db={"employees":[{"_id":1,office:1,jobTitles:"SOFTWARE ENGINEER"},{"_id":2,office:1,jobTitles:"CEO"},{"_id":3,office:1,jobTitles:"CFO"},{"_id":4,office:2,jobTitles:"UX DESIGNER"},{"_id":5,office:2,jobTitles:"BUSINESS ANALYST"},{"_id":6,office:2,jobTitles:"BUSINESS ANALYST"}],"offices":[{_id:1,city:"Berlin"},{_id:2,city:"Prague"}]}

db.offices.aggregate([{"$lookup":{"from":"employees","localField":"_id","foreignField":"office","as":"jobTitles"}},{"$set":{"jobTitles":"$jobTitles.jobTitles"}},{"$unwind":"$jobTitles"},{"$group":{"_id":"$_id","city":{"$first":"$city"},jobTitles:{$addToSet:"$jobTitles"}}}])