如何在asp.net mvc EF代码中使用linq查询关系1：N和N：N（在3个实体之间）从数据库中获取数据？

Question

i have a linq query for fetch data from 3 table (Post, Category, Tag).

i want show latest posts (10 num) in default page of a blog: post title, post tags name and post category name, post short description. i type this linq query

List<Post> IBlogRepository.PostsForList(int pageNo, int pageSize)
    {
        using (var context = new MJBweblogContext())
        {
            IQueryable posts = from Post in context.Posts
                        where Post.Published == true
                        select new
                        {
                            Post.Title,
                            Post.ShortDescription,
                            Post.Description,
                            Post.MetaData,
                            Post.PostedOn,
                            Post.UrlSlug,
                            Post.Published,
                            Post.Modified,
                            Post.Category,
                            Post.Tags,
                            Post.Category.Name,
                        };
            if (posts != null)
            {
                return posts.OfType<Post>() // i give error if i remove OfType<Post>()
                    .OrderByDescending(p => p.PostedOn)
                    .Skip(pageNo * pageSize)
                    .Take(pageSize)
                    .ToList();
            }
            else
            {
                return null;
            }
        }
    }

but i have two problem:

1. if i remove OfType() VS2017 tell me:

   Error CS0266 Cannot implicitly convert type 'System.Linq.IQueryable' to 'System.Collections.Generic.List'. An explicit conversion exists (are you missing a cast?) MJBweblog C:\\Users\\mjb\\source\\repos\\MJBweblog\\MJBweblog\\DAL\\BlogRepository.cs 44 Active

   2. I don't know what code I should write to fetch tags for each post and their category? in the linq query? (i try to show user something like this template of each post in default page )

this is What I want to show for users in default page. (a list of this)

Answer 1

If your Post class is an entity class then you don't need to create another anonymous type with select. Below Linq chain should work for what you are trying to achieve. Also as per the comments - it is not a good practice to return null for a list, the caller can check the Count property to see if the list is empty.

        using (var context = new MJBweblogContext())
        {
            return context.Posts.Where(post => post.Published)
                .OrderByDescending(p => p.PostedOn)
                .Skip(pageNo * pageSize)
                .Take(pageSize)
                .ToList();
        }

Answer 2

You can't just skip all of that other stuff?

List<Post> IBlogRepository.PostsForList(int pageNo, int pageSize)
    {
        using (var context = new MJBweblogContext())
        {
            return context.Posts.Where(a => a.Published == true)
                                .OrderByDescending(p => p.PostedOn)
                                .Skip(pageNo * pageSize)
                                .Take(pageSize)
                                .ToList();
        }
    }

if you're not getting your category information, you should be able to use INCLUDE

List<Post> IBlogRepository.PostsForList(int pageNo, int pageSize)
    {
        using (var context = new MJBweblogContext())
        {
            return context.Posts.Include("Category").Where(a => a.Published == true)
                                .OrderByDescending(p => p.PostedOn)
                                .Skip(pageNo * pageSize)
                                .Take(pageSize)
                                .ToList();
        }
    }