Database
db={"comments":[{"createdOn":"2020-03-08T04:19:20.276Z","comment":"First comment in the app","likesCount":1,"_id":ObjectId("5e6472c32fe18a59b1068f46"),"userId":"5e60ec371dc3d30e61c6805b","postId":"5e6356546d284c2cdfe1ad92","__v":0},{"createdOn":"2020-03-08T04:19:20.276Z","comment":"First comment in the app11","likesCount":1,"_id":ObjectId("5e6472c32fe18a59b1068111"),"userId":"5e60ec371dc3d30e61c6805b11","postId":"111356546d284c2cdfe1a111","__v":0}],"commentLikes":[{"createdOn":"2020-03-08T06:47:58.855Z","_id":ObjectId("5e64955abb6056610f802159"),"userId":"5e60ec371dc3d30e61c6805b","commentId":"5e6472c32fe18a59b1068f46","__v":0},{"createdOn":"2020-03-08T06:47:58.855Z","_id":ObjectId("5e64955abb6056610f802111"),"userId":"5e60ec371dc3d30e61c6805b","commentId":"5e6472c32fe18a59b1068111","__v":0}]}
Query
db.comments.aggregate([{$match:{postId:"5e6356546d284c2cdfe1ad92"}},{$lookup:{from:"commentLikes",let:{id:{$toString:"$_id"}},pipeline:[{$match:{$expr:{$eq:["$$id","$commentId"]}}}],as:"likes"}}])