UpdateOneCommand implementation.

[maheshrajamani]

Issue #51 and also enable array and subdoc update.

[tatu-at-datastax]

This seems different from what Mongo returns as per https://www.mongodb.com/docs/manual/reference/method/db.collection.updateOne/, which says

The method returns a document that contains:

    'matchedCount' containing the number of matched documents
    'modifiedCount' containing the number of modified documents
    'upsertedId' containing the _id for the upserted document.

    A 'boolean' acknowledged as true if the operation ran with write concern or false if write concern was disabled

we can talk about this tomorrow, change in follow-up PR if needs changing.

[vkarpov15]

I agree. The distinction between matchedCount and modifiedCount is somewhat important as well depending on how you've implemented updatedIds.

matchedCount is the number of documents that matched the filter. 0 or 1 for updateOne. And modifiedCount is the number of documents that changed.

You may have a case where matchedCount = 1 but modifiedCount = 0 if either (1) no properties in the document changed, for example $set: { answer: 42 } when answer is already 42, or (2) update operators didn't apply, for example $setOnInsert: { answer: 42 } if no upsert occurred.

[tatu-at-datastax]

Good point on details wrt modifiedCount. This needs to be considered on server side, so that update operations need to indicate if any changes were made.

[vkarpov15]

I'd also advise against using updatedIds because that might mean we need to have separate return values for updateOne() and updateMany(). updateMany() may update millions of documents - sending all those ids over the network may be extremely slow.

[maheshrajamani]

@vkarpov15 Are you suggesting not to return updateIds for updateOne() also?

[vkarpov15]

Yes. For the sake of consistency between updateOne() and updateMany().

[tatu-at-datastax]

Looks good for everything else expect for return value for operation which I think might be different from what Mongo returns.