I am using java/morphia to deal with mongodb. The default ObjectId is not very convenient to use from Java layer. I would like to make it a String type while keep the key generation process using ObjectId, say _id = new ObjectId.toString().
I want to know if there is any side effects doing it this way? For example, will it impact the database performance or causing key conflicts in any means? Will it affect the sharding environment ...
You can use any type of value for an _id field (except for Arrays). If you choose not to use ObjectId, you'll have to somehow guarantee uniqueness of values (casting ObjectId to string will do). If you try to insert duplicate key, error will occur and you'll have to deal with it.
I'm not sure what effect will it have on sharded cluster when you attempt to insert two documents with the same _id to different shards. I suspect that it will let you insert, but this will bite you later. (I'll have to test this).
That said, you should have no troubles with _id = (new ObjectId).toString().