Reference keys are transformed into associations.
Generated associations are all bidirectional. You have to customize them according to your own needs.
Unlike associations, reference keys have no multiplicity on their ends. They just have cardinality. The default value is 0..*. You have to customize it in order to generate different kinds of association.
Generating many-to-many association is just the reverse process of transforming many-to-many association. You just need to:
If your database reference keys cannot be transformed into many-to-many associations, they will be transformed into one-to-many associations. If you want to generate one-to-one association, you need to set the maximum cardinality to 1 (cardinality 0..1 or 1..1).
If the reference key is mandatory, the minimal multiplicity of one side of the generated association will be 1.