为什么 JSON 说缺少序列化挂钩？

Question

运行 cpanm --look DBIx::Class ; cd examples/Schema/ 使用示例数据库。

use 5.024;
use strictures;
use JSON::MaybeXS qw(encode_json);
use MyApp::Schema qw();
use Sub::Install qw();

my $s = MyApp::Schema->connect('dbi:SQLite:db/example.db');
# Yes, I know Helper::Row::ToJSON exists.
Sub::Install::install_sub({
    code => sub {
        my ($self) = @_;
        return { map {$_ => $self->$_} keys %{ $self->columns_info } };
    },
    into => $s->source('Track')->result_class,
    as   => 'TO_JSON',
});

my ($t) = $s->resultset('Cd')->first->tracks;
say ref $t->can('TO_JSON'); # 'CODE', ok
say ref $t->TO_JSON;        # 'HASH', ok
say encode_json $t;
# encountered object 'MyApp::Schema::Result::Track=HASH(0x1a53b48)',
# but neither allow_blessed, convert_blessed nor allow_tags settings
# are enabled (or TO_JSON/FREEZE method missing) at …

我希望序列化器找到安装的钩子并使用它，但我却收到上面的错误。怎么了？

Answer 1

为了使 JSON::XS 考虑 TO_JSON，您必须明确启用 convert_blessed 选项：

my $coder = JSON::XS->new;
$coder->convert_blessed(1);
say $coder->encode($t);

根据docs：

$json = $json->convert_blessed ([$enable])
$enabled = $json->get_convert_blessed
See "OBJECT SERIALISATION" for details.

If $enable is true (or missing), then encode, upon encountering a blessed object, will check for the availability of the TO_JSON method on the object's class. If found, it will be called in scalar context and the resulting scalar will be encoded instead of the object.

The TO_JSON method may safely call die if it wants. If TO_JSON returns other blessed objects, those will be handled in the same way. TO_JSON must take care of not causing an endless recursion cycle (== crash) in this case. The name of TO_JSON was chosen because other methods called by the Perl core (== not by the user of the object) are usually in upper case letters and to avoid collisions with any to_json function or method.

If $enable is false (the default), then encode will not consider this type of conversion.

This setting has no effect on decode.

（强调我的）

为什么 JSON 说缺少序列化挂钩？

Why does JSON say the serialisation hook is missing?

json

perl

dbix-class