App Engine Many To Many Relationships JAVA

Question

I have been banging my head against the wall trying to correctly configure a many to many relationship which should be straight forward.

I have A shopping list class, and an items class. I simply want shopping lists to be able to hold a list of items.

Please Please don't reply with the Google App engine documentation link, I'm looking for a working example that someone has used within their own project, not the theory Google loosely throws around.

Answer 1

It's easy.

Create a class that holds the many-to-many relationship. In your case:

class ShoppingListItems(){

  @Id
  private String id;
  private Long shoppingListId;
  private Long itemId;

  public ShoppingListItems(Long shoppingListId, Long itemId) {
    this.id = shoppingListId.toString() + "-" + itemId.toString();
    this.shoppingListId = shoppingListId;
    this.itemId = itemId;
  }

  public String getId() {
    return id;
  }

  public static void addItemToList(Long shoppingListId, Long itemId, DAO dao) {
        // add shopping list and itemsr IDs to the ShoppingListItems 'join' table
        ShoppingListItems record = new ShoppingListItems(shoppingListId, itemId);
        dao.ofy().put(record);   
  }

Here @Id and dao.ofy().put(record) comes from Objectify lingo but you can use JPA or JDO as you prefer.