Populate Document in Aggregate Function

I want to populate document in aggregate function. For Example:- 2 collections Candidates, Score score having the reference or foriegn key as candidateId that js mongoDB objectID . In case when i want to find out the avg score of the candidare, i want to display the avg score and and name of the candiadte using aggregate function.

In this i got the CandidateId but how can i Populate this in aggregate function?