无法使用 FirstOrDefault() 和条件编写包含查询

Question

我正在编写一个实体框架查询，它需要根据条件加载多个级别。

var blogs1 = context.Blogs
    .Include(x => x.Posts.FirstOrDefault(h => h.Author == "Me"))
    .Include(x => x.Comment)
    .FirstOrDefault();

public class Blog
{
    public int BlogId { get; set; }
    public virtual ICollection<Post> Posts { get; set; }
}


public class Post
{
    public int PostId { get; set; }
    public string Author { get; set; }  
    public int BlogId { get; set; }
    public virtual ICollection<Comment> Comments { get; set; }
}

public class Comment
{
    public int PostId
    public int CommentId { get; set; }
    public string CommentValue { get; set;}
}
var blogs2 = context.Blogs
                        .Include("Posts.Comments")
                        .ToList(); 

我希望结果有第一个或默认博客以及作者“我”为该博客发布的第一个或默认帖子以及所有 cmets 的列表。

当查询blogs1被执行时，我看到以下异常 blogs2 查询按预期工作

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

Answer 1

FirstOrDefault 执行查询，您不能在 Include 中使用它，因为它的目的是包含导航属性。您需要将查询修改为以下两种方式之一：

方法一：分两步：

var blogs1 = context.Blogs
    .Include(x => x.Posts.Select(p => p.Comments))
**//     .Include(x => x.Comment) // This include is incorrect.**
    .FirstOrDefault(x => x.Posts.Any(h => h.Author == "Me"));

var myPosts = blogs1?.Posts.Where(p => p.Author == "Me");

方法二：

var myPosts = context.Posts.Include(p => p.Blog).Include(p => p.Comments).Where(p => p.Author == "Me");