How to avoid Duplicate objects in DB4o database

Question

Student s1 = new Student();
        s1.rollNo = "44";
        s1.name = "kk";
db4o().save(s1);

Student s2 = new Student();
        s2.rollNo = "44";
        s2.name = "kk";
db4o().save(s2);

here I am saving two object s1 and s2 in DB4o Database and both object are saved even though they have duplicate information , what I want is same rollNo student should be saved only once just like relational databases using primary key. I know that DB4o save objects based on reference address, correct me if I am wrong. Please let me know if there is any way to achieve primary key functionality to avoid Data Redundancy in DB4o.

Thanks

Answer 1

db4o tracks object by their identity. So if you create a new object with 'new', it will be considered a new object. If you want to update a object, you need to get that object and change it. So here:

Student toUpdate = db4o.query(new Predicate<Student>() {
    @Override
    public boolean match(Student student) {
        return pilot.rollNo.equals("44");
    }
}).get(0);

toUpdate.name ="newName";

db4o.store(toUpdate); // Updated

That's it. db4o allway's tracks the actual objects. Not the values.

Note that you can add unique constrains to avoid mistakes. For more about updating, see documentation: http://community.versant.com/documentation/reference/db4o-8.1/java/reference/Content/basics/update_concept.htm

Answer 2

Basically you have 2 options:

1. Add a UniqueFieldValueConstraint to your configuration (in this case violations will be reported as exceptions)

2. Execute a query (which is basically what the UniqueFieldValueConstraint will do anyway) and check if such object already exists in the db.

Hope this helps.