Weird hmt + polimorphic association behavior

The problemic model: http://pastie.caboo.se/196775

u.profile.observable_artists_as_string
ActiveRecord::StatementInvalid: Mysql::Error: Unknown column
‘users.user_id’ in’where clause’: SELECT observations.* FROM
observations INNER JOIN users ON obervations.user_id = users.id
WHERE ((users.user_id = 1) AND ((observable_typ=‘Artist’)))

The problem

users.user_id should be observations.user_id

Is it a bug in Rails core?