无法在内部联接上隐式转换类型system.linq.iqueryable

Question

im a rookie when it comes to asp.net but im trying to create a restfull api in asp.net. in the code below im trying to get data from Saldo and Vereniging.

the saldo table contains:

-saldoId -VerenigingId -LidId -Bedrag

the Vereniging table contains:

VerenigingId naam locatie facebookgroupId

what im trying to create is that the return statement returns all the Verenigingen from 1 LidId.

public class LibraryRepository : ILibraryRepository
{
    private LibraryContext _context;

    public LibraryRepository(LibraryContext context)
    {
        _context = context;
    }

    public bool Save()
    {
        return (_context.SaveChanges() >= 0);
    }

    public IEnumerable<Vereniging> GetVerenigingenperLid(int lidId)
    {

        return  _context.Vereniging    // your starting point - table in the "from" statement
                    .Join(_context.Saldo, // the source table of the inner join
                    a => a.verenigingId,        // Select the primary key (the first part of the "on" clause in an sql "join" statement)
                    b => b.verenigingId,   // Select the foreign key (the second part of the "on" clause)
                    (a, b) => new { Vereniging = a, Saldo = b }) // selection
                    .Where(c => c.Saldo.lidId == lidId);

    }

}

i get the error: Cannot implicitly conver type 'system.linq.IQueryable "<<"anonymous type: Library.API.Entities.Vereniging Vereniging, Library.API.Entities.Saldo Saldo>> to 'System.Collections.Generic.IEnumerable "<"Library.API.Entities.Vereniging>. An explicit conversion exists (are u missing a cast?).

I understand the problem im having. its because the return value consists of a piece Saldo and a Piece Vereniging.

but i dont know how to fix this or avoid it.

Answer 1

Your return type should match the type in projection. see illustration below.

Add a Select(...) to the end of the query:

   // ...
   .Where(c => c.Saldo.lidId == lidId)
   // for example
   .Select(c => c.Vereniging);

Answer 2

Return type of GetVerenigingenperLid is IEnumerable<Vereniging> , while you are returning (a, b) => new { Vereniging = a, Saldo = b } in the select.

To fix this, you need to declare new viewmodel class say Viewmodel which will contain Vereniging and Saldo as properties.

public class ViewModel
{
  public Vereniging {get;set;}
  public Saldo {get;set;}
}

public IEnumerable<ViewModel> GetVerenigingenperLid(int lidId)
{

    return  _context.Vereniging    // your starting point - table in the "from" statement
                .Join(_context.Saldo, // the source table of the inner join
                a => a.verenigingId,        // Select the primary key (the first part of the "on" clause in an sql "join" statement)
                b => b.verenigingId,   // Select the foreign key (the second part of the "on" clause)
                (a, b) => new ViewModel { Vereniging = a, Saldo = b }) // selection
                .Where(c => c.Saldo.lidId == lidId);

}

Hope it helps