如何将linq与Aggregation，join和Group by一起使用

Question

I am trying to get averages of restaurant Ratings by id along with the Names of resteraunts.

Review Class

  public int ReviewId { get; set; }
        public double   Rating { get; set; }
        [ForeignKey("ResterauntId")]
        Resteraunt  Resteraunt { get; set; }
        public int ResterauntId { get; set; }

Resteraunt class

  public int ResterauntId { get; set; }
        public string Name { get; set; }

        public virtual ICollection<Review> Reviews { get; set; }
        public virtual Review  reviews{ get; set; }

Here is what I tried. However this will not compile [g.Key.Rating.Average()] And I don't know how to select the names without grouping by name as well

var ratings = (from rev in reviewsList
                 join res in resteraunts on rev.ResterauntId equals
                 res.ResterauntId

                 group new { rev, res } by new { res.ResterauntId, rev.Rating } into g

                 select new { Rating = g.Key.Rating, AveraRating =   g.Key.Rating.Average() }).ToList();

Answer 1

You need to group by restaurant (id) and average the ratings for each. Since LINQ has Group Join, you can just use the join to get the grouping:

var avgRatings = (from res in resteraunts
                  join rev in reviewsList on res.ResterauntId equals
                  rev.ResterauntId into revj
                  select new { res.ResterauntId, AveraRating = revj.Average(r => r.Rating) }).ToList();

Looking more closely at your (partial) classes, I see you are apparently using Entity Framework, in which case you don't need to manually join:

var avgRatings = (from res in resteraunts
                  select new {
                      res.ResterauntId,
                      AveraRating = res.Reviews.Average(r => (double?)r.Rating)
                  }).ToList();

Improvement on Average thanks to @gnud.