db={"data":[{"_id":"8e7b3fa0-4230-448c-8f70-1d7300632834","data":[{"animal":"7d44251a-b308-4deb-875a-33ef0a69fe2b","place":"Chennai"},{"animal":"fcfdd527-5885-48b0-a91f-03f72f78528f","place":"Kolkata"}]}],"animal":[{"_id":"7d44251a-b308-4deb-875a-33ef0a69fe2b","name":"Dog"},{"_id":"fcfdd527-5885-48b0-a91f-03f72f78528f","name":"Cat"}]}

db.data.aggregate([{"$unwind":"$data"},{"$lookup":{"from":"animal","localField":"data.animal","foreignField":"_id","as":"doc"}},{"$project":{"_id":1,"data.animal":{$first:"$doc.name"},"data.place":1}},{$group:{_id:"$_id",data:{$push:"$data"}}}])