db={tours:[{"_id":ObjectId("5f3122f4d8d57e3b9650e5b4"),"title":"tour 1","locations":[{"destination":{"id":"5ec5ae9037ea99f20a79071a"},"services":{"hotel":true}},{"destination":{"id":"5ec5ae8e37ea99f20a78ef8c"},"services":{"hotel":true}}]},{"_id":ObjectId("5f2d65e68bc6e9155310d147"),"title":"tour 2","locations":[{"destination":{"id":"5ecf994435c3a6025d5bf126"},"services":{"hotel":true}}]},{"_id":ObjectId("5f2d66398bc6e9155310d161"),"title":"tour 3","locations":[{"destination":{"id":"5ec5ae8e37ea99f20a78ef8d"},"services":{"hotel":true}}]}],destinations:[{"_id":ObjectId("5ec5ae9037ea99f20a79071a"),"name":"dest 1","country":"country name"},{"_id":ObjectId("5ec5ae8e37ea99f20a78ef8c"),"name":"dest 2","country":"country name"},{"_id":ObjectId("5ec5ae8e37ea99f20a78ef8d"),"name":"dest 3","country":"country name"},{"_id":ObjectId("5ecf994435c3a6025d5bf126"),"name":"dest 4","country":"country name"}]}

db.tours.aggregate([{$unwind:"$locations"},{$addFields:{"locations.destination.id":{$toObjectId:"$locations.destination.id"}}},{$lookup:{from:"destinations",as:"locations.destination",localField:"locations.destination.id",foreignField:"_id"}},{$unwind:{path:"$locations.destination"}},{$group:{_id:"$_id",locations:{$push:"$locations"},title:{$first:"$title"}}}])