Database
db={"comments":[{"createdOn":"2020-03-08T04:19:20.276Z","comment":"First comment in the app","likesCount":1,"_id":"5e6472c32fe18a59b1068f46","userId":"5e60ec371dc3d30e61c6805b","postId":"5e6356546d284c2cdfe1ad92","__v":0}],"commentLikes":[{"createdOn":"2020-03-08T06:47:58.855Z","_id":"5e64955abb6056610f802159","userId":"5e60ec371dc3d30e61c6805b","commentId":"5e6472c32fe18a59b1068f46","__v":0}]}
Query
db.comments.aggregate([{$match:{postId:"5e6356546d284c2cdfe1ad92"}},{$lookup:{from:"commentLikes",localField:"_id",foreignField:"commentId",as:"likes"}}])