Entity Framework 中的条件包含 ()

Question

我看过一些类似问题的答案，但我似乎无法弄清楚如何将答案应用到我的问题中。

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（类型贡献者）上传。我想要做的，只是获取附件所有者类型为作者的附件。

我知道这行不通并出现错误：

.Include(s=>aa.Attachments.Where(o=>o.Owner is Author))

我在此处阅读了过滤投影

编辑 - link 文章： : http://blogs.msdn.com/b/alexj/archive/2009/10/13/tip-37-how-to-do-a-conditional-include.aspx,

但我无法理解它。

我不想在最后的 where 子句中包含过滤器，因为我想要所有帖子，但我只想检索属于作者的那些帖子的附件。

编辑 2：- Post 请求模式

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; }
}

Answer 1

Include() 中的 Lambda 可能只指向一个属性:

.Include(a => a.Attachments)
.Include(a => a.Attachments.Owner);

你的条件对我来说没有意义，因为 Include() 意味着 join 而你要么做要么不做。而且不是有条件的。

你如何用原始格式写这个 SQL？

为什么不只是这个：

context.Attachments
       .Where(a => a.Owner.Id == postAuthorId &&
                   a.Owner.Type == authorType);

?

Answer 2

根据您发布的 link，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，因此它完全适用。这是您应该拥有的查询：

//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();

Answer 3

您可以使用 this implementation 的扩展方法（例如）Include2()。之后，您可以调用：

_context.Posts.Include2(post => post.Attachments.Where(a => a.OwnerId == 1))

以上代码仅包含 Attachment.OwnerId == 1.

的附件

Answer 4

从 Attachments 导航属性中删除 virtual 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 Posts，一个用于附件，然后让关系修复完成其余的工作：

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()

关系修复意味着您可以通过 Post 的导航访问这些过滤后的附件属性

第二种方法：先查询数据库，再查询内存：

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)
        }
    );

我会在这里使用匿名类型

var postsWithAuthoredAttachments = query.ToList()

或者我会创建一个 ViewModel class 来避免匿名类型：

List<MyDisplayTemplate> postsWithAuthoredAttachments = 
     //query as above but use new PostWithAuthoredAttachments in the Select

或者，如果您真的想打开 Posts：

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()

Answer 5

假设 "a" 属于 "YourType" 类型，条件包含可以通过使用方法扩展来解决，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)

Answer 6

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

Answer 7

试试这个

var allposts = _context.Posts
        .Include(p => p.Comments)
        .Include(a => a.PostAuthor)
        .Where(t => t.PostAuthor.Id == postAuthorId).ToList();

_context.Attachments.Where(o=>o.Owner is Author).ToList();

Answer 8

对于网芯

https://docs.microsoft.com/ru-ru/ef/core/querying/related-data/explicit

var allposts = _context.Posts
        .Include(p => p.Comments)
        .Include(a => a.PostAuthor)
        .Where(t => t.PostAuthor.Id == postAuthorId).ToList();

_context.Entry(allposts)
        .Collection(e => e.Attachments)
        .Query()
        .Where(e=> e.Owner is Author)
        .Load();

它对 sql 进行了 2 次查询。

Entity Framework 中的条件包含 ()

Conditional Include() in Entity Framework

c#

entity-framework

linq

linq-to-entities