在另一个表上使用嵌套查询查找平均值

我有一个常见的问题，我无法通过查看互联网上的几个论坛来解决。

我有一个使用ASP.NET WebAPI的餐厅评级应用程序。该应用程序有两张桌子Restaurant和Comments桌子。每个餐厅可能有多条评论，每条评论都有一个评级值。我正试图在WebAPI中组合一种方法，通过查看Comments表，从Restaurant表中提取详细信息，以及每个Restaurants的Average Rating值。到目前为止，我的尝试是在下面的代码中，但它不起作用。我尝试过使用聚合函数、平均函数、嵌套查询、联接等，但仍然无法获取平均值。如果有人能帮忙，我会很感激的？

public IQueryable<RestaurantView> GetRestaurants(string All)
    {
        var query = from x in db.Restaurants
                    select new RestaurantView
                    {
                        RestaurantID = x.RestaurantID,
                        RestaurantName = x.RestaurantName,
                        RestaurantDecription = x.RestaurantDecription
                        RestaurantRatingAverage = (from a in db.Restaurants
                         join b in db.Comments on a.RestaurantID equals b.CommentsRestaurantID into z
                         from c in z
                         group c by c.CommentsRestaurantID into g
                         select new
                         {
                            RatingAverage = Convert.ToDouble(g.Average(a => a.CommentsRating))
                         };)
                    };

        return query;
    }

更新：使用Jonathan的技术（见下文）

public IQueryable<RestaurantView> GetRestaurants(string All)
    {
        var query = from x in db.Restaurants
                    select new RestaurantView
                    {
                        RestaurantID = x.RestaurantID,
                        RestaurantName = x.RestaurantName,
                        RestaurantDecription = x.RestaurantDecription,
                        RestaurantRatingAverage = (from a in db.Comments
                   where a.CommentsRestaurantID.Equals(x.RestaurantID)select a.CommentsRating).Average()
                    };

        return query;
    }

但我现在得到以下异常

 An error has occurred.","ExceptionMessage":"The cast to value type 'Double' failed because the materialized value is null. Either the result type's generic parameter or the query must use a nullable type."

void Main()
{
    var restaurants = new List<Restaurant>();
    restaurants.Add(new Restaurant(1, "McDonalds"));
    restaurants.Add(new Restaurant(2, "Wendy's"));
    restaurants.Add(new Restaurant(3, "KFC"));
    var comments = new List<Comment>();
    comments.Add(new Comment(1, 1, "I love clowns!", 9.5));
    comments.Add(new Comment(2, 1, "Disgusting", 1.0));
    comments.Add(new Comment(3, 1, "Average", 5.0));
    comments.Add(new Comment(4, 2, "Hmmm tasty", 8.5));
    comments.Add(new Comment(5, 2, "Yuck", 4.0));
    // Edit - removed comment for KFC, updated code below to handle nulls
    var restaurantsWithRatings = restaurants.Select(r => new {
            RestaurantId = r.RestaurantId,
            Name = r.Name,
            Rating = (
                comments.Where(c => c.RestaurantId == r.RestaurantId)
                    .Select(c => c.Rating)
                    .DefaultIfEmpty(0)
            ).Average()
        });
    foreach(var r in restaurantsWithRatings)
        Console.WriteLine("{0}: {1}", r.Name, r.Rating);
}
class Restaurant
{
    public Restaurant(int restaurantId, string name)
    {
        RestaurantId = restaurantId;
        Name = name;
    }
    public int RestaurantId { get; set; }
    public string Name { get; set; }
}
class Comment
{
    public Comment(int commentId, int restaurantId, string message, double rating)
    {
        CommentId = commentId;
        RestaurantId = restaurantId;
        Message = message;
        Rating = rating;
    }
    public int CommentId { get; set; }
    public int RestaurantId { get; set; }
    public string Message { get; set; }
    public double Rating { get; set; }
}