Entity Framework Core: Query only a few properties of related entity

Question

I have related entities Teams and Playlists , where a Team can hold multiple Playlists . Now, from a given team, I need all the playlists, but only the Id and PlaylistName properties.

I tried this, but it returns Anonymous Types within the list, that is hard to map to my PlaylistDTO.

var data = await (from team in _context.Teams
    where team.Id == teamId //teamId comes from incoming request
    select new
    {
       Id = team.Playlists.Select(pl => pl.Id),
       PlayListName = team.Playlists.Select(pl => pl.PlayListName)
    }).ToListAsync();

data is of type List<'a> , 'a is a new { IEnumerable<Int> Id, IEnumerable<string> PlaylistName }

The statement below works, but is not what I want, since it gets me all the properties from the Playlist entity.

Team team = await _context.Teams
    .AsNoTracking()
    .Include(t => t.Playlists)
    .Where(t => t.Id == teamId)
    .FirstOrDefaultAsync();

My two entities:
Team

public class Team
{
   public int Id {get; set; }
   public string TeamName { get; set; }
   //... more properties
   public ICollection<Playlist> Playlists { get; set; }
}

Playlist

public class Playlist
{
   public int Id { get; set; }
   public string PlaylistName {get; set; }
   // .. other properties
   public int? TeamId { get; set; }
   public virtual Team Team { get; set; }
}

How can I get only the desired properties of the related entity, without being returned an Anonymous Type?

Thanks for the help!

Answer 1

You should not mix the classes from your domain and the responses that your system gives.

I would create a new class to represent this DTO (Data Transfer Object)

public class ResponseDTO
{
   public int Id { get; set; }
   public ICollection<Playlist> Playlists { get; set; }
}

In your query use this new class instead of the anonymous object

var data = await (from team in _context.Teams
    where team.Id == teamId
    select new ResponseDTO
    {
       Id = team.Playlists.Select(pl => pl.Id),
       PlayListName = team.Playlists.Select(pl => pl.PlayListName)
    }).ToListAsync();

Take a look in the concept of Domain Driven Design to understand the importance of leaving the classes that compose your domain only responsible for that.