Calculate average of value using two tables

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C#

LINQ

In project i have two tables

- Restaurant (RestaurantId, Name, Adress)
- RestaurantReview (ReviewId, RestaurantId, Mark)
One restaurant can have several opinions. I would like my application to display restaurant and the average of its ratings. I create code:

var resReviews = (
  from x in _context.Restaurant
  join y in _context.Reviews on x.IdRestauracji equals y.RestaurantId into z
  from a in z
  group a by a.RestaurantId into g
  select new
  {

     RatingAverage = g.Average(x => Convert.ToInt32(x.Mark))
  }
).ToList();

In list are saved calculated average mark. I would also like to have restaurant information saved on the list. I try create something like this, but it doesn't work:

var resReviews = (
  from x in _context.Restaurant
  join y in _context.Reviews on x.IdRestauracji equals y.RestaurantId into z
  from a in z
  group a by a.RestaurantId into g
  select new
  {
     Restaurant = g.FirstOrDefault();
     RatingAverage = g.Average(x => Convert.ToInt32(x.Mark))
  }
).ToList();

What I have tried:

var lj = (from x in _context.Restaurant
                      join y in _context.Reviews on x.RestaurantId equals y.RestaurantId into z
                      select new
                      {
                          Name= x.Name,
                          Total = z.Average(x => Convert.ToInt32(x.Mark))
                      }).ToList();

RETURN Error:

System.InvalidOperationException: „Processing of the LINQ expression 'GroupJoin<Restaurant, Reviews, int, <>f__AnonymousType4<string, double>>(
    outer: DbSet<Restaurant>, 
    inner: DbSet<Reviews>, 
    outerKeySelector: (x) => x.RestaurantId, 
    innerKeySelector: (y) => y.RestaurantId, 
    resultSelector: (x, z) => new { 
        Name = x.Name, 
        Total = Average<Review>(
            source: z, 
            selector: (x) => ToInt32(x.Marks))
     })' by 'NavigationExpandingExpressionVisitor' failed.