Document how to set "undef_column_value"

[Laz80UK]

Hi,

I couldn't find any documentation on how to set the "undef_column_value".
It causes problems when trying to convert the output to JSON as my expected field type is an integer which could be NULL if not available, however Log::Log4perl::Layout::PatternLayout converts everything that's undef to a string, i.e. "[undef]".
Many thanks.

[mohawk2]

A quick git grep shows me it's used in Log::Log4perl::Layout::PatternLayout, and its value from Log::Log4perl::Appender::DBI always appears to be undef. I think this needs a bit of docs and testing (and maybe fixing)! Want to help out?

[Laz80UK]

Kinda busy here, but trying my best...
I documented and suggested a workaround here:
mschout/Log-Log4perl-Layout-JSON#6

[mohawk2]

This seems to me like an undef is getting prematurely coerced into a string, which isn't always appropriate as you identify. My feeling is that it should be left as-is, or at least be overridable to a real undef. I'm very open to a backwards-compatible PR implementing this! (with tests and docs, naturally)

[abraxxa]

We'd need this to be able to prefix the message by the NDC which should default to an empty string in case there is no NDC set.

[abraxxa]

My search for a workaround showed that NDC doesn't care about this config option: https://metacpan.org/dist/Log-Log4perl/source/lib/Log/Log4perl/NDC.pm#L19

[abraxxa]

This is what I've come up with:

use Log::Log4perl;
use Log::Log4perl::Layout::PatternLayout;
$Log::Log4perl::ALLOW_CODE_IN_CONFIG_FILE = 1;
Log::Log4perl::Layout::PatternLayout::add_global_cspec('U', sub {
    my $ndc = Log::Log4perl::NDC->get();
    return $ndc eq '[undef]'
        ? ''
        : "$ndc ";
});
$Log::Log4perl::ALLOW_CODE_IN_CONFIG_FILE = 0;

And use %U instead of %x, with no whitespace to the next placeholder, which is %m in my case.
I haven't decided if that's ugly or genius...

[mohawk2]

@abraxxa Doc patches welcome? :-)