Updating a document using the default ObjectId

Here are a few facts about keys and the way MongoDB deals with them.

• Documents in MongoDB require a key, _id, which uniquely identifies them.  
• This is a 12-byte binary value, read more from the source of truth, ObjectId.
• When inserting a new document, MongoDB will generate an ObjectId where one is not specified.
• There is a 'reasonable' chance that this will be unique at the time of creation.  
• All of the officially-supported MongoDB drivers use this type by default for _id values.

An annoyance I had with MongoDB, when I first started to use it, was the explicit update required on the default _id key/value pair.  To illusrate this here is a concrete example of what I mean.

I wanted to do this:

	DBCollection collection = db.getCollection("collection");
	collection.save((DBObject)JSON.parse(json));

view raw updatingadocument1.java hosted with ❤ by GitHub

But I had to do this:

	DBCollection collection = db.getCollection("collection");
	DBObject dbo = (DBObject)JSON.parse(json);
	ObjectId oid = new ObjectId((String)dbo.get("_id"));
	dbo.put("_id", oid);
	collection.save(dbo);

view raw updatingadocument2.java hosted with ❤ by GitHub

In this case, json is a JSON document passed from an HTML form via HTTP PUT, eventually ending up here at the DAO.  I was using the default _id implementation and thus would have expected save to do the figure that out and do the necessary work for me.

I just can't live those extraneous lines of code, it's too messy and too much to type every time.  I'll pull it out into a fromJson call, as I can see the pattern emerging and the odds of this reoccurring high.

I must have missed something in the API Docs as this cannot be an unusual requirement.  I would of at least expect to see it in a Util class...

Factory snippet:

	public class DBObjectFactory {
	public final static DBObject fromJson(String json) {
	DBObject dbo = (DBObject)JSON.parse(json);
	ObjectId oid = new ObjectId((String)dbo.get("_id"));
	dbo.put("_id", oid);
	return dbo;
	}
	}

view raw updatingadocument3.java hosted with ❤ by GitHub

Usage snippet:

	DBCollection collection = db.getCollection("collection");
	collection.save(DBObjectFactory.fromJson(json));

view raw updatingadocument4.java hosted with ❤ by GitHub

I get to do what I wanted to now, but it still feels wrong.  It feels a little bit dirty.  It feels like a hack-o-la.  How can I do it better?