Oh, I don't know about the state of the art, I was just speculating. I'd imagine this technique would only useful if you want to support a MongoDB app at the same time as building new features with Postgres, and then gradually phase out the MongoDB interface (e.g. gradually transitioning between a v1 prototype and a v2 rewrite)
If it's not much data (eg ~100k or something), and you don't need any sort of gradual transition, then I'd do something really KISS like dump into a CSV or something and then re-import with whatever the new database management system has for importing files
If it's not much data (eg ~100k or something), and you don't need any sort of gradual transition, then I'd do something really KISS like dump into a CSV or something and then re-import with whatever the new database management system has for importing files