Unable to write an Include query with FirstOrDefault() and condition

问题

I'm writing an entity framework query which needs Eager loading multiple levels based on condition.

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();

I expect result to have first or default Blog and first or default Post for that blog by author "Me" and a list of all comments.

When query for blogs1is executed I see following exception blogs2 query works as expected

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

回答1:

FirstOrDefault executes the query and you can not use it inside Include as its purpose is to include navigational properties. You will need to modify the query to one of the below two ways:

Method 1: Its two two step process:

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");

Method 2:

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

来源：https://stackoverflow.com/questions/58347487/unable-to-write-an-include-query-with-firstordefault-and-condition

标签

entity-framework-6