Are you guys running for money? Just omitting more example scenarios for aggregation framework? You documentation is incomplete making users to beg for solutions in other places! If you do some work do it fully or why waste our time? Can you feel the responsibility yourself? Do you feel humans or you feel business? Money or humans?

I don't find example for following collection scenario using lookup and aggregation.

Many companies focussing on source code rather than documentation which is actually needed to add more customers. If I'm frustrated with help then I will go for a tool where documentation is fully available.

Just giving new tools to humans Is not enough. How you help them later matters.

If you really feel your responsibility, then add several examples to cover all possible scenarios in the documentation.

 
a { id: objectId of b }

b { id: objectId of c }

c { id: objectId in an array pointing to d, id: objectId in an array pointing to e, id: objectId in an array pointing to f }

d { }

e { }

f { }