# Pastebin 85rQaqqt
use Red;

model Comment {
    has UInt $!id               is serial;
    has Str  $.body             is column;
    has UInt $!commentable-id   is column is required;
    has Str  $!commentable-type is column is required;
}

role Commentable {
    method comments {
        my $table = self.^table;
        self.^rs.join-model:
            :name<comments>,
            Comment,
            -> $entity, $comment {
                $entity.^id-hash.values.head == $comment.commentable-id && $comment.commentable-type eq $table
            }
    }
}

model Post does Commentable {
    has UInt $.id is serial;
    has Str  $.title is unique;
}

model Photo does Commentable {
    has UInt $.id is serial;
    has Str  $.path is unique;
}

red-defaults "SQLite";

my $*RED-DEBUG = True;
schema(Comment, Post, Photo).create;

my $post = Post.^create: :title<bla>;
$post.comments.create: :body<blabla>;

my $photo = Photo.^create: :path<ble>;
$photo.comments.create: :body<bleble>;