When filtering on an attribute of type Object Relation the results are not as expected.
where 'recipient' is an attribute of datatype 'Object Relation')
This seems to be due to the fact that the searchService only takes into consideration the database fields sort_key_int or sort_key_string (depending onthe datatype).
But in the converter class for Relation the sortKey is hardcoded to "false" which leads to empty entries in the column of ezcontentobject_attribute table rows for Object Relations.
My quick fix was to replace
which seems to fix the problem. Since i'm no kernel-expert on eZ5 i'm not sure if this is the best solution, though.