I draw table A, add column "prim" as primary key not null,
then draw table B and create 1:N relation which creates foreign key "B.prim" also with not null constraint.
Then, I change the relation to be on A side 0..1, on B side 1..n, i.e. B does not need any A record. At this moment, the foreign key "B.prim" field should not be not null anymore, but it is still kept as not null unless I change it manually.
The FK "not null" constraint should be treated automatically in respect to the relation cardinality.
Thanks a lot