[{rId:1,"allocation":[{"slots":[{}],"_id":"606a8509ec68ae1614ac5e73","date":20190701}],"_active":true,"__v":0}]

db.collection.update({rId:1,"allocation.date":20190701},{"$push":{"allocation.$[a].slots":{a:1,b:2}}},{arrayFilters:[{"a.date":20190701}]})