@OneToMany和@ManyToOne

Question

I need to resolve this problem:

@Entity
public class Team
{

    @OneToMany // how mapped it ?
    private List<Match> matches; // all played and coming matches
}

@Entity
public class Match
{

    @ManyToOne
    @Column( nullable = false )
    private Team teamA;

    @ManyToOne
    @Column( nullable = false )
    private Team teamB;
}

If I had one field like Team team; it would be easier by using mappedBy = team; .

I could use List teams instead of two fields and add @ManyToMany annotation, but it isn't good solution imho.

Answer 1

Assuming that your Match table has a team_id (foreign key) primary key of the Team table. This code might help you. A tam has many matches and many teams are associated to one match.

In your Team entity :

@OneToMany(mappedBy = "team", cascade = CascadeType.ALL)
List<Match> matches= []

In your Match entity :

@ManyToOne
@JoinColumn(name = "TEAM_ID")
Team team

Answer 2

It is a fact that many matches can be associated to one team, and many teams (exactly 2) can be associated to a match.

Since you want one list with all matches the team is associated to without splitting into two mappings in team (for A or B) I would suggest that you use a @ManyToMany relation mapping between them.

Answer 3

Your problem here is that you need to keep in mind that some matches are "at home" and some matches are "away". Thinking of it like that homeTeam and awayTeam will be easier than teamA and teamB .

Each team will have a OneToMany set of home matches and another OneToMany set for away matches. Then, you can add a convenience method to get all matches if want.

@Entity
public class Team {

    @OneToMany(mappedBy="homeTeam")
    private Set<Match> homeMatches = new HashSet<>(); 

    @OneToMany(mappedBy="awayTeam")
    private Set<Match> awayMatches = new HashSet<>(); 

    // getters and setters


    // special getter if you want it
    public Set<Match> getAllMatches() {
        Set<Match> allMatches = new HashSet<>(); 
        allMatches.addAll(homeMatches);
        allMatches.addAll(awayMatches);
        return allMatches;
    }

}

@Entity
public class Match {

    @ManyToOne
    @JoinColumn(name = "HOME_TEAM_ID", referencedColumnName = "TEAM_ID")
    private Team homeTeam;

    @ManyToOne
    @JoinColumn(name = "AWAY_TEAM_ID", referencedColumnName = "TEAM_ID")
    private Team awayTeam;
}