[Dbix-class] one issue in cookBook example

[Mike.G]

Hi, every boday,
I am newer to DBIx::Class
I am readding the CookeBook
but there is a issue in that example:

SELECT DISTINCT with multiple
columns<http://search.cpan.org/%7Eash/DBIx-Class-0.08010/lib/DBIx/Class/Man=
ual/Cookbook.pod#___top>

  my $rs =3D $schema->resultset('Foo')->search(
    {},
    {
      select =3D> [
        { distinct =3D> [ $source->columns ] }
      ],
      as =3D> [ $source->columns ] # remember 'as' is not the same as SQL A=
S :-)
    }
  );

  my $count =3D $rs->next->get_column('count');

so , here. that variable $source, what's mean about the $source?
and where's we defined it?

who can help me explain that variable?

thanks very much

Mike.G
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