Question

I am a new to db4o. I have a big problem with persistance of a graph of objects. I am trying to migrate from old persistance component to new, using db4o.

Before I peristed all objects its graph looked like below (Take a look at Zrodlo.Metadane.abstrakt string field with focused value) [its view from eclipse debuger] with a code:

  ObjectContainer db=Db4o.openFile(DB_FILE);
    try {
        db.store(encja);
        db.commit();            
    } finally{
        db.close();         
    }

After that, I tried to read it with a code:

ObjectContainer db=Db4o.openFile((DB_FILE));

    try{
        Query q = db.query();
        q.constrain(EncjaDanych.class);
        ObjectSet<Object> objectSet = q.execute();
        logger.debug("objectSet.size" + objectSet.size());
        EncjaDanych encja = (EncjaDanych) objectSet.get(0);
        logger.debug("ENCJA"  + encja.toString());
        return encja;
    }finally{
        db.close();         
    }       

and I got it (picture below) – string field “abstrakt” is null now !!!

I take a look at it using ObjectManager (picture below) and abstrakt field has not-null value there!!! The same value, that on the 1st picture.

Please help me 🙂 It is my second day with db4o. Thanks in advance!

I am attaching some code with structure of persisted class:

public class EncjaDanych{
Map mapaIdRepo = new HashMap();
public Map mapaNazwaRepo = new HashMap(); }

!!!!!!!!UPDATED:
When I tried to read only Metadane object (there was only one such a object), it is all right – it’s string field abstrakt could be read correctly.

        try{
        Query q = db.query();
        q.constrain(Metadane.class);
        ObjectSet<Object> objectSet = q.execute();
        logger.error("objectSet.size" + objectSet.size());
        Metadane meta = (Metadane) objectSet.get(0);

        logger.debu("Metadane"  + meta.toString());
        return meta;
    }finally{
        db.close();         
    }       

Answer 1

This is a common db4o FAQ, an issue with what db4o calls “activation”. db4o won’t instantiate the entire graph you stored when you load an object from an ObjectContainer. By default, objects are instantiated to depth 5. You can change the default configuration to a higher value, but that is not recommended since it will slow down object loading in principle because the depth will be used everywhere you load an object with a query.

Two approaches are possible to solve your issue:

(1) You can activate an object to a desired depth by hand when you need a specific depth.
db.activate(encja, 10) // 10 is arbitrary

(2) You can work with Transparent Activation. There are multiple chapters on how to use Transparent Activation (TA) in the db4o tutorial and in the reference documentation.