如何使用DBIx :: Class进行子选择？

Question

I'm starting with DBIx::Class and i have a subselect that wanted to be in DBIx::Class, but i'm getting confused and can't build the code.

My MySQL select is this one:

Select name from tblCategory where id = (
    Select id from tblCategory where id = (
         Select id from tblRadio where name = "RFM"
    )
);

I read that DBIx::Class don't support subselect; is that true? If so, what do you do in situations like this?

Answer 1

According to the DBIx::Class::Manual::Cookbook there is a new Subquery feature:

my $inside_rs = $schema->resultset('Radio')->search({ name => 'RFM' });

my $rs = $schema->resultset('Category')->search({
    id => { '=' => $inside_rs->get_column('id')->as_query },
});

It is marked EXPERIMENTAL so YMMV.

However also note that SQL::Abstract which DBIx::Class uses when building its queries does have a new subquery feature using -nest.

Answer 2

Couldn't this be represented as a join?

my $rs = $schema->resultset('Category')->search(
    {   
       'Radio.name' => 'RFM' 
    },
    {   
        'join' => 'Radio'
    }   
);

This assumes you have a relationship in Category named 'Radio'. If you don't, there's plenty of documentation to help you setup relationships and learn how to perform joins .

As for subqueries, the cookbook for the most recent version says they are supported , but experimental .

Answer 3

Well, you can always supply a scalar reference to insert literal SQL when you're using DBIC's search() method. For example:

my $rs = $schema->resultset('Category')->search({ 
              id => \"(Select id from tblRadio where name = 'RFM')" 
});

That's what I've had to do in the past when I needed more expressiveness than DBIC supported out-of-the-box. I don't know, though, whether that's the "right" thing to do in this case.

Answer 4

after some fight with DBIC i win in the end :P (YEAH!)

Had to rewrite some stuff, and had to forget the subselects and done the relationships well.

I know that this code don't represent the inicial question, but if i had to rewrite all again, i'm showing you the other part of my "project", where i had again other dificulties.

Next is what i've done:

DBIx::Class Schema

package DB::Esquema::Passwords;

use strict;
use warnings;

use base 'DBIx::Class';

__PACKAGE__->load_components("Core");
__PACKAGE__->table("Passwords");
__PACKAGE__->add_columns(
  "pswd",
  { data_type => "INT", default_value => undef, is_nullable => 0, size => 11 },
  "password",
  {
    data_type => "VARCHAR",
    default_value => undef,
    is_nullable => 1,
    size => 20,
  },
  "utilizadorid",
  { data_type => "INT", default_value => undef, is_nullable => 1, size => 11 },
);
__PACKAGE__->set_primary_key("pswd");
__PACKAGE__->belongs_to('utilizadorid' => 'DB::Esquema::Utilizadores');
#belongs_to is not autogenerated, done by hand

Has a relationship with Utilizadores (users)

package DB::Esquema::Utilizadores;

use strict;
use warnings;

use base 'DBIx::Class';

__PACKAGE__->load_components("Core");
__PACKAGE__->table("Utilizadores");
__PACKAGE__->add_columns(
  "utilizador",
  { data_type => "INT", default_value => undef, is_nullable => 0, size => 11 },
  "nome",
  {
    data_type => "VARCHAR",
    default_value => undef,
    is_nullable => 1,
    size => 20,
  },
  "mail",
  {
    data_type => "VARCHAR",
    default_value => undef,
    is_nullable => 1,
    size => 30,
  },
);
__PACKAGE__->set_primary_key("utilizador");
__PACKAGE__->has_one('utilizador' => 'DB::Esquema::Passwords', 'utilizadorid');

NEXT (the script to make it work)

#!/usr/bin/perl -w

use strict;
use diagnostics; #was important to understand
use lib '/var/www/projectox/lib'; #is where the schema is
use DB::Esquema; #use the Schema

system('clear'); # clear the screen

my $esquema = DB::Esquema->connect("dbi:mysql:dbname=dbswiak","root","");
    $esquema->storage->debug(1);

    #HAD TO USE PREFETCH
    my $resultado = $esquema->resultset('Utilizadores')->search(
    undef,{
         prefetch => { 'utilizador' => 'utilizadorid' }
      }
    )->next();

The Result:

    SELECT me.utilizador, me.nome, me.mail, utilizador.pswd, 
utilizador.password, utilizador.utilizadorid, utilizadorid.utilizador, utilizadorid.nome, utilizadorid.mail 
FROM Utilizadores me JOIN Passwords utilizador 
ON utilizador.utilizadorid = me.utilizador 
JOIN Utilizadores utilizadorid ON utilizadorid.utilizador = utilizador.utilizadorid: 

Wasn't what i really wanted, but is the nearest, the goal is to select only the columns i want... maybe i will reach that goal

Answer 5

Subselects are experimental feature now and useful if you need to be able to allow them at compile time. However I find the following to be a good approach:

1. most subselects can be done as a (faster) join - so use a join where you can
2. failing that, create a view in the database and a schema class for the view.