I have two collections Post (belongs to posts database) and User (belongs to account database). My requirements to do join on these two collection. But I am unable to reproduce my requirements.
I am expecting joins on two collections.
This is currently not supported. There is a requirement in the backlog of MongoDB (https://jira.mongodb.org/browse/SERVER-34935) though.
As of now the only option you have is to manually query the two different databases and merge the results together as needed. Nevertheless from my point of view it's a little bit odd to have different databases for related collections. Maybe you can also think about redesigning your database design, if possible.