[Dbix-class] table-per-subclass - how it works?
Alex Erzin
eaa-home at yandex.ru
Fri Jun 15 08:30:02 GMT 2012
I'm confused with "package Host; extends 'Base';"
I expect "extends 'Target'", not 'Base'. Is it correct?
15.06.2012, 12:06, "Hailin Hu" <i at h2l.name>:
> I have no idea about "standard" solution, but another idea for reference :)
>
> Package Base;
> use base 'DBIx::Class::Core';
>
> # parents' name should be a defined has_one or belong_to relationship
> has 'parents' => (
> is => 'rw',
> isa => 'ArrayRef[Str]',
> builder => _build_parents,
> );
>
> sub inflate_result {
> my $self = shift;
> my $ret = $self->next::method( @_ );
> foreach my $relationship ( @{ $ret->parents } ) {
> my $parent = $ret->$relationship;
> my @columns = ... # @columns_in_$parent - @column_in_$ret
> foreach my $column ( @columns ) {
> # stolen from DBIx::Class's source
> no strict 'refs';
> no warnings 'redefine';
> my $method = join '::', ref $ret, $column;
> *$method = Sub::Name::subname( $method, sub {
> return $ret->$parent->$column;
> });
> }
> }
> }
> ...
>
> package Target;
> extends 'Base';
> ...
>
> package Host;
> extends 'Base';
> __PACKAGE__->belong_to('target' => 'Target', 'target_id');
> sub _build_parents {[qw/target/]}
> ...
>
> so just declare the attribute 'parents' in subclass is ok.
> the task is not typical for me, but interesting.
>
> On Fri, Jun 15, 2012 at 3:59 PM, Alex Erzin <eaa-home at yandex.ru> wrote:
>
>> Yes, in some cases you are right, but you are saying about "long inheritance chains" and other cases where inheritance is not enought flexible,
>> but i say about just one case in one place where inheritance is preferred for me, and i try to find solution _how_ to implement inheritance, but not about _why_not_ :)
>>
>> 15.06.2012, 10:43, "Ben Tilly" <btilly at gmail.com>:
>>> The benefits of OOP are mostly in information hiding.
>>>
>>> If you're creating a bunch of long inheritance chains, you're probably
>>> doing OOP poorly. Composition is preferred. And maps *much* more
>>> naturally onto a relational database.
>>>
>>> On Thu, Jun 14, 2012 at 11:37 PM, Alex Erzin <eaa-home at yandex.ru> wrote:
>>>> Hm... it really works, but where are benefits of OOP? ;)
>>>> There is no inheritance at all, and to access parent's properties I should call parent directly, so inheritance just replaced with incapsulation, and it's no one what i try to find.
>>>>
>>>> Some years ago we have implemented inheritance in ORM in some commercial project in Perl, later I have swithed to java and have found Hibernate it really cool with ORM.
>>>> But now I have returned to Perl and I think that DBIx::Class can help me with ORM, but right now i'm confused that cannot find "standard" solution for typical task.
>>>>
>>>> 15.06.2012, 10:25, "Hailin Hu" <i at h2l.name>:
>>>>> Is it acceptable like something below
>>>>>
>>>>> package Target;
>>>>> ...
>>>>> package Host;
>>>>> ...
>>>>> __PACKAGE__->belong_to('target' => 'Target', 'target_id');
>>>>> sub column_only_exists_in_target {
>>>>> my $self = shift;
>>>>> return $self->target->column_only_exists_in_target;
>>>>> }
>>>>>
>>>>> well, it is not a good solution, but it could work, I think :)
>>>>>
>>>>> On Fri, Jun 15, 2012 at 3:04 PM, Alex Erzin <eaa-home at yandex.ru> wrote:
>>>>>> Hello,
>>>>>>
>>>>>> I need to implement table-per-class inheritance, as it simple made in Hibernate in Java (more details can be found here http://docs.jboss.org/hibernate/orm/4.1/manual/en-US/html/ch10.html#inheritance-tablepersubclass),
>>>>>> but cannot find right way how to do it in DBIx::Class.
>>>>>>
>>>>>> For example, I have two objects: Target (with properties ID, Description) and Host (ID, Hostname, IP), Host inherited from Target, and on table-level there are two tables Host and Target, joined by ID field.
>>>>>>
>>>>>> CREATE TABLE `mydb`.`target` (
>>>>>> `target_id` INT NOT NULL ,
>>>>>> `description` VARCHAR(45) NULL ,
>>>>>> PRIMARY KEY (`target_id`) )
>>>>>>
>>>>>> CREATE TABLE `mydb`.`host` (
>>>>>> `target_id` INT NOT NULL ,
>>>>>> `hostname` VARCHAR(45) NULL ,
>>>>>> `ip` VARCHAR(45) NULL ,
>>>>>> PRIMARY KEY (`target_id`) ,
>>>>>> CONSTRAINT `fk_host_target`
>>>>>> FOREIGN KEY (`target_id` )
>>>>>> REFERENCES `mydb`.`target` (`target_id` )
>>>>>> )
>>>>>>
>>>>>> How to implemeng there classes Target and Host, so it possible to write
>>>>>>
>>>>>> print $target->id(), $targer->description()
>>>>>> print $host->id(), $host->description(), $host->hostname(), $host->ip()
>>>>>>
>>>>>> ?
>>>>>>
>>>>>> I have no ideas how to implement inheritance, and all my experiments are failed - from Host i cannot get access to parent properties like description.
>>>>>> Could you help me please (with examples)?
>>>>>>
>>>>>> Thanks.
>>>>>>
>>>>>> --
>>>>>> eaa@
>>>>>>
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>>>>> _______________________________________________
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>>>> --
>>>> eaa@
>>>>
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>>> _______________________________________________
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>> --
>> eaa@
>>
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>
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--
eaa@
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