EntityFramework Core Group By with Sum

Question

Haven't written any code in a while and I'm trying to shake the rust off by writing a small app for my kids to track the books they've read over the summer. The DB is simple, only 3 tables. 1 for books (title, author, pages etc), 1 for reader names and 1 is the actual reading log (1 row for each book they read, foreign key relationship to book table and reader table).

Trying to use EntityFramework core to write a query that will return the total pages read by reader. I can run this in SSMS and it works fine

select r.Name, sum(b.Pages) as PagesRead
from ReadingLog rl
inner join Readers r on rl.ReaderId = r.Id
inner join Books b on rl.BookId = b.Id
where rl.IsActive = 1
group by r.Name

I tried writing it in EF as this:

var pagesByReader = _context.ReadingLog
                            .Where(m => m.IsActive)
                            .GroupBy(m => m.Reader.Name)
                            .Select(m => new GraphViewModel()
                                   {
                                      Reader = m.Key,
                                      PagesRead = m.Sum(b => b.Book.Pages)
                                   })
                            .ToList();

but I get this error at runtime: ).Book.Pages' could not be translated. Either rewrite the query in a form that can be translated, or switch to client evaluation explicitly by inserting a call to either AsEnumerable(), AsAsyncEnumerable(), ToList(), or ToListAsync()

I think I'm just writing the EF query wrong but I can't figure out why. Any help is appreciated

Answer 1

Looks like you have m.Key in the select . When using GroupBy , all values in a select that are not defined in the group by must be in an aggregate function.

Try changing m.Key to m.Key.First()

Answer 2

As mentioned in the comment, the sql is tranlated to Linq, like the following code:

var result = (from rl in _context.ReadingLog 
            join r in _context.Readers on rl.ReaderId equals r.Id 
            join b in _context.Books on rl.BookId equals b.Id 
            where rl.IsActive 
            group new {r,b} by r.Name int g 
            select 
            new GraphViewModel 
            { 
                Reader = g.Key, 
                PagesRead = g.Sum(x => x.b.Pages) 
            }).ToList();