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Re: BUILD and how to create a subclass object when creating a parent

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Arjan Widlak - United Knowledge
April 25, 2018 20:42
Re: BUILD and how to create a subclass object when creating a parent
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Hi Karen,

Thank you for your reply. Sorry if it's not clear, let me try again with 
a more concrete example.

I have a class called Asset, with subclasses like so:


Assets are objects to be displayed, Containers are Assets, but can 
contain other Assets and Knowledge Delivery Units are a special kind of 

I have a similar group of classes with subclasses like so:


A Response is some response on an Asset by a user. The 
Response::Container subclass sees if all Assets in a Container have 
received a Response. It's like a summary of it's parent class.

Now the problem. If I create a Response, I might like to create a 
Response::Container instance as well. If the Response I am about to 
create is the last Response needed for all Assets in the Container to 
have received a Response, than I would like to create a 
Response::Container subclass instance.

So creation of an instance of a subclass - Response::Container - is only 
dependent on the creation of the parent class - Response. Therefore it 
seems like a good design choice to couple this to the create method of 
Response. (Where the create method does new, update the database, and 
new_by_id, to re-instantiate from the database.)

How does the Moose framework envision such a case? I expected BUILD to 
be the answer. It causes a deep recursion, so it's definitely not, as is 
well documented. Around also causes a deep recursion by default if one 
does this, that can be avoided though by creating an empty subroutine in 
the subclass. Quite error prone, so not the obvious choice.

Since many of the ideas in Moose - Roles, Method modifiers, etc. - are 
design ideas, I thought there is probably a view on cases like this. 
However I cannot find the obvious way to to couple the creation of a 
subclass to the creation of a parent class in Moose. How would one be 
expected to do this in Moose?

Kind regards,

On 25/04/18 19:54, Karen Etheridge wrote:
> I'm not sure you mean by create a subclass instance on creation of its 
> superclass. Are you intending to have the superclass object have an 
> attribute that holds an instance of the subclass? Perhaps can you show 
> a diagram of what you intend your data to look like after construction 
> is complete?
> On Mon, Apr 23, 2018 at 7:53 AM, Arjan Widlak - United Knowledge 
> < <>> wrote:
>     Hi,
>     I tried to use the BUILD method to create an instance of a
>     subclass on creating an instance of the parent. This does not work
>     and the documentation clearly explains why:
>     <>
>     BUILD methods are called from parent to child and hence this
>     triggers a loop.
>     I am able to do this with the around method modifier, however I
>     need to override the behaviour in the subclass. This is also seems
>     error prone and bad practice.
>     But if this cannot be done this way, how should it be done? How
>     can I always create a subclass instance on creation of it's
>     superclass?
>     Kind regards,
>     Arjan

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