model.Page = db.Pages .Where(p => p.PageId == Id) .Include(p => p.Series .Select(c => c.Comics .Select(col => col.Collection))) .SingleOrDefault();
虽然我现在需要通过名为“ReadingOrder”的属性来订购漫画,但这很有用.
我试过了:
model.Page = db.Pages .Where(p => p.PageId == Id) .Include(p => p.Series.Select(c => c.Comics.OrderBy(o => o.ReadingOrder) .Select(col => col.Collection))) .SingleOrDefault();
但这会导致以下错误:
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. Parameter name: path
任何想法这个错误意味着什么?
提前致谢
编辑:
我的模特:
public class Page { public int PageId { get; set; } public string Title { get; set; } public ICollection<Series> Series { get; set; } } public class Series { public int SeriesId { get; set; } public int PageId { get; set; } public string Title { get; set; } public Page Page { get; set; } public ICollection<Comic> Comics { get; set; } } public class Comic { public int ComicId { get; set; } public string Title { get; set; } public int ReadingOrder { get; set; } public string Subtitle { get; set; } public int CollectionId { get; set; } public Collection Collection { get; set; } } public class Collection { public int CollectionId { get; set; } public string Title { get; set; } public ICollection<Comic> Comics { get; set; } }
解决方法
实际上,EF不支持在预先加载语句(Include)中应用排序(以及过滤).
所以,你可以做什么?
选项1:
加载实体后在内存中排序:
model.Page = db.Pages .Where(p => p.PageId == Id) .Include(p => p.Series.Select(c => c.Comics .Select(col => col.Collection))) .SingleOrDefault(); if (model.Page != null) { foreach (var series in model.Page.Series) series.Comics = series.Comics.OrderBy(c => c.ReadingOrder).ToList(); }
很丑,但是因为你显然只是通过id加载一个Page对象,它可能比以下选项更快(LINQ to Objects in memory)(如果Series和Comics集合不是特别长).
选项2:
在混合了eager和explicite加载的部分中细分查询:
model.Page = db.Pages .Where(p => p.PageId == Id) .Include(p => p.Series) // only Series collection is included .SingleOrDefault(); if (model.Page != null) { foreach (var series in model.Page.Series) db.Entry(series).Collection(s => s.Comics).Query() .Include(c => c.Collection) .OrderBy(c => c.ReadingOrder) .Load(); // one new DB query for each series in loop }
选项3:
投影?
Here和here顺便说一下复杂的危险包括多个导航属性链.它可以加载大量重复数据. Include确保您只有一次DB往返,但可能需要更多传输数据. Explicite加载有多个往返,但总共可能有更少的数据.
(我知道,我给了你这个包括……选择……选择……选择……链,但我怎么知道你会认真对待我:).那么,根据嵌套集合的大小,它仍然是最佳选择.)
原文地址:https://www.jb51.cc/aspnet/249324.html
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