I have noticed that when I wish to create a relationship between two tables, I must make the index on the child Exclusive, and after I have created the relationship, I can then change it to non-exclusive, to get the 1-to-many characteristic.
I am using Data Modeler 1.9.0.2, and working on a FireBird DB.
you don't need to make an exclusive index on child table to create a relationship. Maybe you are creating an identity relationship? For 1-to-many, use non-identity relationships.