Add some db resilience...

Replace the dbh attribute with a get_dbh method.  The method checks
whether the connection is good before returning it.

lib/MyModel.pm

@@ -1,25 +1,29 @@
 package MyModel;
 use feature qw( signatures );
 use Moose;
+use Mojo::Log;
 no warnings qw( experimental::signatures );
 
+our $logger = Mojo::Log->new;
+
 # Share this among all model instances.
 our $dbh;
 
-has dbh => (
-   is => 'ro',
-   isa => 'DBI::db',
-   required => 1,
-   lazy => 1,
-   builder => '_build_dbh',
-);
-
 sub _build_dbh ($self) {
+   $dbh = DBI->connect(
+      "dbi:Pg:", '', '',
+      { AutoCommit => 0, RaiseError => 1, PrintError => 0 },
+   );
+   return;
+}
+
+sub get_dbh ($self) {
    if (!defined $dbh) {
-      $dbh = DBI->connect(
-         "dbi:Pg:", '', '',
-         { AutoCommit => 0, RaiseError => 1, PrintError => 0 },
-      );
+      $logger->info("Establishing database connection.");
+      $self->_build_dbh;
+   } elsif (!$dbh->ping) {
+      $logger->warn("Database connection lost, reconnecting.");
+      $self->_build_dbh;
    }
    return $dbh;
 }

lib/MyModel/Book.pm

@@ -5,7 +5,7 @@ extends 'MyModel';
 no warnings qw( experimental::signatures );
 
 sub count_all ($self) {
-   my ($book_count) = $self->dbh->selectrow_array(
+   my ($book_count) = $self->get_dbh->selectrow_array(
       'SELECT COUNT(*) FROM book'
    );
    return $book_count;