Re: Re-indexing a SQL resultset by id

[Juerd]

in reply to Re-indexing a SQL resultset by id

It seems to me that your database design (or the name of the 'id' column) is broken if you have non-unique ID's.

As for supporting this in the module: I'm not inclined to support something in DBIx::Simple that makes the value type returned dependent on the contents of the result set. That results in rather fragile code. You could just add a method, though:

# untested

package DBIx::Simple::Result::Metaperl;
our @ISA = qw(DBIx::Simple::Result);

sub map_arrays_of_hashes {
    my ($self, $column_name) = @_;
    my %arrays;
    for my $row ($self->hashes) {
        push @{ $arrays{$row->{id}} }, $row;
    }
    return wantarray ? %arrays : \%arrays;
}

...

$db->result_class = 'DBIx::Simple::Result::Metaperl';

my %arrays_of_hashes = $db->query(...)->map_arrays_of_hashes('id');

To keep DBIx::Simple simple, I'm not adding a proliferation of variants; a vast number map_foos_of_bars permutations could be thought of and I'm sure there's they could all be useful in someone's code somewhere, but let's draw a line... :)

[metaperl]

It seems to me that your database design (or the name of the 'id' column) is broken if you have non-unique ID's.

FALSE:

CREATE TABLE students
  id PRIMARY KEY SERIAL
  firstname TEXT
  lastname TEXT
;
CREATE TABLE classes
  id PRIMARY KEY SERIAL
  name text
;
CREATE TABLE student_classes
  studentid  INT
  classid INT
;

and now:

SELECT  * FROM student_classes;

Will you get several instances of each id? Yes? Is the database design un-normalized? No. Conclusion. There are reasonable queries leading to multiple instances of certain fields by which one might want to collate results. In this case, I might want a hash keyed by student id and an array ref of all results with that student id. Call it "collation".

[Juerd]

SELECT * FROM student_classes;
Will you get several instances of each id? Yes?

Yes, but these fields are not called 'id'; here, it's just a suffix. A join, however, could result in multiple rows of the result set with the same id, even in a nice database design, so I do retract my observation about the database perhaps not being well-designed.