public class PatientModel
{
    public Guid ID { get; set; }
    public virtual PatientMedicalHistoryModel PatientMedicalHistory { get; set; }
}
public class PatientMedicalHistoryModel
{
    public Guid ID { get; set; }
    public virtual PatientModel Patient { get; set; }
}

modelBuilder.Entity<PatientMedicalHistoryModel>().HasRequired(r => r.Patient).WithOptional(o => o.PatientMedicalHistory);

//this.PatientMedicalHistory.ID = patient.ID;
patient.PatientMedicalHistory = new PatientMedicalHistoryModel()
{
    HasHyperTension = this.PatientMedicalHistory.HasHyperTension,
    HyperTensionMedication = this.PatientMedicalHistory.HyperTensionMedication
};

ServiceFactory.PatientService().updatePatient(patient, oldPatient);