From: Eli Bendersky <eliben@...>
Date: 2006-05-07T05:21:11+09:00
Subject: Re: Creating sub-classes using Struct

Logan Capaldo wrote:
> On May 6, 2006, at 2:55 PM, Eli Bendersky wrote:
> 
>>> arbitrary classes and not just Object. Btw, if feels a bit strange to
>> to do. Simplifying:
>>
>> I have a flat-file full of records. The way records are stored can be
>> customized via a separate configuration file. I want to have a class
>> Database which is a container of Record classes. The Record is best
>> implemented as a Struct, IMHO, and I want to create the class
>> dynamically after reading the configuration (which says which fields
>> there are in a Record). I think it would be convenient for a Record to
>> be a subclass of Database, since there is no point having Records
>> without a Database.
>>
> 
> Eh, I'm not sure you get the is-a relationship. Saying that a Record
> is a subclass of Database implies that anywhere you would use a
> Database you could use a Record instead. That doesn't really make
> sense. It would also seem to imply that a Record could contain many
> records. Instead of
> 
> class Database
>    ...
> end
> 
> class Record < Database
>    ...
> end

It seems that I managed to completely confuse everyone using an 
inaccurate description. By sub-class I meant a nested class here, not a 
child in is-a inheritence. I don't know what I was thinking :-)

> 
> 
> I would suggest
> 
> class Database
>    def self.new(file_containing_info)
>        fields = parse_field_format(file_containing_info)
>        Database.const_set('Record", Struct.new(*fields))
>        super
>    end

Excellent ! const_set is what I was missing to get my solution working. 
No more "dynamic constant assignment" errors for me :-)

> 
>    def initialize(file)
>       @records = []
>       read_each_record_in_file(file) do |record_data|
>            record = Database::Record.new
>            extract_record_data_into(record_data, record)
>            @records << record
>       end
>   end
> end

This looks like what I need. Many thanks

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