NHibernate.PropertyValueException : not-null property references a null or transient - nhibernate

Question

I am getting the following exception.
NHibernate.PropertyValueException : not-null property references a null or transient
Here are my mapping files.
Product
<class name="Product" table="Products">
<id name="Id" type="Int32" column="Id" unsaved-value="0">
<generator class="identity"/>
</id>
<set name="PriceBreaks" table="PriceBreaks" generic="true" cascade="all" inverse="true" >
<key column="ProductId" />
<one-to-many class="EStore.Domain.Model.PriceBreak, EStore.Domain" />
</set>
</class>
Price Breaks
<class name="PriceBreak" table="PriceBreaks">
<id name="Id" type="Int32" column="Id" unsaved-value="0">
<generator class="identity"/>
</id>
<property name="ProductId" column="ProductId" type="Int32" not-null="true" />
<many-to-one name="Product" column="ProductId" not-null="true" cascade="all" class="EStore.Domain.Model.Product, EStore.Domain" />
</class>
I get the exception on the following method
[Test]
public void Can_Add_Price_Break()
{
IPriceBreakRepository repo = new PriceBreakRepository();
var priceBreak = new PriceBreak();
priceBreak.ProductId = 19;
repo.Add(priceBreak);
Assert.Greater(priceBreak.Id, 0);
}
Following up on Jan reply. I've removed the ProductId from priceBreak map. This works!!
public int AddPriceBreak(Product product, PriceBreak priceBreak)
{
using (ISession session = EStore.Domain.Helpers.NHibernateHelper.OpenSession())
using (ITransaction transaction = session.BeginTransaction())
{
product.AddPriceBreak(priceBreak);
session.SaveOrUpdate(product);
transaction.Commit();
}
return priceBreak.Id;
}

Answer 1

Remove the ProductId property from the mapping and from the PriceBreak class. And use the PriceBreaks collection to add PriceBreaks, you don't need the PriceBreakRepository, but only a ProductRepository.
Example:
using (var session = sessionFactory.OpenSession())
{
using (var tx = session.BeginTransaction())
{
var product = session.Get<Product>(19);
product.AddPriceBreak(new PriceBreak());
tx.Commit();
}
}
And in the Product:
class Product
{
// ...
public void AddPriceBreak(PriceBreak pb)
{
pb.Product = this;
PriceBreaks.Add(pb);
}
}

Answer 2

Your usage of Id properties along with the actual references is incorrect.
First, remove this line:
<property name="ProductId" column="ProductId" type="Int32" not-null="true" />
Then, instead of assigning ProductId (you should remove that property completely), use:
priceBreak.Product = session.Load<Product>(19);
(You might need to add the Load method to your repository).