Here's the case:
1. The insert of the data into the two parent tables never fails.
2. The insert of the child table data fails once in a while.

Here's the DB details:
Two Parent Tables:
* TRANSACTION (pk: ORDER_ID)
* INVOICE (pk: INVOICE_ID)

One Child Table:
* TRANSACTION_LINE (fk:ORDER_ID; fk: INVOICE_ID)

Here's the logic Description:
1. I create a new Transaction Object with a null Transaction Line Collection.
2. I create a new Invoice Object with a populated Transaction Line Collection.
3. I issue a hibernate session.save() on the Transaction first, then the Invoice second.

Problem: The result is that every once in a while, the children aren't saved.

The one-to-many relationships are bags with with inverse=true in the parent.

Any thoughts?
Could it be the null reference to the collection in the Transaction?

Here are some mappings:

Transaction.hbm.xml
<bag name="transactionLine" inverse="true" cascade="save-update">
<key foreign-key="FK_N_TRANSA_RELATION__N_TRANSA">
<column name="ORDER_ID" not-null="true" precision="22" scale="0"/>
</key>
<one-to-many entity-name="org.hibernate.com.model.TransactionLine"/>
</bag>

Invoice.hbm.xml
<bag name="transactionLine" inverse="true" cascade="save-update">
<key foreign-key="FK_N_TRANSA_LINE_INVOICE_ID">
<column name="INVOICE_ID" not-null="false" precision="22" scale="0" />
</key>
<one-to-many entity-name="org.hibernate.com.model.TransactionLine" />
</bag>

TransactionLine.hbm.xml
<many-to-one name="orderId" entity-name="org.hibernate.com.model.Transaction" cascade="none" foreign-key="FK_N_TRANSA_RELATION__N_TRANSA">
<column name="ORDER_ID" not-null="true" precision="22" scale="0"/>
</many-to-one>

<many-to-one name="invoiceId" not-null="true" entity-name="org.hibernate.com.model.Invoice" cascade="none" foreign-key="FK_N_TRANSA_LINE_INVOICE_ID">
<column name="INVOICE_ID" precision="22" scale="0"/>
</many-to-one>