log_context.pl

use Mojolicious::Lite -signatures;

use Mojo::JSON 'to_json';
use Scalar::Util 'weaken';

{
  package Mojo::Log::WithContext;
  use Mojo::Base 'Mojo::Log';
  has context => sub { {} };
}

helper 'log' => sub ($c) {
  return $c->stash->{'log.logger'} if $c->stash->{'log.logger'};

  my $log = Mojo::Log::WithContext->new;
  $log->context->{request_id} = $c->tx->req->request_id;

  $log->unsubscribe('message');

  $log->format(sub ($log, $level, @lines) {
    return join '|', @lines, to_json($log->context); 
  });
  
  weaken $c;
  $log->on(message => sub ($log, $level, @lines) {
    return unless $c;
    # pass up to app's logger after formatting
    $c->app->log->$level($log->format->($log, $level, @lines));
  });

  return $c->stash->{'log.logger'} = $log;
};

get '/' => sub ($c) {
  $c->log->context->{user} = 'Joel'; # add some context
  $c->log->info('Requested /'); # log some message
  $c->render(text => 'done');
};

app->start;