=head2 field_names
Read-only method to return a list or array ref of the field names. Returns undef
-or an empty list if the table has no fields set. Usefull if you want to
+or an empty list if the table has no fields set. Useful if you want to
avoid the overload magic of the Field objects returned by the get_fields method.
my @names = $constraint->field_names;