[Dbix-class] relationship custom join condition using relation in foreign rel

[Peter Mottram]

I'm using a Message class which has various m2m rels + link tables to
store things such as ProductReview and OrderComment.

I'd like to create an accessor in User which returns only the Message
rows that are ProductReviews as opposed to OrderComments/whatever. The
SQL I'm hoping to build is along these lines:

SELECT messages.*
  FROM users me
    JOIN messages messages ON me.users_id = messages.author
    JOIN product_reviews product_reviews ON messages.messages_id =
product_reviews.messages_id
  WHERE me.users_id = ?

I was hoping to use a custom join condition on a has_many relationship
but am getting nowhere. Maybe I should just add a simple sub to User
instead that uses the existing messages rel + some extra join
conditions? Suggestions welcome - simplified classes follow...

TIA
PeteM

## User class

package User;

add_columns( qw/users_id username/ );

has_many( messages => "Message", { 'foreign.author' => 'self.users_id' } );

has_many( reviews => "Message", sub { ** what goes here? ** });

## Message class used for many things including product reviews

package Message;

add_columns( qw/messages_id author/ );

might_have( product_review => 'ProductReview', 'messages_id' );

belongs_to( author => 'User', { 'foreign.users_id' => 'self.author' }, {
join_type => 'left' } );

## ProductReview class isa (kind of) Message

package ProductReview; # link table Product - Message

add_columns( qw/messages_id sku/ );

belongs_to( message => 'Message', "messages_id" );
belongs_to( product => 'Product', "sku" );

## Product class (only shown for clarity)

package Product;

add_columns( qw/sku name/ );

has_many( product_reviews => "ProductReview", "sku" );

many_to_many("reviews", "product_reviews", "message");