Question

From the MongoDB console, I can:

>  db.log.insert({ dt : new Date })
>  db.log.find().sort({ $natural : -1 }).limit(1)
{ "_id" : ObjectId("50caae2cadd0e471af0b3941"), "dt" : ISODate("2012-12-14T04:42:20.560Z") } 

How can I do the same from the Perl MongoDB driver?

Background

I am using MongoDB with a capped collection for logging. I understand that the ObjectID contains a timestamp, but displaying it and querying it is not simple. I would therefore like to add a server-based timestamp to each entry, but have been unable to figure out how to pass literal commands via the Perl driver.

Answer 1

The Perl MongoDB Driver uses DateTime objects to store Dates, and returns DateTime object back to you if you read dates from the database.

#!/usr/bin/env perl
use strict;
use warnings;
use MongoDB;
use MongoDB::OID;
use DateTime;

# database connection & collection
my $conn  = MongoDB::Connection->new;
my $db    = $conn->dates;
my $log   = $db->log;

# Delete Collection
$log->drop;

# Insert one
my $oid = $log->insert({ dt => DateTime->now });

# Show all
my $all = $log->find;
while ( my $entry = $all->next ) {
    printf "_id: %s dt: %s\n", $entry->{_id}->to_string, $entry->{dt}->format_cldr("dd.MM.yyyy HH:mm:ss");
}

If you execute it, it prints something like

_id: 50cb1b9321d30efd17000000 dt: 14.12.2012 12:29:07

You should also read the DateTime Documentation to understand what you can do with it: http://search.cpan.org/perldoc?DateTime

On the Connection startup you can also set which Date object you want.

http://search.cpan.org/perldoc?MongoDB::Connection#dt_type

If you set it to DateTime::Tiny for example it will be faster. But you should first understand the difference between DateTime and DateTime::Tiny. Read the Documentation of booth and decide in which case the one or the other is better.

---

The example above creates the client time. If you want the time from the server instead of your client, because client/server are not the same. You can do two things.

At first, create a JavaScript function that returns a Date object und evaluate the function on the server.

# JavaScript function that return a Date object
my $now = MongoDB::Code->new(code => qq{function(){
    return new Date
}});

# later...
my $oid = $log->insert({ 
    dt => $dt,
    st => $db->eval($now),
});

The second possibility. The default “_id” object from MongoDB already contains a timestamp when the object will be created. At least the documentation says it comes from the server, and i hope it really comes from the server and will not created by the driver. But if $entry is your MongoDB result you can get a DateTime object from it this way:

my $dt = DateTime->from_epoch(epoch => $entry->{_id}->get_time);

Example:

#!/usr/bin/env perl
use strict;
use warnings;
use MongoDB;
use MongoDB::OID;
use MongoDB::Code;
use DateTime;

# database connection & collection
my $conn  = MongoDB::Connection->new;
my $db    = $conn->dates;
my $log = $db->log;

# Delete Collection
$log->drop;

# Client Time
my $dt = DateTime->now;

# JavaScript function that return a Date object
my $now = MongoDB::Code->new(code => qq{function(){
    return new Date
}});

# wait 2 seconds to see a difference between
# $dt and the $now function
sleep 2;

# Insert one
$log->insert({ 
    dt => $dt,
    st => $db->eval($now),
});

sleep 2;

$log->insert({ 
    dt => $dt,
    st => $db->eval($now),
});

# Show all
my $dtf = "dd.MM.yyyy HH:mm:ss";
my $all = $log->find;
while ( my $entry = $all->next ) {
    printf "dt: %s\n", $entry->{dt}->format_cldr($dtf);
    printf "st: %s\n", $entry->{st}->format_cldr($dtf);
    printf "_id time: %s\n", DateTime->from_epoch(epoch => $entry->{_id}->get_time)->format_cldr($dtf);
    print "\n";
}

Output:

dt: 14.12.2012 16:29:28
st: 14.12.2012 16:29:30
_id time: 14.12.2012 16:29:30

dt: 14.12.2012 16:29:28
st: 14.12.2012 16:29:32
_id time: 14.12.2012 16:29:32