Re: dynamic creation of Class::DBI derived classes

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Hi,

my first thought would be to look at CDBI::Loader. Abaut the way to
check if the item is a reference or a value that should be used
directly, I would try something with the infaltion subroutine for the
column, but I may be wrong.

Emile Aben wrote:
| Hi,
|
| I've only used Class::DBI the last few weeks, but it
| already has increased my productivity tremendously.
| Unfortunately the last few days I'm held up by it (and
| my lack of skills?) because of the following:
| Part of my database schema looks like this:
| 3 tables:
| Item            (id, name, description)
| ItemAttribute   (id, name, description, ... )
| ItemData        (id, fk_item, fk_itemattribute, value,
| start_date, end_date)
|
| ItemData contains foreign keys to Item and
| ItemAttribute and I've been able to get this working
| with no problems using Many-to-Many mappings. This way
| I can give Item attributes that are valid for a
| specific period of time.
|
| ItemAttribute has a few more columns that are the
| cause of my problem. There is a 'is_reference' column,
| if this is not NULL then the value in 'name' should be
| interpreted as the name of another table that contains
| more information on what possible values there are for
| this attribute. Say there is a ItemAttribute row:
| id   10,
| name Contact,
| is_reference 1
|
| This would mean the thing stored in ItemData with
| itemattribute 10 is the primary key of the table
| 'Contact' (which might look like: (id, first_name,
| last_name) or something).
|
| What I can do without problems is for every external
| table I know I can create an extra class, but since
| the application that I'm writing will be distributed
| over quite a few machines, I don't want to update the
| app itself everytime an extra attribute that refers to
| an extra table is added.
|
| So I'm looking for a way to dynamically create these
| Class::DBI classes. Unfortunately the 'table' that is
| set with:
| package AuxTables;
| __PACKAGE__->table($table);
|
| is Class-data and not Instance-data (correct me if I'm
| wrong, I've never received formal programming
| training) and so this won't work.
|
| I've also tried several other approaches:
| * overload all CDBI methods like this:
|  sub retrieve {
| 	my ($class,$table,@args) = @_;
| 	__PACKAGE__->table($table);
| 	__PACKAGE__->columns( All => MyDBI->get_columns(
| $table ) );
| 	return $class->SUPER::retrieve(@args);
|  }
| couldn't get this to work, and is also very ugly.
| Tried to find a single 'point of entry' where I had to
| only rewrite 1 function, but couldn't find 1 single
| function that had this property.
| * use a DIY-object (stolen from CDBI wiki):
|         my $sql = "SELECT * FROM " . $tablename ;
| 	my $dbh = $this->db_Main();
| 	my $sth = $dbh->prepare_cached($sql);
| 	my @results = AuxTables->sth_to_objects($sth);
| couldn't get this to work, does only return the first
| value from the specific auxilliary table.
| * looked into Class::DBI::Factory
|  seems to need a configuration, in advance (so not
| based on data in tables itself), to be useful
| * eval approaches
|  won't work because 'package $class;' is invalid
| syntax
|
| Has anybody ever used a similar schema with CDBI
| before and got this type of 'dynamic' subclasses to
| work as beautifully as the rest of CDBI?
| Anybody any tips on what else I could try to get this
| working?
|
| thanks,
| E
|
|
| 	
| 		
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- --
Grüße,

Andreas Fromm

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