The relationship could not be changed because one or more of the foreign-key properties is non-nullable?

Question

According to the accepted answer here: The relationship could not be changed because one or more of the foreign-key properties is non-nullable

I've already deleted all child items before updating the entity using entity.Children.Clear():

public static JsonViewData AddOrUpdate(ModelDBContext context, GENCompanyViewModel companyModel, string userName, string userId)
        {
            try
            {
                var company = new GENCompany();


                if (companyModel.Id > 0) //Update
                {
                    company = context.Companies.Find(companyModel.Id);
                    context.Entry(company).State = EntityState.Modified;

                    company.Vehicles.Clear();
                    company.Phones.Clear();
                    company.Emails.Clear();

                    company.InjectFrom(companyModel);
                    company.DateUpdated = DateTime.Now;
                }
                else //Add
                {
                    company.InjectFrom(companyModel);
                    company.CreatedById = new Guid(userId);
                    context.Companies.Add(company);

                    context.SaveChanges(userName);

                }




                if (companyModel.Vehicles.Any())
                {
                    foreach (var vehicleModel in companyModel.Vehicles)
                    {
                        var vehicle = context.Vehicles.Find(vehicleModel.Id);
                        context.Entry(vehicle).State = EntityState.Unchanged;
                        company.Vehicles.Add(vehicle);
                    }
                }


                if (companyModel.Phones.Any())
                {
                    foreach (var phoneModel in companyModel.Phones)
                    {
                        var phone = new GENCompanyPhone();
                        phone.InjectFrom(phoneModel);
                        company.Phones.Add(phone);
                    }
                }

                if (companyModel.Emails.Any())
                {
                    foreach (var emailModel in companyModel.Emails)
                    {
                        var email = new GENCompanyEmail();
                        email.InjectFrom(emailModel);
                        company.Emails.Add(email);
                    }
                }



                context.SaveChanges(userName);
                return new JsonViewData { IsSuccess = true, Message = "Added Successfully" };
            }
            catch (Exception ex)
            {
                return new JsonViewData { IsSuccess = false, Message = ex.InnerException?.InnerException?.Message ?? ex.InnerException?.Message ?? ex.Message };
            }
        }

and I get this error:

The operation failed: The relationship could not be changed because one or more of the foreign-key properties is non-nullable. When a change is made to a relationship, the related foreign-key property is set to a null value. If the foreign-key does not support null values, a new relationship must be defined, the foreign-key property must be assigned another non-null value, or the unrelated object must be deleted.

Answer 1

The problem occurs because you are trying to remove something from one end of a relationship but not the other and the thing on the other end is non nullable.

The easiest solution to this is to enable cascade deletes, then let EF worry about the details but if you have to handle this then what you need to do is remove all the items using as you are but also on each item set the related item to null and if this cannot be null you'll have to remove the related item too.