Multiple randomly selection Linq

Question

I have Questions table and List of CategoryIds. I have to get one question from each CategoryId randomly. Right now I'm doing like this:

var randomQuestions = new List<Question>();
foreach(int id in categoryIds){

randomQuestions.add(questions.Where(o => o.CategoryId== id).OrderBy(o => Guid.NewGuid()).FirstOrDefault());
}

But is there way to do it only with Linq? Thanks

I hope questions variable not a DbSet<> (if so it's totally awful). Try that: https://stackoverflow.com/questions/7258824/c-elegant-code-for-getting-a-random-value-from-an-ienumerable — Alexbogs, Jan 30 '20 at 16:09
Why you say "get one question randomly" where you are using this checking o.Id == id? — ilyes, Jan 30 '20 at 16:14

score 3 · Accepted Answer · answered Jan 30 '20 at 16:20

This should do what you want in one query

var randomQuestions = questions
    .Where(q => categoryIds.Contains(q.CategoryId))
    .GroupBy(q = > q.CategoryId)
    .Select(grp => grp.OrderBy(_ => Guid.NewGuid()).First())
    .ToList();

That will first filter only questions in the categories you care about, then it groups on the category id and for each group it randomly picks one.

Multiple randomly selection Linq

1 Answers1