How to group join with linQ

Question

I need to join two lists by an Id in order to map values to an User entity.

For that part, I think I've managed to do it with :

 var concernedParticipantIds = baseValues.Select(x => x.ParticipantId).Distinct().ToList();
                var queryParticipants =_unitOfWorkMt.GetMTReadonly<Participant>().GetQuery(tenantId).Where(x => concernedParticipantIds.Contains(x.Id));

                var participantAndValues = from value in baseValues
                    join participant in queryParticipants on value.ParticipantId equals participant.Id 
                    select new {Participant = participant, Value = value};

But it results in one line per Value like this :

user_1 => value_1
user_1 => value_2
user_1 => value_3
user_2 => value_1
user_2 => value_2
user_2 => value_3
...

Is there a way to group by the user? I'd like something like :

- user_1 => value_1  
          value_2
          value_3
- user_2 => value_1
          value_2
          value_3

The only thing I came up with is something like

 var results = from p in participantAndValues
               group p.Value by p.Participant into temp
               select new { Person = temp.Key, Values = temp.ToList() };

Seems to work but not sure it's the right way, and maybe there's a way to do it in one query?

Answer 1

Use GroupJoin:

var participantAndValues = from value in baseValues
    join participant in queryParticipants on value.ParticipantId equals participant.Id into paricipantJ
    select new {Participant = participant, Values = participantJ };

Answer 2

You can achieve it like this way

var results = from p in participantAndValues
               group p by p.Participant into g
               select new { Person = g.Key, Values = g.ToList() };

Or as a non-query expression:

var results = participantAndValues.GroupBy(p => p.Participant)
                                  .Select(g => new { Person  = g.Key, Values = g.ToList() });