I am bit confusion of list index type,my mapping file has like below

`<list name="transactionItems" cascade="save-update,delete-orphan"
 lazy="false">
 <key column="TRANSACTION_ID" />
 <list-index column="IDX" />
 <one-to-many class="TransactionItem" />
 </list>`

whenever hibernate load a mapped object,its through exception **null index column for collection:transactionItems** please suggest me what can be the problem here. can you exaplain a bit about list-index?