How to get and write the ID of a document from one collection when adding a document from another collection?

Question

MongoDB

Hello!
I use Go + MongoDB + Martini.
When adding a document to Collection 1 via a form, it is necessary to be able to select a document from Collection 2 (by select or which search form).
Currently save handler:
https://play.golang.org/p/taCYS-NYkDL
Thank you!

Answer 1

Switched to PostgreSQL.)