db={"posts":[{_id:ObjectId("611142303c409b5dc826e111"),comments:[{owner:ObjectId("611142303c409b5dc826e563"),description:"my description"},{owner:ObjectId("611142303c409b5dc826e333"),description:"my description2"}]}],"owners":[{_id:ObjectId("611142303c409b5dc826e563"),name:"foo"},{_id:ObjectId("611142303c409b5dc826e333"),name:"figthers"},]}

db.posts.aggregate([{$unwind:"$comments"},{$lookup:{from:"owners",localField:"comments.owner",foreignField:"_id",as:"owners"}},{$group:{_id:"$_id",root:{$first:"$$ROOT"},comments:{$push:{owner:"$comments.owner",description:"$comments.description",owner_profile:{"$arrayElemAt":["$owners",0]},}}}},{$replaceRoot:{newRoot:{"$mergeObjects":["$root",{comments:"$comments",}]}}},{$project:{owners:0}}])