Strange behaviour in $round about mingo HOT 3 CLOSED

It seems the B value is correct in mingo. see https://runkit.com/embed/l8o7ew3s2m2r

from mingo.

This is the equivalent from MongoDB. https://mongoplayground.net/p/HBcC9QA5sRB

[
  {
    "A": 1.602,
    "B": 1.6016,
    "_id": ObjectId("5a934e000102030405000000")
  }
]

It should round to 2 as the last digit for A and leave B untouched. There is a bug in mingo implementation here. The condition check should be >= 5 to fix the value for A. As for B we need to check what the default places in MongoDB is. Since this library mostly reverse engineer from behaviour, there are a few gaps here and there.

Feel free to submit a PR.

from mingo.

There is more to the fix here. From MongoDB docs

When rounding on a value of 5,
$round
rounds to the nearest even value.

from mingo.