Is it possible to use a secondary index in embedded persistent class

Question

Hi,
I'm new to Berkely DB Java Edition and have just started playing with it.
To express a relation between two entities Foo and Bar, I am trying with an assocation class FooBarAssociation that is not an entity but rather a @Persistent-annotated embedded class in Foo (Foo has a collection of FooBarAssociations). Now I'm looking for a way to find all Foos associated with a given Bar id by use of an index. Can this be done? Is the use of the SecondaryKey in the embedded class at all correct? If this is not the way to go, can it be done any other way (except from using three entities)?
@Entity
class Bar {
    @PrimaryKey
    private int id;
@Persistent
class FooBarAssocication {
    @SecondaryKey(relatedEntity=Bar.class, relate=Relationship.ONE_TO_ONE)
    private int barId;
@Entity
class Foo {
    @PrimaryKey
    private int id;
    private Set<FooBarAssocication> fooBarAssociations = new HashSet<FooBarAssocication>();
}Any help much appreciated.
/Martin Söderström

Answer 1

Martin,
So could this be done with the embedded persistent class like this, or should I give up and make the FooBarAssociation class an entity of its own?A separate relationship entity class will definitely work, as described in the SecondaryIndex javadoc.
But SecondaryKey fields have to be at the top level of an entity class, so an embedded class that contains the secondary key (and other attributes) won't work.
Logically, you have a one-to-many relationship from Foo-to-Bar, and the relationship has attributes.
If you store the relationship in the Bar entity, then it's pretty easy because there is one Bar instance per relationship and other attributes can easily be stored there. I know you said you want to put it in the Foo class because it logically belongs there, but I'll show this alternative anyway for completeness:
@Entity
class Bar {
    @PrimaryKey
    private int id;
    @SecondaryKey(relatedEntity=Foo.class, relate=Relationship.MANY_TO_ONE)
    private int fooId;
    private Date createTime;
@Entity
class Foo {
    @PrimaryKey
    private int id;
}If it's really important to put the relationship fields into the Foo class, then you can do something like the following. It's messy to attach relationship attributes, but it can be done with either a parallel list or map.
Also, be sure to read the "Key Placement with Many-to-Many for Related Entities" section of the SecondaryIndex javadoc page.
@Entity
class Bar {
    @PrimaryKey
    private int id;
@Entity
class Foo {
    @PrimaryKey
    private int id;
    // Use one of the following, either this:
    @SecondaryKey(relatedEntity=Bar.class, relate=Relationship.ONE_TO_MANY)
    private List<Integer> barIds = new ArrayList<Integer>;
    private List<Date> barCreateTimes = new ArrayList<Date>;
    // Or this:
    @SecondaryKey(relatedEntity=Bar.class, relate=Relationship.ONE_TO_MANY)
    private Set<Integer> barIds = new HashSet<Integer>;
    private Map<Integer, Date> barCreateTimes = new HashMap<Integer, Date>;
From reading the documentation I also get the feeling that I could use the Base API and set up a SecondaryDatabase to create a secondary index for a case like this (correct?).Yes, you could do that, but I would hate to see you move to the base API just for this issue. The DPL is much easier to use (and easier for us to support).
Anyway, so far the BDBJE looks very promising to us. It may replace our current Hibernate solution.Great. We'd love to hear more about what you're doing, why JE works well for you, etc, either on the forum or privately if you prefer (mark.hayes at o.com). It helps us a lot to know what people are doing with JE.
--mark