Hello Stuart Hemming,
It seems the two classes do not have identity properties which may cause such behavior. Please select the TrainingSession.ID and TrainingSessionAttendee.ID members and from the Properties pane change their Identity
option to true
. Then you should be able to drag an association from the TrainingSessionAttendee to the TrainingSession class and see only one row in the Define Relationship dialog where the TrainingSessionAttendee.SessionID and TrainingSession.ID properties can be mapped.
All the best,
the Telerik team