The relationships between tables when it is one-one both ways, where one-way it is optional and the inverse way it is required, such a relationship could not be modeled. It allows me to select, so I have a relationship cardinality of 1..1 - 0..1, but it will not let me save it.
WORKAROUND:
None
STATUS:
Waiting for fix to be released in a future version of Toad Data Modeler.
You can get it to show a 1..1 - 0..1 cardinality if you uncheck dependency on the left hand side and uncheck mandatory on the right hand side.
