Relashionship configuration of multiple navigation properties of same type?

Question

1) I am trying to establish a relationship between two classes. So, I have the following class

 public class Team
{
    [Key]        
    public int Id { get; set; }

    public string Team { get; set; }

    public List<MatchGame> MatchGames { get; set; }
}

and

public class MatchGame
{
    [Key]
    public int Id { get; set; }

    public int HomeTeamId { get; set; }
    public Team HomeTeam { get; set; }
   
    public int AwayTeamId { get; set; }
    public Team AwayTeam { get; set; }

}

The configuration that I tried to perform the relashioship is

   protected override void OnModelCreating(ModelBuilder modelBuilder)
    {

        modelBuilder.Entity<MatchGame>()
            .HasOne(h => h.HomeTeam)
            .WithMany(m => m.MatchGames)
            .HasForeignKey(k => k.HomeTeamId)
            .OnDelete(DeleteBehavior.Cascade);

        modelBuilder.Entity<MatchGame>()
            .HasOne(h => h.AwayTeam)
            .WithMany(m => m.MatchGames)
            .HasForeignKey(k => k.AwayTeamId)
            .OnDelete(DeleteBehavior.NoAction);
    }

the produced error is:

Cannot create a relationship between 'Team.MatchGames' and 'MatchGame.AwayTeam' because a relationship already exists between 'Team.MatchGames' and 'MatchGame.HomeTeam'. Navigation properties can only participate in a single relationship. If you want to override an existing relationship call 'Ignore' on the navigation 'MatchGame.AwayTeam' first in 'OnModelCreating'.

I have also seen the following threads but couldn't find the reason why the relationship couldn't establish.

EF code first: one-to-many twice to same collection type

https://docs.microsoft.com/en-us/ef/ef6/modeling/code-first/fluent/relationships

EFCore - How to have multiple navigation properties to the same type?

2) Also, in order not to create a new post: I wanted to have the public string Team { get; set; } public string Team { get; set; } public string Team { get; set; } in the Team Class to be Unique. I tried some DataAnnotation that I have seen but didn't work. what do I have to use for this purpose.

Answer 1

The problem is you try to use one navigation property MatchGames for defining relationship. Try create two separate navigation properties like below.

public class Team
{
    public int Id { get; set; }
    public string TeamName { get; set; }
    public List<Match> HomeMatches { get; set; }
    public List<Match> AwayMatches { get; set; }
}

public class MatchGame
{
    public int Id { get; set; }
    public int HomeTeamId { get; set; }
    public Team HomeTeam { get; set; }
    public int AwayTeamId { get; set; }
    public Team AwayTeam { get; set; }
}

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<MatchGame>(entity =>
    {
        entity.HasOne(m => m.HomeTeam)
           .WithMany(t => t.HomeMatches)
           .HasForeignKey(m => m.HomeTeamId)
           .IsRequired()
           .OnDelete(DeleteBehavior.Cascade);

        entity.HasOne(m => m.AwayTeam)
           .WithMany(t => t.AwayMatches)
           .HasForeignKey(m => m.AwayTeamId)
           .IsRequired()
           .OnDelete(DeleteBehavior.Cascade);
    });

}

Ad 2. You need to create unique index on TeamName :

modelBuilder.Entity<Team>(e => e.HasIndex(t => t.TeamName).IsUnique());

Also you can consider to add MaxLength attributes in your model and so on.