entity framework core add new entity with related entity

Question

I have a Net Core 3.1 MVC project where a Playlist can be created and added to an existing Team. I run in to problems when I try to save the Playlist entity with _context.Add(playlist) and then _context.SaveChangesAsync() with the following error:

Cannot insert duplicate key row in object 'dbo.Teams' with unique index 'IX_Teams_TeamName'. The duplicate key value is (Team Name Four).
The statement has been terminated.

My code:

PlaylistDTO dto = new PlaylistDTO();
dto.Name = "new playlist with related team";
dto.Team = _context.Team.FirstOrDefault(t => t.Id == id) // id comes from viewmodel
Playlist storeToDb = _mapper.Map<Playlist>(dto)
_context.Playlists.Add(storeToDb);
                    
await _context.SaveChangesAsync(); // throws error cannot insert duplicates.

My entities:

public class Playlist: AuditEntity
{
    // id comes from AuditEntity
    public string PlayListName { get; set; }
    public string Description { get; set; }

    public Team Team { get; set; }
}

public class Team: AuditEntity
{
    // id comes from AuditEntity
    public string TeamName {get; set; }
    // other properties, including other related entities
    public ICollection<Playlist> Playlists {get; set;}
}

My DbConfig file

public class TeamDbConfig : IEntityTypeConfiguration<Team>
{
    public void Configure(EntityTypeBuilder<Team> builder)
    {
        builder.ToTable("Teams");

        builder.HasIndex(t => t.TeamName)
            .IsUnique();

        // one2many rel for playlist. One team can have multiple playlists
        builder.HasMany(t => t.Playlists)
            .WithOne(pl => pl.Team)
            .IsRequired(false)
            .OnDelete(DeleteBehavior.Restrict);
            
    }
}

In a similar post it was explained that with .Add() EF will treat any objects as new (and so it cannot add an entity with restricted columns twice). However, I don't know how to get this to work. Loading it untracked vs tracked, setting the Entry().EntityState to Modified or Unchanged doesn't seem to do anything.

This seems to be a pretty standard thing to do, yet I cannot get it done. So, I have a few questions:

Given what I want (a user can add an existing team to a new playlist), do I have the correct relationships defined between Team and Playlist?
What do I need to use as a statement instead (or in addition to) the Add() statement that I now have?

Can you post the real code pls? It is vital if you want to get help. You have posted _context.Playlists.Add(storeToDb); but storeToDb is not even defined. — Serge
– Serge, Commented Oct 16, 2021 at 21:35
The error is not about id by the way, error says unique index (Name column) is violated. And where you configure primary key (Id column?) for your Team entity? — Evk
– Evk, Commented Oct 16, 2021 at 21:36
What happens in _mapper.Map<Playlist>(dto)? It seems that there a new Team object is created. — Gert Arnold
– Gert Arnold, Commented Oct 17, 2021 at 9:24

Serge · Accepted Answer · 2021-10-16 22:10:08Z

Since you are using net 3.1 you have to add TeamId to

public class Playlist: AuditEntity
{
    // id comes from AuditEntity
    public string PlayListName { get; set; }
    public string Description { get; set; }

     public int? TeamId { get; set; }
    [ForeignKey(nameof(TeamId))]
    [InverseProperty("Playlists")]
    public virtual Team Team { get; set; }
}

public class Team: AuditEntity
{
    // id comes from AuditEntity
    public string TeamName {get; set; }
    // other properties, including other related entities

    [InverseProperty(nameof(Playlist.Team))]
    public ICollection<Playlist> Playlists {get; set;}
}

[InverseProperty(nameof(FirstClass.SecondClass))]


modelBuilder.Entity<PlayList>(entity =>
            {
                entity.HasOne(d => d.Team)
                   .WithMany(p => p.Playlists)
                   .HasForeignKey(d => d.TeamId);
            });

and use it to assign to playlist

PlaylistDTO dto = new PlaylistDTO();
dto.Name = "new playlist with related team";
dto.TeamId=id;  // id comes from viewmodel

Playlist storeToDb = _mapper.Map<Playlist>(dto)
_context.Playlists.Add(storeToDb);
await _context.SaveChangesAsync();

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