This could be done fairly easily, perhaps more easily, as a standalone program, i think. Something that exposes a MongoDB-compatible API, and translates requests into libpq to go to a PostgreSQL backend.

You could run it either on the PG host, or on the same host as the apps wanting to talk to PG.

My understanding of all of this is beginner level at best, but isn't that more or less what waterline does? https://github.com/balderdashy/waterline-docs

Based on a quick look at the documentation, i think the API Waterline exposes to clients is not an imitation of the MongoDB API, which would allow PostgreSQL to be used as a drop-in replacement for MongoDB. Rather, it's its own thing.

Waterline does support both MongoDB and PostgreSQL, though, so you could use it as part of a migration strategy: start with MongoDB, install Waterline backed by MongoDB, migrate your app to use Waterline rather than using MongoDB directly, replace MongoDB with PostgreSQL.

The guy behind plv8 has already started such a project. I have no idea if it is production ready.

https://github.com/umitanuki/mongres

this is very cool. I wonder what happened to it.