[Catalyst] General Web- and OO-question

I wouldn't. Just instantiate your object at the entry controller and store
the instance in the $c->session, or wherever you find appropiate depending
on the object characteristics and the kind of state handling you need.

Do your entry validation as part of the attribute checking validation, as in
Moose:

package MyClass;
use Moose;
use Moose::Util::TypeConstraints;
subtype 'AcceptedAge' => as 'Int' => where { $_ >= 16 } => message { 'You
must be at least 16 to use this.' };
subtype 'NameStr' => as 'Str' => where { length($_) > 1 } => message { "What
kinda name is '$_'?" };
has 'name' => (is=>'rw', isa=>'NameStr', required => 1 );
has 'age' => (is=>'rw', isa=>'AcceptedAge', required => 1 );
has 'sex' => (is=>'rw', isa=>'Str', required => 1 );

In your (overly-simplified) controller:

sub submit : Local {
$c->session->{obj} = new MyClass( $c->req->params ); # dies if
constraint fails, so trap errors
}

Then, just finish validating your object in the "exit" controller.

sub wrap_it_up : Local {
...
if( $c->session->{obj}->validate ) {
## yup
} else {
## nope
}
}

Back in your class, do the more complex validations:

sub validate {
return 1 if( ( $self->sex eq 'F' && $self->age >=16 ) ## girls
mature earlier...
}



TIMTOWTDI

Moose will give you a lot of ammo to help you with this, things such as
coersion and roles. Check with the moosers in their maling list for further
OO questions.

On the other hand, if you need client-side validation, take a peek at
HTML::FormFu, as it may help you with putting some of those rules on the
client side. Catalyst::Model::Adaptor may also come in handy if you feel you
need to turn your class into a model.

-rodrigo

Hi Rodrigo,

thank you for answering.
But that' s my problem: One one web page I do not get all attributes I need
to instantiate the object correctly. Simple unrealistic example. One one page
the user can enter a "name" on the second the "age". The object can be instatiated
if you provide a valid name and age.

After the first page I do have the name, but not the age yet. But I can't instatiate
the object because it throws an exception because age is not valid, isn't it?
Or does Moose check constraints only on provided attributes. This can't be true
with "required" attributes, can it?
I start to hate this perl slogan... ;-)
I want a "good" way.
What do you mean with that?

Best regards
Andreas

Well, just instantiate an empty object then use the accessors Moose creates
for you automagically. Accessors validate as well:

sub start_up : Local {
$c->session->{obj} = new MyClass();
}
sub submit_name : Local {
$c->session->{obj}->name( $c->req->params->{name} );
}
sub submit_age : Local {
$c->session->{obj}->age( $c->req->params->{age} );
}

As long as the attribute is not "required => 1", no exception is thrown.

First, read up on the Model part of MVC, that's where most of your
application storage should end up. Have you decided what are you going to do
with the object once it's filled up?

--rodrigo

2009/3/27 Rodrigo <[email protected]>
I think you are missing his point. He wants to instantiate an object from
the form, but if he uses his Moose object then the things he wants to
validate (errors) will prevent him from getting an instance of his class.
If "name" is required or isa 'Str' then if it gets an arrayref, he can't
use the object (as either it missing or being the wrong type throws an
exception).

I don't know if this is a good idea yet, but it's fairly easy to create a
"proxy" object for a given class, less all the validation:

http://www.pastie.org/429140

YMMV, but it would do what you want, as you could "promote" it to a real
value iteratively after validation...

Why try to make your objects work harder than they have to?

If you have a multi-step process, collect all your data along each
step then initialize your object with the data at the end. If you need
to do validation along the way, you could reuse the moose type
constraints in the classes meta information without constructing an
object.

Oops, that doesn't work, but you get the idea.

Well, if the object gets an arrayref when it was expecting an Str then he
can't use the object and it should throw an exception.

2009/3/27 Rodrigo <[email protected]>
Picking this one to respond to, since Scott said the same thing.

I think the OPs point was to it's really easy to go from request params to a
constructed object. He seems to think this a great Way To Do Things�. His
problem was that you can't do that if the object throws an exception when he
tries to instantiate it. This short-circuits his intent to use the type
constraints in Moose implicitly through the constructor.

The replies all point to the idea of mating the params to their attributes
and checking the constraints before instantiating. These are all great, but
they don't solve his problem of using the object back on the form, think:

<input type="text" value="[% obj.attribute %]"...>

As for the OPs stated question: There's no one true way. There's a bunch of
opinions. :)

I've thought a few times about making an attribute metaclass that lets you ask
for validity on-demand rather than always checking at object creation time; the
problem with that is that it's a pretty big shift from how objects normally
work, and I'm not sure what kind of subtle weird behaviors might arise.

Another option mentioned elsewhere in the thread is a proxy object. Moose
certainly exposes enough metainformation to make this possible, but I think
there'd still be a good amount of fiddly work required; for example, you'd
either have to use AUTOLOAD or generate anonymous classes on the fly, or switch
to an explicit proxy API.

I don't think any of this is insurmountable; the biggest reason I haven't tried
one or all of these is that I'm lazy.

hdp.

No doubt someone could expand upon MooseX::Params::Validate along with
using the metaclass for the Moose Class.

FWIW, http://github.com/jrockway/moosex-partialobject/blob/49df9adf6b5fcecc517dc5be9439a8af75b9d455/lib/MooseX/PartialObject.pm

Patches welcome. I hacked this together at OSCON last year, but haven't
actually used it.

Regards,
Jonathan Rockway

Hi Scott, hi all,

thank you for your answers. They bring (little) light to my problem. ;-)
This is a nice start point. The only thing I'm missing at the moment is
the other questions: Where do I collect the data? In a simple hash?
How can I address the type constraints related to an attribute?
Would you persist the partly collected data to the session?

I'm also intersted in that proxy solution. Is someone using that approach?

Best regards
Andreas

And how are you solving that problem being lazy. My first question was partly motivated
by laziness too as I wanted to use the objects's knowledge about attribute checking
outside the object (forms validation). The second motivations was getting a hint to
do it in a conceptual nice way.

So I still ask for solutions for the problem of instatiating complex objects while gathering
informations on multiple web pages. Problems are:
1) I may not have all required attributes at a time to instatiate an object correctly, therefor
instatiation can't be used to validate single attributes.

2) I can't create a "partly" valid object as storage for the meanwhile gathered attributes =>
I need another way to store the attributes.

I would really appreciate to hear more (real world) solutions to this dilemma.
How are you all out there doing it in a OO-fashion?

Thank you all.

Best regards
Andreas