JPA Mapping a Map of Entities

Question

Coming off my way-too-popular question from yesterday:

I'm trying to map a structure that is essentially the following:

Map<User, List< POJOWithComposite >>

Where User is a basic model, similar to the following:

@Entity
public class User extends Model {
   public String username;
   public String password;
   ...
}

and POJOWithComposite is another entity, that has some mapped entities and some primitives, similar to:

@Entity
public class POJOWithComposite extends Model {
   public int someIntField;
   public OtherModel compositeEntity;
   ...
}

The structure could be analogous to a shopping cart. So the User holds the account info, the OtherModel could be a purchasable item, and POJOWithComposite is a shopping cart that holds items and quantities, say. My analogy breaks down a little when it comes to the main object, but humour me, and let's say for every session we add the shopping cart the user paid for to the list, which will then be archived, say.

Firstly, if the list belongs to each user I could just add it to the User model, although I feel that this isn't very modular (i.e. the information isn't really user account information). ATM, this map sits in a utility-like shopping class, which takes care of matching shopping carts with shoppers, to save me from making the User model too big. (Is there a better pattern to use here perhaps?)

Assuming that I continue down this road, Let's try and map this Map. Ideally, I would like to be able to map it as is. Although I have had very little luck with that.

I happened upon some talk of mapping ArrayList easily because it's Serializable, although didn't manage to implement that here.

So I made a wrapper entity:

@Entity
public class POJOWithCompositeList extends Model {
   @OneToMany
   List<POJOWithComposite> list = new ArrayList<POJOWithComposite>();
}

This works. When I use this entity like this, I can map my new:

@ManyToMany
Map<User, POJOWithCompositeList>

Although I don't really like this solution because I add another mapped entity, i.e. the wrapping entity means another table in the database, which means another join every query etc...

So my two questions are:

From the design perspective, is sticking this Map in a util class acceptable?
Assuming we do map the models like this, how can I improve the mapping?

@JB Nizet, its own entity, which should be a singleton-like entity (i.e. only one instance and that instance holds the map). Admittedly not pretty. — davin, Mar 11 '11 at 19:57

score 2 · Answer 1 · edited Mar 18 '11 at 11:14

There is no point in creating a singleton entity. You should simply map a one-to-many association between User and POJOWithComposite.

Get a User (using session.get()) and call user.getPOJOWithComposites() to get all the POJOWithComposites of a given user. If you want them for several users, then find these users with HQL, and iterate through the found users and navigate into the association. The singleton map is the hibernate session. Use its methods to query the database.

score 1 · Accepted Answer · answered Mar 16 '11 at 16:47

I ended up making a new Entity with @ManyToMany mappings on the composite parts, something like:

@Entity
public class POJOWithComposites extends Model {

   @ManyToMany
   CompositeType compositePtr;

   @ManyToMany
   CompositeOtherType compositeOtherPtr;

   ...
}

That way I indeed left this element of complexity out of my User model, since it wasn't part of the core user entity, and allowed an efficient mapping (foreign keys).

JPA Mapping a Map of Entities

2 Answers2