The DELETE st conflicted with the REF constraint in mvc

Question

hello mvc users,

 

i cant understand that error

 

The DELETE statement conflicted with the REFERENCE constraint "FK_dbo.Appointments_dbo.Patients_PatientId". The conflict occurred in database "ClinicDB", table "dbo.Appointments", column 'PatientId'.

 

The statement has been terminated. And

 

And in patient configuration class

 

there is code below

 

HasMany(p => p.Appointments)

.WithRequired(a => a.Patient)

.WillCascadeOnDelete(true);

 

what else ? anything

how to fix this prob?