title: JPA 2: multiple column usage in foreign keys
body: <p>I am using Hibernate as persistence provider and modelling my entities with JPA 2.</p>
  
  <p>Now a question came up and i hope you can help me. </p>
  
  <p>In my application you can open up a Game, create groups of players in it and walk around on the map (tiles (2d)). </p>
  
  <p>First my entity definitions:
  Game:</p>
  
  <pre><code>@Entity
  public class Game implements Serializable {
  @Id
  @SequenceGenerator(name = "gen_gameid", sequenceName = "seq_gameid")
  @GeneratedValue(generator="gen_gameid")
  private long gameid;
  
  /**
   * Playing Characters.
   */
  @OneToMany(mappedBy = "game")
  private List&lt;Character&gt; characters;
  private int round = 0;
  
  @OneToMany(mappedBy="game")
  private List&lt;Tile&gt; tiles;
  
  @OneToMany(mappedBy="game")
  private List&lt;Group&gt; group;
  </code></pre>
  
  <p>Tile (a tile will be created from a template and belongs to just one game): </p>
  
  <pre><code>@Entity @IdClass(TileId.class)
  public class Tile implements Serializable{
      private static final long serialVersionUID = 2039974286110729270L;
  
      @Id
      private int x;
  
      @Id
      private int y;
  
      @Id @ManyToOne @JoinColumn(name="gameid")
      private Game game;
  
      @OneToOne(mappedBy="tile")
      private Character character;
  }
  </code></pre>
  
  <p>Character:</p>
  
  <pre><code>@ManyToOne
  @JoinColumn(name="gameid", referencedColumnName = "gameid")
  private Game game;
  
  @ManyToOne
  @JoinColumns({
      @JoinColumn(name="groupgameid", referencedColumnName = "gameid"),
      @JoinColumn(name="groupTag", referencedColumnName = "grouptag")
  })
  private Group group;
  
      @OneToOne
      @JoinColumns({    
        @JoinColumn(name = "x", referencedColumnName = "x"),
        @JoinColumn(name = "y", referencedColumnName = "y"),
        @JoinColumn(name = "tilegameid", referencedColumnName = "gameid")
      })
      private Tile tile;
  </code></pre>
  
  <p>As you can see i had to rename the gameid column to groupgameid and tilegameid. This is not very pretty because i would need the gameid in character just once. 
  To mark the fk columns in character from tile and group with insertable=false, updateable=false will allow the sql generation, but i can't change/set this values. </p>
  
  <p>Sure i could introduce an artificial pk in group and tile, but i would need more joins. </p>
  
  <p>Is JPA just limited that i have to allow the gameid column more than once with other names? Or is my design not optimal? </p>
  
  <p>Looking forward to your feedback and thanks in advance.
  greetings
  Markus</p>
  
  <p>PS (edit):
  At Moment i let the shema generate by hibernate at startup till my model is complete. But here is the generated shema (bit simplified and cutted out some unimportant fields):</p>
  
  <pre><code>CREATE TABLE Character
  (
    charid bigint NOT NULL,
    gameid bigint,
    grouptag character varying(255),
    x integer,
    y integer,
    CONSTRAINT hero_pkey PRIMARY KEY (charid),
    CONSTRAINT fkd4addb09308bc3b822441a FOREIGN KEY (gameid)
    REFERENCES game (gameid) MATCH SIMPLE
    ON UPDATE NO ACTION ON DELETE NO ACTION,
    CONSTRAINT fkd4addb093091cb6522441a FOREIGN KEY (gameid, x, y)
    REFERENCES tile (gameid, x, y) MATCH SIMPLE
    ON UPDATE NO ACTION ON DELETE NO ACTION,
    CONSTRAINT fkd4addb09c018f3ae22441a FOREIGN KEY (gameid, grouptag)
    REFERENCES gamegroup (gameid, grouptag) MATCH SIMPLE
    ON UPDATE NO ACTION ON DELETE NO ACTION
  ) 
  
  CREATE TABLE tile (
    x integer NOT NULL,
    y integer NOT NULL,
    gameid bigint NOT NULL,
    CONSTRAINT tile_pkey PRIMARY KEY (gameid, x, y),
    CONSTRAINT fk27c6ce308bc3b8 FOREIGN KEY (gameid)
    REFERENCES game (gameid) MATCH SIMPLE
    ON UPDATE NO ACTION ON DELETE NO ACTION)
  
  CREATE TABLE gamegroup
  (
    grouptag character varying(255) NOT NULL,
    gameid bigint NOT NULL,
       CONSTRAINT gamegroup_pkey PRIMARY KEY (gameid, grouptag),
    CONSTRAINT fk3c1c51cd308bc3b8 FOREIGN KEY (gameid)
    REFERENCES game (gameid) MATCH SIMPLE
    ON UPDATE NO ACTION ON DELETE NO ACTION
  )
  </code></pre>
  
  <p>PS 2: I already played around with <code>, insertable = false, updatable = false</code>.
  For example when i change the group JoinColumns to:</p>
  
  <pre><code>@ManyToOne
  @JoinColumns({
      @JoinColumn(name="gameid", referencedColumnName = "gameid", insertable = false, updatable = false ),
      @JoinColumn(name="groupTag", referencedColumnName = "grouptag")
  })
  private Group group;
  </code></pre>
  
  <p>I get an Error that mixing is not allowed: 
      Caused by: org.hibernate.AnnotationException: Mixing insertable and non insertable columns in a property is not allowed: net.hq.model.Charactergroup</p>
  
  <p>When i make both insertable=false i am not able to set the group tag anymore. It groupTag stays empty after insertion and gameid is set. :-/</p>
  
  <p>The way i add a Character to a group:</p>
  
  <pre><code>// Create game
  Game game = new Game();
  game.addCharacter(max);
  em.persist(game);
  
  // Group
  Group heroGroup = new Group(game, "HEROES");
  heroGroup.addCharacter(max);
  em.persist(game);
  </code></pre>
  
  <p>Method in Group Class:</p>
  
  <pre><code>public void addCharacter(Character character){
      if(this.characters == null)
          this.characters = new ArrayList&lt;Character&gt;();
  
      this.characters.add(character);
      character.setGroup(this);
  }
  </code></pre>