Question

Object-Relational-Mappers have been created to help applications (which think in terms of objects) deal with stored data in a more application-friendly way like every other class/object.

However, I have never seen a OKM (Object-Key/Value-Mapper) for NoSQL “Key/Value” storage systems. Which seems odd because the need should be far greater given the fact that more value-relations will have to be hard-coded into the app than a regular, single SQL table row object.

four requests:
user:id
user:id:name
user:id:email
user:id:created

vs one request:
user = [id => ..., name => ..., email => ...]

Plus you must keep track of “lists” (post has_many comments) since you don’t have has_many through tables or foreign keys.

INSERT INTO user_groups (user_id, group_id) VALUES (23, 54)

vs

usergroups:user_id = {54,108,32,..}
groupsuser:group_id = {23,12,645,..}

And there are lots more examples of the added logic that an application would need to replicate some basic features that normal relational databases use. All of these reasons make the idea of a OKM sound like a shoe-in.

Are there any? Are there any reasons there are not any?

Answer 1

Ruby’s DataMapper project is an ORM and will happily talk to a key-value store through the use of an adapter.

Redis and MongoDB have adapters that already exist. CouchDB has an adapter — it’s not maintained, but at one point it worked pretty well. I don’t think anyone’s done anything with Cassandra yet, but there’s no reason it couldn’t be done. The Dubious framework for Google App Engine takes a very similar approach to Data Mapper to make the Data Store available to applications.

So it’s very possible to do ORM with key-value stores. The ORM just really needs to avoid the assumption that SQL is its primary vocabulary.