ObjectId to long and back

[Octavian Covalschi]

Hi there,

I'm trying to implement a simple recommendation engine using Mahout. I'm using MongoDB to store all information and of course all my _ids are of ObjectId type... The problem is that Mahout is looking for longs for userId and itemId in order to make it's magic....

So, I'm looking a simple way to convert ObjectId to long and back. A way In order to keep uniqueness of course. So far I've come up with a solution, but it involves saving inc part of the id. Basically in order to restore an ObjectId from a long, I need to save long value and inc part...


    static long objectIdToLong(ObjectId id) {
        ByteBuffer bb = ByteBuffer.wrap(id.toByteArray())
        return bb.getLong().longValue()
    }

    static ObjectId longToObjectId(long l, int inc) {
        byte[] b  = new byte[12]
        ByteBuffer bb2 = ByteBuffer.wrap(b);
        bb2.putLong(l);
        bb2.putInt(inc);
        return new ObjectId(b)
    }

This is doing what I need, but I'm not sure if it's the right way to do it...

Thanks.

[Scott Hernandez]

What about just using longs for the _id values? You don't need to use
the default ObjectId for the _id field.

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[Octavian Covalschi]

Yes, I know, but I thought it's recommended to use ObjectId and the app is already in production...

Also, since I'm using Grails framework, not sure how well will it work  with long instead of ObjectId, its MongoDB plugin is still in a beta like mode... and don't want to mess to much with it...

Thanks!

[Octavian Covalschi]

OK, looks like i'll have to have additional column for long id or an additional collection with ObjectId <-> long mapping... for specific tasks... basically when I'll generate mahout's input file, I'll generate long numbers and save them to that collection...

The code I have posted before it's not working as I expect, basically the long number that comes out of 

Thanks.

[Octavian Covalschi]

Sorry did not finish my thought...

For some reason this piece of code, will generate the same long number for different ObjectId(yes, they are different) in my case I have 7 collisions... 

 ByteBuffer.wrap(id.toByteArray()).getLong().longValue()

Thanks again.

[Scott Hernandez]

Well yes, an ObjectId is 12 bytes and a long is 8 bytes so you will
have collisions when reducing the amount of bits being used.

On Thu, Aug 11, 2011 at 11:32 AM, Octavian Covalschi

[Gergely Nagy]

On Thu, Aug 11, 2011 at 17:32, Octavian Covalschi
<octavian....@gmail.com> wrote:
> Sorry did not finish my thought...
> For some reason this piece of code, will generate the same long number for
> different ObjectId(yes, they are different) in my case I have 7
> collisions...

That's because an ObjectId is longer than a 64-bit long. While an
ObjectId is 96 bits (or 12 bytes), a 64bit long is well, 64 bits only.
So you have 32 bits that are truncated. It's no wonder you get
collisions.

--
|8]

[Octavian Covalschi]

I see, it makes sense now.  It's been a while since I worked with bits and paid attention to bytes..

Thanks for you time.

[Samita Mahajan]

 static long objectIdToLong(ObjectId id) {

        ByteBuffer bb = ByteBuffer.wrap(id.toByteArray());

        return bb.getLong();

    }

    static ObjectId longToObjectId(long l, int inc) {

   

        byte[] b  = new byte[12];

        ByteBuffer bb2 = ByteBuffer.wrap(b);

        bb2.putLong(l);

        bb2.putInt(inc);

        return new ObjectId(b);

    }

public static void main(String[] args) {

ObjectId tt = new ObjectId("575feb0adaf8e8f1fa424746");

long value = objectIdToLong(tt);

System.out.println(value);

ObjectId id = longToObjectId(value , 32);

System.out.println(id);

}

It  shows wrong result 

My object id : 575feb0adaf8e8f1fa424746

Converting Long value : 6296009235943188721

Result object id : 575feb0adaf8e8f100000020