How to concat multiple selects with LINQ?

Question

I have main select which looks like this:

protected IQueryable<Answers> GetActualAnswers<TAns>(DateTime? start, DateTime? end, long? statusId) where TAns: AnswersBase
{
_contex.Set<TAns>.Where(x => x.Type == VoteType.Good)
.Select(vv => new Answers
        {
             CreatedAt = vv.CreatedAt,
             StatusId = vv.StatusId,
             Type = vv.Type ,
             AnswerInGuideStatusId = vv.AnswerInGuideStatusId
        }
}

I'm using this method in two simple queries:

 var result1 = GetActualAnswers<JournalAnswers>(start, end, statusId)
    .Select(j => new UnitedAnswers
    {
         Question = j.Question,
    }

    var result2 = GetActualAnswers<BoAnswers>(start, end, statusId)
    .Select(b => new UnitedAnswers
    {
         Prospects = b.Prospects ,
    }

var mainResult = result1.Concat(result2);

I get errors:

Sql = Sql = '((System.Data.Entity.Infrastructure.DbQuery<UnitedAnswers>)result1).Sql' threw an exception of type 'System.NotSupportedException'
Sql = Sql = '((System.Data.Entity.Infrastructure.DbQuery<UnitedAnswers>)result2).Sql' threw an exception of type 'System.NotSupportedException'
Sql = Sql = '((System.Data.Entity.Infrastructure.DbQuery<UnitedAnswers>)mainResult).Sql' threw an exception of type 'System.NotSupportedException'

Is it possible to use several Selects? May be someone can give advice with this query?

Do the two queries result1 and result2 work, when executed separatly ? "Not supported might be something in the 4! classes in your queries you are using. Especially the Answers class does not have a Question or Prospects properties set, how can you copy them ? And why you first copy to Answers, than to UnitedAnswers ? Why not to UnitedAnswers right away, would cause less complication. — Holger, Dec 27 '19 at 11:16
Where do you think `j.Question` and `b.Prospects` will come from? — NetMage, Dec 27 '19 at 21:37

score 0 · Answer 1 · answered Dec 30 '19 at 02:33

0

Firstly, I was wondering where the property j.Question and b.Prospects you get, While in the method GetActualAnswers you did not get the value of 2 properties above.

Secondly, At the method GetActualAnswers you were returning IQueryable, so you should check empty instead of null value

Then your case might look like this

var mainResult = Enumerable.Concat(
resut1 ?? Enumerable.Empty<UnitedAnswers>(),
resut2 ?? Enumerable.Empty<UnitedAnswers>()

or

  var mainResult = Enumerable.Concat(
    result1.AsEnumerable(),
    result2.AsEnumerable());

The following links are useful for you.

answered Dec 30 '19 at 02:33

Nguyễn Văn Phong

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Does this answer your question? Please let me know if you need any help. @TAD – Nguyễn Văn Phong Dec 30 '19 at 06:17
Unfortunately this answer didn't help me. I had changed conception to bypass this problem. Thank you for helping. – TAD Jan 08 '20 at 09:31
`I had changed conception to bypass this problem`, You mean my code worked but you changed your solution instead of `concat multiple selects`? – Nguyễn Văn Phong Jan 08 '20 at 09:33
Your code doesn't help. There is problem with field. It should be simple dto object. So I use Automapper to generate ef query before execute them. – TAD Jan 08 '20 at 11:45

How to concat multiple selects with LINQ?

1 Answers1