使用 Linq 对 4 个表进行分组

Question

我有以下要求：

• 一个评分可以有零个或多个 RatingPictures
• 一个评分可以有零个或多个评论
• 一个评论属于一个用户

这是我目前所拥有的：

from rating in Ratings
where rating.LocationID == 1007
    join ratingpicture in RatingPictures
    on rating.ID equals ratingpicture.RatingID into j3
    from ratingpicture in j3.DefaultIfEmpty()
        join comment in Comments
        on rating.ID equals comment.RatingID into j1
        from comment in j1.DefaultIfEmpty()
            join user in Users
            on comment.UserID equals user.ID into j2
            from user in j2.DefaultIfEmpty()
group new { ratingpicture, comment, user } by rating into g
select new { rating = g.Key, ratingpicture= g.Key, comment = g.ToList() }

Answer 1

如果您像这样为实体类建模：

public class Comment
{
    public int CommentId { get; set; }
    public int RatingId { get; set; }
    public virtual Rating Rating { get; set; }

    public int UserId { get; set; }
    public virtual User User { get; set; }
}

public class User
{
    public int UserId { get; set; }

}

public class Rating
{
    public Rating()
    {
        RatingPictures = new HashSet<RatingPicture>();
        Comments = new HashSet<Comment>();
    }

    public int RatingId { get; set; }
    public virtual ICollection<RatingPicture> RatingPictures { get; set; }
    public virtual ICollection<Comment> Comments { get; set; }
    public int LocationID { get; set; }
}

public class RatingPicture
{
    public int RatingPictureId { get; set; }
    public int RatingId { get; set; }
    public virtual Rating Rating { get; set; }

}

那么你的查询就这么简单：

var query = context.Ratings.Where(r => r.LocationID == 1007)
    .Include(r => r.RatingPictures)
    .Include(r => r.Comments.Select(c => c.User));

var result = query.ToList();

在后台，此查询将被转换为连接查询。但这就是 ORM 的美妙之处，我们可以在更抽象的层次上工作。

查看this reference，了解有关实体框架中的关系和导航属性的更多信息。