EntityManager.merge inserts duplicate entities

Question

Hi all,

I've got a pretty special setup: I create all the classes in Java, connect them in my application (several ManyToOne-relationships).

Then, I'd like to iterate over my objects and save them into the database. Sometimes, an object is already in the database, then it should not be persisted again.

I implemented the hashCode() and equals()-method correct, but my em.merge() inserts the objects nevertheless.

Again:

I create some objects, i.e. I create some player and set in which team they are. the teams may be different objects in Java, but according to their "equals"-method, they are the same. So if I save a player, the team should be saved accordingly (that works), but if the team exists in the database (according to the equals-method), it should not be inserted again, but the relationship should be set, of course.

What I'm I doing wrong? More information needed?

    private static void saveModels(final Set<?> models) {
    EntityManagerFactory factory = null;

    factory = Persistence.createEntityManagerFactory("sqlite");

    EntityManager manager = factory.createEntityManager();

    manager.getTransaction().begin();

    for (Object object : models) {
        manager.merge(object);
    }

    manager.getTransaction().commit();

    manager.close();
    factory.close();
}

edit

@Entity
public class Team {

    private long id;
    private String description;

    @Id
    @GeneratedValue
    public long getId() {
        return id;
    }

    public void setId(long id) {
        this.id = id;
    }

    public String getDescription() {
        return description;
    }

    public void setDescription(String description) {
        this.description= description;
    }

    /*
     * (non-Javadoc)
     * 
     * @see java.lang.Object#hashCode()
     */
    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + description.length();
        return result;
    }

    /*
     * (non-Javadoc)
     * 
     * @see java.lang.Object#equals(java.lang.Object)
     */
    @Override
    public boolean equals(final Object obj) {
        if (this == obj) {
            return true;
        }
        if (obj == null) {
            return false;
        }
        if (getClass() != obj.getClass()) {
            return false;
        }
        Team other = (Team) obj;
        if (!description.equals(other.getDescription())) {
            return false;
        }
        return true;
    }
}


@Entity
public class Player {
    private long id;
    private Team team;
    private String name;

    @Id
    @GeneratedValue
    public long getId() {
        return id;
    }

    public void setId(long id) {
        this.id = id;
    }

    @ManyToOne(cascade = { CascadeType.PERSIST, CascadeType.MERGE }, targetEntity = Team.class)
    @JoinColumn(name = "team_id")
    public Team getTeam() {
        return team;
    }

    public void setTeam(Team team) {
        this.team = team;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }


    @Override
    public int hashCode() {
        return name.length();
    }

    @Override
    public boolean equals(Object obj) {
        if (!(obj instanceof Player)) {
            return false;
        }

        Player other = (Player) obj;
        return other.getName().equals(name);
    }
}

Answer 1

Try making the relationship bi-directional by given the Team class a Collection of Players as a field. This would be annotated like so

@OneToMany(cascade = { CascadeType.PERSIST, CascadeType.MERGE }, targetEntity = Team.class)
Collection<Player> getPlayers() {
  return this.players;
}

Answer 2

JPA uses the @Id field to do the merges, it won't use the equals and hashCode methods to check if an entity already exists in the database.

Add a @OneToMany mapping on the Team, like Bozho suggests, although I'd do it like this.

@OneToMany(cascade = { CascadeType.ALL })
private List<Player> players = new ArrayList<Player>();

public void addPlayer(Player player) {
    player.setTeam(this);
    players.add(player);
}

public Collection<Player> getPlayers() {
    return new ArrayList<Player>(this.players);
}

As you're doing merging lots of entites at once, I'm assuming you're doing a bulk import from a CSV or something. Then, rather than creating a new Team/Player for every line in the CSV, keep a Map of Teams keyed by the name and just add the players to the relevant Team.

So, instead of

Team t = new Team();
t.setName(teamName)
Player p = new Player();
p.setName(playerName);
p.setTeam(t);

Do

Map<String, Team> teams = new HashMap<String,Team>();

...

if (!teams.containsKey(teamName)) {
    Team t = new Team();
    t.setDescription(teamName);
    teams.put(teamName, t)
}

Player p = new Player();
p.setName(p);
teams.get(teamName).addPlayer(p);

...

saveModels(teams.values());