There's a library here that implements a lot of database features and can be used on top of any sorted transactional K/V store, with FoundationDB being an exemplar backend:
https://github.com/permazen/permazen
It's pretty sophisticated, but the author uses it in his own projects and then just open sources it without trying to build a community around it so you may have to dig in to see that. It gives object mapping, indexing, composite indexing, triggers, query building and so on.
It's not "hard" to implement this stuff per se but it's a lot of work, especially to build enough test coverage to be convincing. I used to be quite into this idea of FoundationDB layers and especially Permazen, which I think is easily one of the best such layers even though it's not well known. I even wrote a SQL binding using Calcite so you could query your Permazen object stores in FDB using SQL!
I will say though, that in the recent past I started a job at Oracle Labs where I ended up using their database in a project, and that kind of gave me a new perspective on all this stuff. For example: scaling. Like a lot of people who spend too much time on Hacker News I used to think Postgres was state of the art, that RDBMS didn't scale well by design, and if you wanted one that did you'd need to use something exotic like layers on a FoundationDB cluster. But no. FoundationDB scales up to a few hundred nodes at most, and Oracle RAC/ExaData clusters can scale up that far too. There are people storing data from particle accelerators in ExaData clusters. The difference is the latter is a full SQL database with all the features you need to build an app right there already, instead of requiring you to depend on questionably well maintained upper layers that are very feature-light.
One place this hits you immediately is joins. Build out an ExaData cluster and you can model your data naturally whilst joining to your heart's content. The DB has lots of ways that it optimizes complex queries e.g. it pushes down predicates to the disk servers, it can read cached data directly out of other node's RAM over RDMA on a dedicated backbone network, and a whole lot more. Nearly every app requires complex queries, so this is a big deal. If you look at FoundationDB layers, then, well:
https://github.com/permazen/permazen/issues/31
Now in the last few years FoundationDB added for a very, very simple kind of push-down predicate in which a storage server can dereference a key to form another key, but if you look closely (a) it's actually a layering violation in which the core system understands data formats used by the Record layer specifically so it messes up their nice architecture, (b) the upper layers don't really support it anyway and (c) this is very, very far from the convenience or efficiency of a SQL join.
Another big problem I found with modeling real apps was the five second transaction timeout. This is not, as you might expect, a configurable value. It's hard-coded into the servers and clients. This turns into a hugely awkward limitation and routinely wrecks your application logic and forces you to implement very tricky concurrency algorithms inside your app, just to do basic tasks. For example, computing most reports over a large dataset does not work with FoundationDB because you can't get a consistent snapshot for more than five seconds! There are size limits on writes too. When I talked to the Permazen author about how he handled this, he told me he dumps his production database into an offline MySQL in order to do analytics queries. Well. This did cool my ardour for the idea somewhat.
There are nonetheless two big differences or advantages to FoundationDB. One is that Apple has generously made it open source, so it's free. If you're the kind of guy who is willing to self-support a self-hosted KV storage cluster without any backing from the team that makes it, this is a cost advantage. Most people aren't though so this is actually a downside because there's no company that will sell you a support contract, and your database is the core of the business so you don't want to take risks there usually. The second is it supports fully serializable transactions within that five second window, which Oracle doesn't. I used to think this was a killer advantage, and I still do love the simplicity of strict serializability, but the five second window largely kills off most of the benefits because the moment you even run the risk of going beyond it, you have to break up your transactions and lose all atomicity. It also requires care to achieve full idempotency. Regular read committed or snapshot isolation transactions offer a lower consistency level, but they can last as long as you need, don't require looping and in practice that's often easier to work with.