpPost]
[ValidateAntiForgeryToken]
public async Task<IActionResult> Create(UserProject userProject)
{
if (ModelState.IsValid)
{
AppUser appUser = await userManager.GetUserAsync(HttpContext.User);
appUser.Projects.Add(userProject);
context.Projects.Add(userProject);
await context.SaveChangesAsync();
return RedirectToAction("Index");
}
return View(userProject);
}

但是，这context.Projects.Add(userProject);会导致错误NullReferenceException: Object reference not set to an instance of an object.有人可以告诉我出了什么问题以及如何实现我想要做的事情吗？

数据库上下文

public class ScrumApplicationContext : IdentityDbContext<AppUser>
{
public ScrumApplicationContext(DbContextOptions<ScrumApplicationContext> options)
: base(options)
{
}
public DbSet<UserProject> Projects { get; set; }
}

启动配置服务

public Startup(IConfiguration configuration)
{
Configuration = configuration;
}
public IConfiguration Configuration { get; }
// This method gets called by the runtime. Use this method to add services to the container.
public void ConfigureServices(IServiceCollection services)
{
services.AddControllersWithViews();
services.AddDbContext<ScrumApplicationContext>(options => options.UseSqlServer(Configuration.GetConnectionString("ScrumApplicationContext")));
services.AddIdentity<AppUser, IdentityRole>()
.AddEntityFrameworkStores<ScrumApplicationContext>()
.AddDefaultTokenProviders();
}
// This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
public void Configure(IApplicationBuilder app, IWebHostEnvironment env, IServiceProvider serviceProvider)
{
if (env.IsDevelopment())
{
app.UseDeveloperExceptionPage();
}
else
{
app.UseExceptionHandler("/Home/Error");
// The default HSTS value is 30 days. You may want to change this for production scenarios, see https://aka.ms/aspnetcore-hsts.
app.UseHsts();
}
app.UseHttpsRedirection();
app.UseStaticFiles();
app.UseRouting();
app.UseAuthentication();
app.UseAuthorization();
app.UseEndpoints(endpoints =>
{
endpoints.MapControllerRoute(
name: "areas",
pattern: "{area:exists}/{controller=Home}/{action=Index}/{id?}"
);
endpoints.MapControllerRoute(
name: "default",
pattern: "{controller=Home}/{action=Index}/{id?}"
);
});
CreateAdminRole(serviceProvider);
CreateAdminUserAsync(serviceProvider);
}

创建视图

@model ScrumApp.Models.UserProject
@{
ViewData["Title"] = "Create";
}
<h1>Create</h1>
<div class="row">
<div class="col-md-4">
<form asp-action="Create">
<div asp-validation-summary="ModelOnly" class="text-danger"></div>
<div class="form-group">
<label asp-for="Name" class="control-label"></label>
<input asp-for="Name" class="form-control" />
<span asp-validation-for="Name" class="text-danger"></span>
</div>
<div class="form-group">
<input type="submit" value="Create" class="btn btn-primary" />
</div>
</form>
</div>
</div>
<div>
<a asp-action="Index">Back to List</a>
</div>
@section Scripts {
@{await Html.RenderPartialAsync("_ValidationScriptsPartial");}
}

Answer 1

尝试将外键添加到User，Project添加到UserProject类。

public class UserProject
{
public int Id { get; set; }
public string Name { get; set; }
public int UserId { get; set; }
public int ProjectId { get; set; }
[ForeignKey("UserId")]
public User User { get; set; }
[ForeignKey("ProjectId")]
public Project Project { get; set; }
}

然后，您可以添加实体：

var userProject = new UserProject { UserId=.., ProjectId=.. };
context.UserProjects.Add(userProject);

Answer 2

问题是Projects最初为空。您需要先初始化它：

appUser.Projects ??= new List<UserProject>();
appUser.Projects.Add(userProject);

或者只需在属性上设置默认值：

public ICollection<UserProject> Projects { get; set; } = new List<UserProject>();

至少有一个项目(并在查询中包含关系(可以解决此问题，因为 EF 已经实例化了集合。但是，这并不能解决所有情况下的问题，也不是"解决方案"。您需要适当地规划和处理 null。