我已经看到了类似问题的一些答案,但是我似乎无法弄清楚如何将答案应用于我的问题。
var allposts = _context.Posts
.Include(p => p.Comments)
.Include(aa => aa.Attachments)
.Include(a => a.PostAuthor)
.Where(t => t.PostAuthor.Id == postAuthorId).ToList();
附件可以由作者(作者类型)或贡献者(贡献者类型)上传。我想做的是,仅获取附件所有者为作者类型的附件。
我知道这行不通并给出错误:
.Include(s=>aa.Attachments.Where(o=>o.Owner is Author))
我已在此处阅读有关滤波投影的信息
编辑-链接到文章::http://blogs.msdn.com/b/alexj/archive/2009/10/13/tip-37-how-to-do-a-conditional-include.aspx,
但是我只是无法理解。
我不想在最终的where子句中包含过滤器,因为我想要所有帖子,但是我只想检索属于作者的那些帖子的附件。
编辑2:-请求发布架构
public abstract class Post : IPostable
{
[Key]
public int Id { get; set; }
[Required]
public DateTime PublishDate { get; set; }
[Required]
public String Title { get; set; }
[Required]
public String Description { get; set; }
public Person PostAuthor { get; set; }
public virtual ICollection<Attachment> Attachments { get; set; }
public List<Comment> Comments { get; set; }
}
从您发布的链接中,我可以确认该技巧有效,但仅适用于一对多(或多对一)关系。在这种情况下,您的Post-Attachment
应该是一对多关系,因此完全适用。这是您应该具有的查询:
//this should be disabled temporarily
_context.Configuration.LazyLoadingEnabled = false;
var allposts = _context.Posts.Where(t => t.PostAuthor.Id == postAuthorId)
.Select(e => new {
e,//for later projection
e.Comments,//cache Comments
//cache filtered Attachments
Attachments = e.Attachments.Where(a => a.Owner is Author),
e.PostAuthor//cache PostAuthor
})
.AsEnumerable()
.Select(e => e.e).ToList();
从virtual
导航属性中删除Attachments
关键字以防止延迟加载:
public ICollection<Attachment> Attachments { get; set; }
第一种方法:发出两个单独的查询:一个用于发布,一个用于附件,其余的让关系修复来完成:
List<Post> postsWithAuthoredAttachments = _context.Posts
.Include(p => p.Comments)
.Include(p => p.PostAuthor)
.Where(p => p.PostAuthor.Id == postAuthorId)
.ToList();
List<Attachment> filteredAttachments = _context.Attachments
.Where(a => a.Post.PostAuthor.Id == postAuthorId)
.Where(a => a.Owner is Author)
.ToList()
关系修正意味着您可以通过帖子的导航属性访问这些过滤的附件
第二种方法:一次查询数据库,然后进行内存中查询:
var query = _context.Posts
.Include(p => p.Comments)
.Include(p => p.PostAuthor)
.Where(p => p.PostAuthor.Id == postAuthorId)
.Select(p => new
{
Post = p,
AuthoredAttachments = p.Attachments
Where(a => a.Owner is Author)
}
);
我只在这里使用匿名类型
var postsWithAuthoredAttachments = query.ToList()
或者我将创建一个ViewModel类以避免匿名类型:
List<MyDisplayTemplate> postsWithAuthoredAttachments =
//query as above but use new PostWithAuthoredAttachments in the Select
或者,如果您真的想拆开帖子:
List<Post> postsWithAuthoredAttachments = query.//you could "inline" this variable
.AsEnumerable() //force the database query to run as is - pulling data into memory
.Select(p => p) //unwrap the Posts from the in-memory results
.ToList()
您可以使用扩展方法的this implementation(例如)Include2()
。之后,您可以致电:
_context.Posts.Include2(post => post.Attachments.Where(a => a.OwnerId == 1))
上面的代码仅包含Attachment.OwnerId == 1
处的附件。
EF Core 5.0即将引入过滤包含。
var blogs = context.Blogs
.Include(e => e.Posts.Where(p => p.Title.Contains("Cheese")))
.ToList();
参考: https://docs.microsoft.com/en-us/ef/core/what-is-new/ef-core-5.0/whatsnew#filtered-include
Include()
中的Lambda只能指向属性:
.Include(a => a.Attachments)
.Include(a => a.Attachments.Owner);
您的情况对我来说没有意义,因为Include()
表示join
,您可以执行或不执行。而不是有条件的。
您将如何用原始SQL编写?
为什么不只是这样:
context.Attachments
.Where(a => a.Owner.Id == postAuthorId &&
a.Owner.Type == authorType);
?
假设“ a”的类型为“ YourType”,则可以通过使用方法扩展来解决条件包含,例如]]
public static class QueryableExtensions { public static IQueryable<T> ConditionalInclude<T>(this IQueryable<T> source, bool include) where T : YourType { if (include) { return source .Include(a => a.Attachments) .Include(a => a.Attachments.Owner)); } return source; } }
...然后像使用.include一样使用它,例如
bool yourCondition;
.ConditionalInclude(yourCondition)