The 'Is Mandatory' property does not create a constraint on the database. Database constraints are only created for primary keys and reference attributes.
If true, Platform Server validates, at run-time, if this attribute is filled-in. If false, when the attribute is not filled-in, the default empty values (spaces, 0, … ) are accepted.
as I read it, mandatory FK's should go boom and thus checked in database.
the other stuff will enter null...