Please post sample documents and expected results. Read Formatting code and log snippets in posts first so that we can cut-n-paste your documents in our systems.
Post what you have tried so far and explain where it fails so that we do not orient you in a dead end you already know does not work.
If you have not tried anything yet start by looking at
for the random selection part and at
because you want 5 objects within an array of a single document so you need to unwind first.
The other possible solution will involve a combination of
and probably