[eluser]WanWizard[/eluser]
That is a limitation of MySQL (actually, of the driver).
You can only do cross-database joins when both databases are accessed via the same connection, and then prefix the table names (of the non-default database) in the query with the database name.
This is a not-supported setup in CI (the CI query builder doesn't prefix the table names), and therefore also not in Datamapper (which uses the CI query builder to construct the queries).
