Ah. I really need to look closely.

I need to run more tests, but I think the following should do.

# search for both receiving_user and im_network
@search_msgs = $schema->resultset('Message')->search(
  {
    'user.im_username' => 'linux_rohan',
    'user.im_network' => 'Yahoo',
  },
  { join => { conversation => { 'users' => 'user' } } },
);
ok (scalar @search_msgs == 2, 'join message to users');
[download]

Since I'm already joining into the user table, I can just search for im_network.