But if each user has their posts? All the posts shoved into one collection or one array?
Why? The posts in one collection - it is quite OK. find topics find topics in other collections is also approx.
If you want to implement something more or less normal, it is necessary to write a crutch on a crutch.
Do not write a crutch on a crutch, first, design the database to avoid this.
To insert a document in a collection can only be in the end.
What's the problem (I have a secret: it happens almost every DBMS)? What do you care where it is inserted? Do sample sorting.
For example, insert the post into the collection, and I need to get posts by date.RTFM
write a crutch with skip, limit.
What a bad man you a game said? If you have records 10кк, you just RAM is not enough to make such selections, without skip/limit.
If you need to insert at the beginning of the collection, it would be much easier.
Are you trying to tear out tonsils through the anus, and the other person.
Prompt, please, who knows a better alternative.
Once again, you invented a problem and are trying heroically to solve, but the problem is that you are not properly using the tools we offer mongo, other database you no it does not help