mongodb: java: How to update a field in MongoDB using expression with existing value

Question

I need to update a field using some expression. Given a DBObject query, how can I multiply or divide given field by an argument value?

How can I write a function performing operation something like this, but in Java:

function(DBObject queryObject, String fieldToUpdate, Double argumentValue) { 
    collection.find(queryObject).forEach(function(e) {
        // update field given by input argument "fieldToUpdate", something like..
        // e.put(fieldToUpdate, e.get(fieldToUpdate)/argumentValue); ??
        collection.save(e);
    });
}

Is it possible using "$set" and find() or findAndModify()? If yes, how can update existing value using an expression like existingValue / argumentValue?

Derick · Accepted Answer · 2013-07-03 08:33:16Z

1

At the moment, MongoDB doesn't allow you to update the value of a field according to an existing value of a field. Which means, you can't do the following SQL:

UPDATE foo SET field1 = field1 / 2;

In MongoDB, you will need to do this in your application, but be aware that this is no longer an atomic operation as you need to read and then write.

answered Jul 3, 2013 at 8:33

Derick

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