This is most likely a mapping file mismatch.

Check that your inversed-by and mapped-by attributes are set correctly.

Also check that your one-to-one, many-to-one, one-to-many, 
and many-to-many are correct.

Most common seen with empty mapped-/inversed-by, 
two sides both trying to be the same association 
(many-to-one, on both for instance), or by mismatched attributes.

one-to-one bi-directional should have inversed-by and mapped-by
one-to-many should have mapped-by
many-to-one bi-directional should have inversed-by
many-to-many bi-directional should have inversed-by inside the join, mapped-by outside the join