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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.

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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.

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