如何比较数据库和客户端.Net Core 2中的值 [英] How to compare the value between the one from database and the client .Net Core 2

问题描述

我们有此编辑Razor页面（edit.cshtml），它是从以下页面模型扩展而来的，它非常基本，仅包括PopulateRolesDropDownList：

We have this edit Razor pages (edit.cshtml) which is extended from the following page model and it's very basic only include the PopulateRolesDropDownList:

public class RoleNamePageModel : PageModel
{
    public SelectList RoleNameSL { get; set; }

    public void PopulateRolesDropDownList(ApplicationDbContext _context,
        object selectedRole = null)
    {
        var rolesQuery = from d in _context.Roles
                               orderby d.Name // Sort by name.
                               select d;

        RoleNameSL = new SelectList(rolesQuery,
                    "RoleId", "Name", selectedRole);
    }

}

在此编辑页面中，我们添加：

Also in this Edit page, we added:

<input type="hidden" asp-for="User.UserRoles.ElementAt(0).RoleId" name="User.Current.RoleId" />

我们还在后面的代码中执行了[BindProperty]

We also do the [BindProperty] in the code behind

public ApplicationUser User { get; set; }

我们需要找出此模型是否有变化。这样做的方法是什么？

We need to find out whether there is a change on this model. What is the approach to do this?

环境：

• .NET Core 2.2


• 剃刀页

更新-1 ：

在PostAsync上，我们再次调用了数据库：

On the PostAsync, we made another call to the database:

var userRoleToUpdate = await _context.UserRoles
                                        .FirstOrDefaultAsync(m => m.UserId == id.ToString());

我们只需要将此值与下拉列表中的更改进行比较即可。我们无法工作。

We just need to compare this value with the change on a drop-down list or not. We could not work how.

更新-2：

我们确实做了更改按照@NevilleNazerane的建议，如下：

We did change as per recommend by @NevilleNazerane below:

public class AssignClubUserViewModel
    {
        public string FirstName { get; set; }
        public string LastName { get; set; }

        public string UserName { get; set; }

        public Guid SelectedRoleID { get; set; }

    }

    [BindProperty]
    public AssignClubUserViewModel AssignClubUser { get; set; }

并添加OnGetAsync：

and added OnGetAsync:

public async Task<IActionResult> OnGetAsync(Guid? id)
    {
        if (id == null)
            return NotFound();

        var user = await _context.Users
                        .Include(u => u.ClubApplicationUsers)
                        .FirstOrDefaultAsync(m => m.Id == id.ToString());

        AssignClubUser.FirstName = user.FirstName;
        AssignClubUser.LastName = user.LastName;
        AssignClubUser.UserName = user.UserName;
        AssignClubUser.SelectedClubID = 
        user.ClubApplicationUsers.ElementAt(0).ClubID;

        ....

这是对的吗？我收到错误：NullReferenceException：AssignClubUser.FirstName行上的Arg_NullReferenceException；

Is this right? I got the error: NullReferenceException: Arg_NullReferenceException on line AssignClubUser.FirstName = user.FirstName;

更新-3：

通过创建ModemView然后在OnGetAsync（）上进行查询来固定以确保使用ModelView进行映射：

Fixed by creating a ModemView and then on the OnGetAsync() for query ensure to mapped with the ModelView:

var user = await _context.Users
                            .Include(u => u.ClubApplicationUsers)
                            .Where(t => t.Id == id.ToString())
                            .Select(t => new AssignClubUserViewModel<ApplicationUser>
                            {
                                FirstName = t.FirstName,
                                LastName = t.LastName,
                                UserName = t.UserName,
                                SelectedClubID = t.ClubApplicationUsers.ElementAt(0).ClubID
                            }).SingleAsync();

推荐答案

由于您拥有视图模型，因此建议您简化您的绑定，并让您的背后代码处理其他功能。您可以先创建 SelectedRoleId 属性：

Since you have a view model, I recommend you simplify your bindings and let your behind code handle the other functionalities. You can first make a SelectedRoleId property:

public int SelectedRoleId { get; set; }

在您的视图模型中，可以将此属性的默认值分配给 User .UserRoles.ElementAt（0）.RoleId 在您的构造函数或您的 OnGet 中，取决于您的设置方式。

In your view model, you can assign this property's default value to User.UserRoles.ElementAt(0).RoleId in either your constructor or your OnGet, based on how you need it set up. This way the drop down is bound to a simple property.

对于绑定下拉菜单（HTML选择），. NET Core提供了 asp项

For binding dropdowns (HTML selects) .NET Core provides the asp-items tag helper.

<select asp-for="SelectedRoleId" asp-items="Model.RoleNameSL"></select>

在您的 OnPostAsync 中，您可以使用 SelectedRoleId 访问所选值。

In your OnPostAsync, you can use SelectedRoleId to access the selected value.

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