sub get_by_string {
   my ($this, $field, $property, $value) = @_;
   die "Invalid field $field" if (!exists $this->{$field});
   return grep { $_->{$property} eq $value } @{$this->{$field}};
}

sub get_by_number {
   my ($this, $field, $property, $value) = @_;
   die "Invalid field $field" if (!exists $this->{$field});
   return grep { $_->{$property} == $value } @{$this->{$field}};
}