从使用实体模型转换为使用视图模型

Question

当我使用这个查询和实体模型时，一切正常：

var categories = _context.ProductCategories.Include(e => e.Children).ToList();
var topLevelCategories = categories.Where(e => e.ParentId == null);
return View(topLevelCategories);

实体模型：

public class ProductCategory
{
    public int Id { get; set; }
    public int SortOrder { get; set; }
    public string Title { get; set; }

    [ForeignKey(nameof(ParentCategory))]
    public int? ParentId { get; set; }

    public ProductCategory ParentCategory { get; set; } //nav.prop to parent
    public ICollection<ProductCategory> Children { get; set; } //nav. prop to children

    public List<ProductInCategory> ProductInCategory { get; set; }
}

...但是当我尝试改用视图模型时，我收到以下错误消息：

“InvalidOperationException：传递到 ViewDataDictionary 的模型项的类型为 'System.Collections.Generic.HashSet1[MyStore.Models.ProductCategory]', but this ViewDataDictionary instance requires a model item of type 'System.Collections.Generic.IEnumerable1[MyStore.Models.ViewModels.ViewModelProductCategory]'。”

我知道发送到视图的模型不正确，但我不明白为什么。

我的视图模型查询：

var VMCategories = _context.ProductCategories
                .Include(e => e.Children).ToList()
                .OrderBy(s => s.SortOrder)
                .Where(r => r.ParentId == null)
                    .Select(v => new ViewModelProductCategory
                    {
                        Id = v.Id,
                        Children = v.Children,
                        ParentId = v.ParentId,
                        Title = v.Title,
                        SortOrder = v.SortOrder
                    })
                    .ToList();
return View(VMCategories);

视图模型：

public class ViewModelProductCategory
{
    public int Id { get; set; }
    public int? ParentId { get; set; }
    public string Title { get; set; }
    public int SortOrder { get; set; }

    public string ProductCountInfo =>
        Products != null && Products.Any() ? Products.Count().ToString() : "0";

    public ProductCategory ParentCategory { get; set; } // Nav.prop. to parent
    public IEnumerable<ProductCategory> Children { get; set; } // Nav.prop. to children

    public List<ViewModelProduct> Products { get; set; } // Products in this category
    public List<ViewModelProduct> OrphanProducts { get; set; } // Products with no references in ProductInCategory

}

索引视图：

@model IEnumerable<MyStore.Models.ViewModels.ViewModelProductCategory>

<ul>
    @Html.Partial("_CategoryRecursive", Model)
</ul>

_CategoryRecursive.cshtml：

@model IEnumerable<MyStore.Models.ViewModels.ViewModelProductCategory>

<ul style="list-style:none;padding-left:0px;">
   @if (Model != null)
    {
        foreach (var item in Model)
        {
            if (item.Children != null)
            {
                <li>
                    <ul>
                        @Html.Partial("_CategoryRecursive.cshtml", item.Children)
                    </ul>
                </li>
            }
        }
    }
</ul>

我的错误在哪里？

Answer 1

您的视图模型是实体模型和视图模型的混合搭配。例如，视图模型 Children 集合是实体模型类型，而它也应该是视图模型的集合。保持干净，不要让实体模型泄漏到视图模型中

public class ProductCategoryVM
{
    public int Id { get; set; }
    public int? ParentId { get; set; }
    public string Title { get; set; }
    public int? Products { get; set; }
    public IEnumerable<ProductCategoryVM> Children { get; set; }
}

一旦你准备好了

• 获取所有类别实体模型
• 也计算产品
• 将它们映射到类别视图模型

（知道有多个级别的类别）

var query = from category in context.Categories
            let products = from product in context.Products
                            where product.CategoryId == category.Id
                            select product
            select new ProductCategoryVM
            {
                Id = category.Id,
                ParentId = category.ParentId,
                Title = category.Title,
                Products = products.Count(),
                ...
            };
var viewmodels = query.ToList();

下一步可能是从根开始递归地将子级附加到相应的父级。 （查看gist查看全图）

var root = viewmodels
    .Where(p => p.ParentId == null)
    .FirstOrDefault();
root.Traverse(viewmodels);

public static class Extensions
{
    public static void Traverse(this ProductCategoryVM node, IEnumerable<ProductCategoryVM> nodes)
    {
        var children = nodes.Where(p => p.ParentId == node.Id);
        foreach (var child in children)
        {
            child.Traverse(nodes);
        }

        node.Children = children;
    }
}