扩展DbSet [英] extension for DbSet

问题描述

什么是扩展DbSet？我想补充的方法FindTheLatest。这是我的code：
这是我的code

What is extension for DbSet? I want to add method 'FindTheLatest'. this is my code: this is my code

    public static List FindTheLatest(this DbSet<Review> reviews, int nums)
    {
        return reviews.OrderByDescending(r => r.Created).Take(nums).ToList();
    }

但它不工作。我必须做的？

But it doesn't work. What i must to do?

这是我的扩展类：

public static class RestaurantReviewsExtenstion
{
    public static IEnumerable<Review> FindTheLatest(this IQueryable<Review> reviews, int nums)
    {
        return reviews.OrderByDescending(r => r.Created).Take(nums).ToList();
    }

    public static Review FindById(this System.Data.Entity.DbSet<Review> reviews, int id)
    {
        return reviews.SingleOrDefault(s => s.ReviewId == id);
        //return reviews.Find(item => item.Id == id);
    }

    public static Review FindTheBest(this List<Review> list)
    {
        return list.OrderByDescending(o => o.Rating).FirstOrDefault();
    }
}

这是我的DBContex类：

this is my DBContex Class:

public class OdeToFoodDb: DbContext
{
    public DbSet<Restaurant> Restaurants { get; set; }
    public DbSet<Review> Reviews { get; set; }

    protected override void OnModelCreating(System.Data.Entity.DbModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Restaurant>()
            .HasMany(resturant => resturant.Reviews)
            .WithRequired(review => review.Resturant);
        base.OnModelCreating(modelBuilder);
    }
}

这是我得到错误：

    OdeToFoodDb _db = new OdeToFoodDb();
    public PartialViewResult LatestReview()
    {
        System.Threading.Thread.Sleep(1500);
        //this is where i get error
        var review = _db.Reviews.FindTheLatest(1);
        //************************************
        return PartialView("_Review", review);
    }

对不起信息不畅。

Sorry about poor information.

推荐答案

好了一件事，列表是一个泛型类型。对于另一个 DbSet 不是一套任何的具体的的类型 - 您可能希望 DbSet＆LT; T＆GT; 对于一些适当的 T 。也许你的意思是：

Well for one thing, List is a generic type. For another DbSet isn't a set of any specific type - you probably want DbSet<T> for some appropriate T. Perhaps you mean:

public static List<Review> FindTheLatest(this DbSet<Review> reviews, int nums)
{
    return reviews.OrderByDescending(r => r.Created).Take(nums).ToList();
}

或者

public static List<Review> FindTheLatest(this DbSet reviews, int nums)
{
    return reviews.Cast<Review>()
                  .OrderByDescending(r => r.Created)
                  .Take(nums)
                  .ToList();
}

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