c# - 如何应用 Skip and Take on Include

Question

我有一个名称为 Tag 的类。每个标签都有一些 News 、 Article 和 `Poll'。我想要 Get All Related Post 其中有 Tag 。 为此，我使用此查询:

 public Tag Get(string tagName, int page, int recordsPerPage = 10)
    {
        int skip = page * recordsPerPage;
        return 
            _tags.Where(tag => tag.Title.ToLower() == tagName.ToLower()).Include(row => row.Articles
            .OrderByDescending(n => n.Code).Skip(skip).Take(recordsPerPage).ToList())
            .Include(row => row.News.AsQueryable()).OrderBy(news => news.Code).Skip(skip).Take(recordsPerPage)
            .Include(row => row.Polls).OrderByDescending(poll => poll.Code).Skip(skip).Take(recordsPerPage).FirstOrDefault();
}

但是当我运行 Project 时出现此错误:

The Include path expression must refer to a navigation property defined on the type. Use dotted paths for reference navigation properties and the Select operator for collection navigation properties.

Answer 1

您不能将不是属性表达式的表达式传递给 Include 方法，查看您的代码

.Include(row => row.Articles.OrderByDescending(n => n.Code).Skip(skip).Take(recordsPerPage).ToList())


.Include(row => row.News.AsQueryable())

如您所见，在 include 中，您正在调用许多方法，这是不允许的。

你必须为此改变它:

.Include(row => row.Articles)
.Include(row => row.News)

让我尝试改进您的代码，您当前的代码是这样的:

_tags.Where(tag => tag.Title.ToLower() == tagName.ToLower()).Include(row => row.Articles
            .OrderByDescending(n => n.Code).Skip(skip).Take(recordsPerPage).ToList())
            .Include(row => row.News.AsQueryable()).OrderBy(news => news.Code).Skip(skip).Take(recordsPerPage)
            .Include(row => row.Polls).OrderByDescending(poll => poll.Code).Skip(skip).Take(recordsPerPage).FirstOrDefault();

让我们在换行符中打破它

_tags.Where(tag => tag.Title.ToLower() == tagName.ToLower())
     .Include(row => row.Articles.OrderByDescending(n => n.Code).Skip(skip).Take(recordsPerPage).ToList())
     .Include(row => row.News.AsQueryable())
     .OrderBy(news => news.Code)
     .Skip(skip)
     .Take(recordsPerPage)
     .Include(row => row.Polls)
     .OrderByDescending(poll => poll.Code)
     .Skip(skip)
     .Take(recordsPerPage)
     .FirstOrDefault();

如果你只需要一个标签，你不需要做这个大的方法链，你可以使用这个来简化它:

public Tag Get(string tagName, int page, int recordsPerPage = 10)
{
    return _tags.Include(tag => row.Articles).Include(tag => row.News).Include(tag => row.Polls)
         .OrderBy(tag => news.Code)
         .FirstOrDefault(tag => tag.Title.ToLower() == tagName.ToLower());
}