How to update Collection with more Object?

I have 2 Collection A and B:

A : {_id, aField}
B : {_id, bField}

When I insert the B, A will update with add new _bField object field contain of B doc

A: {
   _bField: {
      bId: {_id:...., bField:........}

And then if I insert new more on B, A update with the new too.
Please help me.

what is the problem of doing it manually? :smile:
what is the need of doing so?

I would like to cache doc back, when I insert the new doc on B, and then update A with the new field object (It will update more via B).