[{_id:1,tags:[1,2,3],observables:[1,2,3]},{_id:2,observables:[2,3,4]}]

db.collection.aggregate([{"$unwind":"$observables"},{"$project":{_id:0,observableId:"$observables",alertId:"$_id"}},{"$merge":{into:{db:"some-db",coll:"some-coll"},on:["observableId","alertId"]}}])