.NET (Core and Framework)

using (var context = new Entities())
            {
                try
                {
                    var record = (from p in context.Patient.Include("PatientAddress")
                                  where p.PatientId == patient.PatientId
                                  select p).FirstOrDefault();
                    
                    if (record == null)
                    {
                        context.AddToPatient(patient);
                    }
                    else
                    {
                        foreach (var item in patient.PatientAddress)
                        {
                            if (item.EntityKey == null)
                            {
                                context.AddToPatientAddress(item); // <--Throws Exception here.
                                continue;
                            }
                            context.ApplyPropertyChanges("PatientAddress", item);
                        }
                        context.ApplyPropertyChanges("Patient", patient);
                    }
                    context.SaveChanges();
                    return true;
                }
                catch
                {
                    return false;
                }
            }