db={"a":[{_id:"123323525245",token:"token_1",points:0},{_id:"3454545334",token:"token_2",points:0}],"b":[{_id:"1324454",lines:[{token:"token_1",points:20},{token:"token_1",points:10},{token:"token_2",points:12}]}]}

db.b.aggregate([{"$unwind":"$lines"},{"$group":{"_id":"$lines.token","points":{"$sum":"$lines.points"}}},{"$lookup":{"from":"a","localField":"_id","foreignField":"token","as":"docs"}},{"$project":{"_id":{"$first":"$docs._id"},"points":"$points","token":"$_id"}}])