Update Entity Framework DbContext cache (navigatio

Question

I have some code that's adding new child records ("installment dates") of a parent record to my DbContext. I've just noticed that after I add one to the set, the parent's InstallmentDates collection (a navigation property) is not updated to contain the new object. This is what I was expecting. But after adding the next one, the InstallmentDates property updates and contains the first one added.

List<PayPlanInstallmentDate> installmentDates = new List<PayPlanInstallmentDates>();

foreach (var installmentDateSpec in payPlanSpec.InstallmentDates)
{
    var dbInstallmentDate = CRMEntity<PayPlanInstallmentDate>.GetOrCreate(installmentDateSpec.ID);

    if (dbInstallmentDate.ID == Guid.Empty) dbInstallmentDate.ID = Guid.NewGuid();
    dbInstallmentDate.fk_PayPlanID = dbPayPlan.ID;
    dbInstallmentDate.InstallmentNumber = installmentDateSpec.InstallmentNumber;
    dbInstallmentDate.DueDate = installmentDateSpec.DueDate;

    installmentDates.Add(dbInstallmentDate);
}

Where CRMEntity<PayPlanInstallmentDate>.GetOrCreate ultimately calls:

public virtual T AddNew(T newEntity)
{
    return DBContext.Set<T>().Add(newEntity);
}

I can see why this might happen since when I Add the new entity, it isn't yet associated by the foreign key. However, the result is that dbPayPlan.PayPlanInstallmentDates is left with all but the last one added. I would be fine with it containing none of the new records or all of them, but all but one could be problematic. Is there a way to make it refresh again without Adding something?