[Dbix-class] Prefetch Problem: Related data can't be fetched

[sindharta_tanuwijaya at yahoo.co.jp]

Hi,

My friend solved the problem. It's weird though and I am still a little bit confused. The way I am trying to access/display the rows is like this:

[% FOREACH railway = railways -%]
  [% railway.railway_station.railway_station_fields %]
[% END -%]

And as Nigel pointed out,

> Single object which returns an array from the has_many accessor.
If I use 'join' only, I could get all the data for railways which only have 1 row of railway_station by using "railway.railway_station", but for railways which have more than 1 rows of railway_station, I had to use something like "railway.railway_station.0". But it always failed for "prefetch"

The way I wanted it, is that, since my search code is like this,

my $railways = $schema->resultset('Railway')->search(
{
 'me.railway_id' => { 'IN' => $selected_railway_ids },
 'railway_station.main_flag' => 1, ## forgot to mention this earlier
},
{
  join => 'railway_station',
  prefetch => 'railway_station',
  }
);
$c->stash->{railways} = [$railways->all];


And although railway has 'has_many' rel, with the condition main_flag=1, only 1 or null railway_station should be returned for each railway, and therefore, I am hoping to access it by  [% railway.railway_station%]

The way that my friend solved it, was to declare a select variable

    my @select = (
        qw/            
            railway_field_1
            railway_field_2
            main_flag   #from railway_station
            railway_station_field1 #from railway_station
          /
    );

and put it in the search condition like this:

my $railways = $schema->resultset('Railway')->search(
 {
  'me.railway_id' => { 'IN' => $selected_railway_ids },
  'railway_station.main_flag' => 1, ## forgot to mention this earlier
 },
 {
  select => \@select,
  join => 'railway_station',
 }
 );

and he wasn't using prefetch at all. Now, I could access 'main flag' field by using [% railway.main_flag.%], instead of 
[% railway.railway_station.main_flag %]
Any ideas why this happened?

Sindharta
 

Jess Robinson <castaway at desert-island.me.uk> wrote: 
On Tue, 26 Feb 2008, sindharta_tanuwijaya at yahoo.co.jp wrote:

> Hi,
>
> I am having a problem, in which I couldn't get the relationship column data by using prefetch.
> My relationship is declared like this, suppose we have two tables: railway and railway_station, which relationship is 1 to many.
>
> #in railway schema
> __PACKAGE__->has_many(
>    "railway_station",
>    "Schema::RailwayStation",
>    { "foreign.railway_station_railway_id" => "self.railway_id" },
> );
>
> #in railway_station schema
> __PACKAGE__->belongs_to(
>  "railway",
>  "Schema::Railway",
>  { "foreign.railway_id" => "self.railway_station_railway_id" },
> );
>
> My prefetch command is as follows:
>
>   my $railways = $schema->resultset('Railway')->search(
>        {
>           'me.railway_id' => { 'IN' => $selected_railway_ids },
>        },
>        {
>            join => 'railway_station',
>            prefetch => 'railway_station',
>        }
>    );
>
> I could get the values of the columns of the railways correctly, but not the columns of railway_stations.

You don't tell us *how* you are trying to get the columns of the railway 
stations, so I really cant tell you where you have gone wrong. Your search 
code is correct.

You can check what SQL it actually runs and try it for yourself, by 
setting DBIC_TRACE=1 in your test script environment.

> If I use 'join' only though, I could get all the data for railways which only have 1 row of railway_station, but not for
> railways which have more than 1 rows of railway_station.
> Any ideas why this happened ?

Sounds like you are accessing the data incorrectly.

> I have a hunch that maybe it's the ID of the railway_station that is causing this problem. In our current database, the primary key of
> railway_station works as a dummy field, in which there are a lot of null values (although they are primary keys).
> Of course, I have my own objections to this current implementation, but I am wondering if that is indeed the problem, since the primary key of railway_station isn't supposed to be used in any query generated above ?

Well, it wont fetch the ones with null relation values, obviously.. but 
would you expect it to?

> Or maybe I am missing something else here.

Like giving us your code? ;)

Jess

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