Nanoseconds lost coming from MongoDB ISODate Object

问题

I'm losing the nanoseconds from the MongoDb interface for the ISODate object. All the nanoseconds are set to zero when I read them in perl.

First, my environment:

MongoDB version: 1.8.2 
perl v5.12.4 
MongoDB perl module version: 0.701.4 

I have a Mongo DB that has rtcTime coded as an ISODate, as follows:

"rtcTime" : ISODate("2013-05-13T18:54:55.918Z")

The code to extract the rtcTime looks something like this:

my @results = $db->get_collection( 'timings' )->find( )->all(); 
foreach my $record ( @results ) 
{ 
    print $record->{rtcTime}->nanoseconds()."\n"; 
}

Output is all 0's.

To fully reproduce the problem, create an ISODate object with arbitrary (non-zero) hires_epoch values in the MongoDB database. Then try to use the MongoDB / DateTime / DateTime::Format::ISO8061 modules to extract any kind of hires time data.

Q: Why can't I get my milliseconds, microseconds, or nanoseconds from the MongoDB ISODate data?

回答1:

This is a bug in the way that the MongoDB Perl driver interacts with DateTime. When ISODate values are retrieved from the database, the driver initializes the DateTime object using its from_epoch constructor. But the nanosecond portion is not passed to the constructor.

DateTime does support passing the full value including nanoseconds, and the driver should be updated to fix that.

I've created a ticket for this bug and I will fix it. But maybe not until after the holiday weekend. :)

回答2:

MongoDB stores documents in BSON format and its specification says:

BSON Date is a 64-bit integer that represents the number of milliseconds since the Unix epoch (Jan 1, 1970).

So, date precision is limited to miliseconds. If you really need to store nanoseconds, you shouldn't use date type. You have to use a long and store the timestamp, so that you'll no lose precision.

来源：https://stackoverflow.com/questions/17476176/nanoseconds-lost-coming-from-mongodb-isodate-object