實體框架多對多關系插入

Question

我有一個帶有實體框架6的Web API應用程序。我有2個實體-Player和Match，它們之間具有多對多關系：

 public class Player {
    public string PlayerId { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public virtual ICollection<Match> Matches { get; set; }

}
 public class Match {
    public string MatchId { get; set; }
    public DateTime DateTime { get; set; }
    public virtual ICollection<Player> Players { get; set; }
}

這是我的DBContext的樣子：

public class FIFA15RankingContext : DbContext {
    public DbSet<Player> Players { get; set; }
    public DbSet<Match> Matches { get; set; }

    protected override void OnModelCreating(DbModelBuilder modelBuilder) {
        modelBuilder.Entity<Player>()
            .HasMany(t => t.Matches)
            .WithMany(t => t.Players)
            .Map(m => {
                m.ToTable("PlayerMatches");
                m.MapLeftKey("PlayerId");
                m.MapRightKey("MatchId");
            }
            );
        base.OnModelCreating(modelBuilder);
    }
}   

EF在幕后為我創建了表PlayerMatches（PlayerId，MatchId）。

這是我的MatchesController（由VS 2013搭建）：

    // POST: api/Matches
    [ResponseType(typeof(Match))]
    public async Task<IHttpActionResult> PostMatch(Match match)
    {
        if (!ModelState.IsValid)
           return BadRequest(ModelState);

        db.Matches.Add(match);

        try
        {
            await db.SaveChangesAsync();
        }
        catch (DbUpdateException)
        {
            if (MatchExists(match.MatchId))
               return Conflict();

            else
               throw;               
        }

        return CreatedAtRoute("DefaultApi", new { id = match.MatchId }, match);
    }

我希望能夠創建新匹配並將其與現有用戶相關聯。 我無法使其以任何其他方式工作，只能在db.Matches.Add（match）之前添加以下代碼（聞起來很糟糕）：

        var playersFromDB = new List<Player>();
        if (match.Players != null) {
            foreach (var p in match.Players) {
                playersFromDB.Add(db.Players.Find(p.PlayerId));
            }
        }
        match.Players = playersFromDB;

我究竟做錯了什么？

Answer 1

如果數據庫中已經存在所有播放器，則可以將它們附加到dbcontext，而不是從db加載。

    if (match.Players != null) {
        foreach (var player in match.Players) {
            db.Players.Attach(player);
        }
    }

Answer 2

手動附加User實體並將其狀態設置為未Unchanged

foreach( var p in match.Players )
{
    db.Entry( p ).State = EntityState.Unchanged;
}