Question

我正在尝试使用一个简单的下拉列表创建一个表单（书），以显示相关表（作者）中的字段。

图书模型：

namespace SimpleDropDownList.Models
{
    public class Book
    {
        [Key]
        public int BookID { get; set; }
        [StringLength(255)]
        [Display(Name = "Book Title")]
        public string Title { get; set; }
        public int AuthorID { get; set; }
        public Author Author { get; set; }
        public AuthorViewModel AuthorViewModel { get; set; }
    }
}

作者模型：

namespace SimpleDropDownList.Models
{
    public class Author
    {
        [Key]
        public int AuthorID { get; set; }
        [Display(Name = "First Name")]
        [StringLength(50)]
        public string FirstName { get; set; }
        [Display(Name = "Last Name")]
        [StringLength(50)]
        public string LastName { get; set; }
        public ICollection<Book> Books { get; set; } = new List<Book>();
    }
}

ViewModel：

namespace SimpleDropDownList.Models
{
    [NotMapped]
    public class AuthorViewModel
    {
        //Property to hold the list of authors in the GET
        public IEnumerable<SelectListItem> AuthorOptions { get; set; }

        //Property to bind the selected author used in the POST
        public List<int> SelectedAuthorIds { get; set; }
    }
}

BooksController上的Create（）方法：

public IActionResult Create()
            {
                //ViewData["AuthorID"] = new SelectList(_context.Set<Author>(), "AuthorID", "AuthorID");
                AuthorViewModel vm = new AuthorViewModel();
                vm.AuthorOptions = _context.Book.Select(x => new SelectListItem()
                { Value = x.AuthorID.ToString(), Text = x.Author.LastName }).ToList();
            return View(vm);
            }

创建视图：

    @model SimpleDropDownList.Models.Book

    @{
        ViewData["Title"] = "Create";
    }

    <h1>Create</h1>

    <h4>Book</h4>
    <hr />
    <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="Title" class="control-label"></label>
                    <input asp-for="Title" class="form-control" />
                    <span asp-validation-for="Title" class="text-danger"></span>
                </div>
                <div class="form-group">
                    <label asp-for="AuthorID" class="control-label"></label>
                    @*<select asp-for="AuthorID" class ="form-control" asp-items="ViewBag.AuthorID"></select>*@

                    <select asp-for="@Model.AuthorViewModel.SelectedAuthorIds" asp-items="@Model.AuthorViewModel.AuthorOptions"></select>
                    
                </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");}
    }
    Model: AuthorViewModel

单击“创建”按钮时，将产生以下错误。

处理请求时发生未处理的异常。 InvalidOperationException：模型项传递到 ViewDataDictionary是类型 'SimpleDropDownList.Models.AuthorViewModel'，但这 ViewDataDictionary实例需要以下类型的模型项 'SimpleDropDownList.Models.Book'。

我的期望很简单...打开“创建书”表单并下拉列表以显示作者ID和姓氏。

Answer 1

将顶部的视图从@model SimpleDropDownList.Models.Book更改为@model SimpleDropDownList.Models.AuthorViewModel

Answer 2

现在可以使用了。将属性从Book类移到AuthorViewModel类。现在如下图所示。

图书模型：

namespace SimpleDropDownList.Models
{
    public class Book
    {
        [Key]
        public int BookID { get; set; }
        [StringLength(255)]
        [Display(Name = "Book Title")]
        public string Title { get; set; }
        public int AuthorID { get; set; }
        public Author Author { get; set; }
        // Removed this link and added property to AuthorViewModel class
        // public AuthorViewModel AuthorViewModel { get; set; }
    }
}

作者模型：

namespace SimpleDropDownList.Models
{
    public class Author
    {
        [Key]
        public int AuthorID { get; set; }
        [Display(Name = "First Name")]
        [StringLength(50)]
        public string FirstName { get; set; }
        [Display(Name = "Last Name")]
        [StringLength(50)]
        public string LastName { get; set; }
        public ICollection<Book> Books { get; set; } = new List<Book>();
    }
}

ViewModel：

namespace SimpleDropDownList.Models
    {
        [NotMapped]
        public class AuthorViewModel
        {
            //Property to hold the list of authors in the GET
            public IEnumerable<SelectListItem> AuthorOptions { get; set; }

            //Property to bind the selected author used in the POST
            public List<int> SelectedAuthorIds { get; set; }

            // Added this property to link back to Book class
            public Book Book { get; set; }
        }
    }

将“创建”视图更改为：

public IActionResult Create()
{
    // The ViewData line auto created by the scaffolding process
    // ViewData["AuthorID"] = new SelectList(_context.Set<Author>(), "AuthorID", "AuthorID");

    AuthorViewModel vm = new AuthorViewModel();
    vm.AuthorOptions = _context.Book.Select(x => new SelectListItem()
    { Value = x.AuthorID.ToString(), Text = x.Author.LastName }).ToList();

    return View(vm);
}

剃刀页面打开并具有相关的下拉列表时出错

2 个答案: