NHibernate one-to-one mapping does an update on the parent when the child is created..

I have been implementing new functionality to an existing system using NHibernate. This application has an existing schema that we need to augment. In the current project I have an existing database that is used for transaction tracking. The main table for tracking transactions is Transaction. This table has an InsteadOf trigger on the table that prevents updates when the transaction has been cancelled or posted. I am adding functionality to the system to show the calculate the transaction sequence number for each transaction for the given account. This sequence is the incrementing count of all non-cancelled transactions, in date order. If a transaction within the sequence is cancelled, all transaction sequences are recalculated and the TransactionSequence table is updated with the results. The cancelled transaction will retain its sequence number, but the next sequence number will be decremented until there is a continuous sequencing of all non-cancelled transactions.

To implement this, I created a TransactionSequence table that has a primary key of TransactionId which is also a foreign key to the Transaction table. I then created a one-to-one mapping between these two entities in my mapping files.

The Transaction mapping file looks like this:

<class name="Transaction" dynamic-update="true">
 <id name="Id" column="ID">
     <generator class="native" />
 id>
 <property name="TransactionTypeId" access="field.camelcase-underscore" />
 <property name="TransactionStatusId" column="DebitDebitStatus" access="field.camelcase-underscore" />

 <one-to-one name="Sequence" class="TransactionSequence" fetch="join"
              lazy="false" constrained="false"> 
 </one-to-one>
class>

The TransactionSequence mapping looks like:

<class name="TransactionSequence" table="TransactionSequence" dynamic-update="true">
 <id name="TransactionId" column="TransactionID" type="Int32">
     <generator class="foreign">
         <param name="property">Transactionparam>
     generator>
 id>
 <version name="Version" column="Version" unsaved-value="-1" access="field.camelcase-underscore" />
    <property name="SequenceNumber" not-null="true" />
 <one-to-one name="Transaction"
             class="Transaction"
             constrained="true"
             foreign-key="fk_Transaction_Sequence" />

When I have calculate the transaction sequences, I do a save of the Transaction within the NHibernate session, but it fails when the transaction sequence does not yet exist because it tries to update the Transaction table with a query like 'UPDATE Transaction SET Id = ? WHERE Id = ?' with the same value for parameters 1 and 2.

How can I get NHibernate to create the child of the one-to-one mapping without updating the Transaction record? Has anyone seen this before?