Add support for sharded collections in mongodb

Such workaround should work, in case if you have already implemented @EmbeddedId or @IdClass in OGM

Yes, these are supported already.

May I also state my opinion about support of sharded collections?

Absolutely, any feedback is more than welcome!

Without sharded collections MongoDB and others such as Cassandra - almost useless.

I wouldn't exactly say that these stores are useless without sharded collections, e.g. MongoDB adds much value by its support for nested/structured data, no need for having a physical schema in the datastore etc. But of course I understand that sharding is an important feature for many, and we will surely add proper support for it. Really it's just a matter of resources and prioritizing. But the good thing is, it's all open source, so you can push for a certain feature just by sending a pull request

Hi Vadim, thanks for opening this issue!

Atm. Hibernate OGM does not yet explicitly support sharded collections in MongoDB, we also don't have any specific tests for that. But it's definitely something we should add. An annotation (or more generically, an "option" which may be given via an annotation or via the fluent configuration API) for specifying the shard key of an entity seems very reasonable. It would also allow to enable sharding of the collection based on that key during schema initialization. Would you be interested in helping out with implementing such feature?

Btw. as a workaround (whether it works or not may depend on your id generation strategy) you may try to use a compound id for your entity which comprises the actual id as well as your shard key. This should add the shard key to the "where clause" of updates.