NHibernate + Multiple foreign keys to same column

Question

I'm trying to map my Team and Match table by using following mappings:

// Team.cs

public class Team
{
    public virtual int ID { get; private set; }
    public virtual string TeamName { get; set; }
    public virtual Cup Cup { get; set; }
    public virtual IList<Match> Matches { get; set; }

    public Team()
    {
        Matches = new List<Match>();
    }
}

public class TeamMap : ClassMap<Team>
{
    public TeamMap()
    {
        Id(x => x.ID);
        Map(x => x.TeamName).Not.Nullable();
        References(x => x.Cup, "CupID");

        HasMany(x => x.Matches)
            .Key(x => x.Columns.Add("Team1ID", "Team2ID"))
            .Inverse().Cascade.AllDeleteOrphan();

        Table("Teams");
    }
}

// Match.cs

public class Match
{
    public virtual int ID { get; private set; }
    public virtual Team Team1 { get; set; }
    public virtual Team Team2 { get; set; }
    public virtual int WinnerID { get; set; }
    public virtual Cup Cup { get; set; }
}

public class MatchMap : ClassMap<Match>
{
    public MatchMap()
    {
        Id(x => x.ID);
        Map(x => x.WinnerID);
        References(x => x.Team1, "Team1ID");
        References(x => x.Team2, "Team2ID");
        References(x => x.Cup, "CupID");
        Table("Matches");
    }
}

However, it throws an exception that says:

Foreign key (FKEFFCA4CA45169AED:Matches [Team1ID, Team2ID])) must have same number of columns as the referenced primary key (Teams [ID])

Any suggestions?

UPDATE:

I was able to solve it by mixing something together based on the comment wrote by @Yads.

My code:

// Team.cs

public class Team
{
    public virtual int ID { get; private set; }
    public virtual string TeamName { get; set; }
    public virtual Cup Cup { get; set; }
    public virtual IList<Match> HomeMatches { get; set; }
    public virtual IList<Match> AwayMatches { get; set; }
    public virtual IList<Match> Matches { get { return HomeMatches.Concat(AwayMatches).ToList(); }}

    public Team()
    {
        HomeMatches = new List<Match>();
        AwayMatches = new List<Match>();
    }
}

public class TeamMap : ClassMap<Team>
{
    public TeamMap()
    {
        Id(x => x.ID);
        Map(x => x.TeamName).Not.Nullable();
        References(x => x.Cup, "CupID");

        HasMany(x => x.HomeMatches).KeyColumn("HomeTeamID")
            .Inverse().Cascade.AllDeleteOrphan();

        HasMany(x => x.AwayMatches).KeyColumn("AwayTeamID")
            .Inverse().Cascade.AllDeleteOrphan();
        Table("Teams");
    }
}

// Match.cs

public class Match
{
    public virtual int ID { get; private set; }
    public virtual Team HomeTeam { get; set; }
    public virtual Team AwayTeam { get; set; }
    public virtual int WinnerID { get; set; }
    public virtual Cup Cup { get; set; }
}

public class MatchMap : ClassMap<Match>
{
    public MatchMap()
    {
        Id(x => x.ID);
        Map(x => x.WinnerID);
        References(x => x.HomeTeam, "HomeTeamID");
        References(x => x.AwayTeam, "AwayTeamID");
        References(x => x.Cup, "CupID");
        Table("Matches");
    }
}

However, I'm not aware of what drawbacks this approach have... The .Concat() seems a bit nasty to me...

Answer 1

I don't know why, but I feel like I've answered this exact question before...

The error you're seeing is occurring because you're creating an association based upon a composite inverse key, which won't be able to be mapped to a single foreign key.

One solution would be to try to shoehorn your "ideal" relational model into your object model, or, alternatively you could enrich your model. I suggest having a rich model to express a team playing in a match, you could call it Participation . This would give you a many to many relationship between teams and matches.

The downside to this is you would need to express the cardinality of the relationship as a business rule (so you can't have three teams playing in the same match).

The advantage is you could have a flag on the participation saying who the winner was, so that you don't need to have a WinnerId field on your object model.

You'd end up with code that would be used like this:

var match = new Match();
match.AddTeam(team1);
match.AddTeam(team2);

var winner = match.Participants.FindWinner();
winner = match.Winner; // alias for the above