Many thanks for your reply, it is definitely an improvement but unfortunately is does not (yet) work.

Here the sql that is generated and the exception that is thrown:

SELECT me.id, me.msg, me.box, me.free_lock, IS_FREE_LOCK( box ) AS fre
+e_lock FROM queue me WHERE ( free_lock ) ORDER BY id: 
DBIx::Class::ResultSet::first(): DBI Exception: DBD::mysql::st execute
+ failed: Unknown column 'me.free_lock' in 'field list' [for Statement
+ "SELECT me.id, me.msg, me.box, me.free_lock, IS_FREE_LOCK( box ) AS 
+free_lock FROM queue me WHERE ( free_lock ) ORDER BY id"]
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As a band-aid I have tried to add a column free_lock to the resultset (which of course would be cheating again) but no no avail.

Unfortunately the Cookbook only has examples for using database functions in the select-clause of the sql, but I need it in the where-clause...