It's my first time using NHibernate.

I had followed the tutorial over a nforge and had get myself started with my project. However I encountered an error which I do not know what is it all about.

Basically there are 2 classes for now. My database has a few more but I am working on this 2 first.
Class 1 -- WaterBody
Class 2 -- Exceedance

the relation between this 2 class is a one waterbody is to many exceedance.

this is my mapping file for the both of them ( I have no idea if I am doing it correctly)

Exceedance.hbm.xml

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
assembly="GEMSystem"
namespace="GEMSystem.Models.Entities">
<class name="Exceedance" table="Exceedance">
<id name ="ExceedanceId" column="ExceedanceId" type="int">
<generator class="sequence"/>
</id>
<property name="WaterBodyId">
<column name="WaterBodyId" unique-key="WaterbodyParameter"/>
</property>
<property name ="Parameter">
<column name="Parameter" unique-key="WaterbodyParameter"/>
</property>

<property name ="Min" />
<property name="Max" />

<many-to-one name="WaterBody"
class="WaterBody"
column="WaterBodyId"
cascade="all"/>
</class>

</hibernate-mapping>

----------------------------------------------------------------------------------------------------------
WaterBody.hbm.xml

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
assembly="GEMSystem"
namespace="GEMSystem.Models.Entities">

<class name="WaterBody">
<id name="WaterBodyId" type="string" length="6">
<generator class="assigned" />
</id>
<property name="Name" />
<property name="WaterBodyType" />
</class>
</hibernate-mapping>
----------------------------------------------------------------------------------------------------------
My SQL Create statement for both tables

CREATE TABLE WaterBody
( WaterBodyId char(6) PRIMARY KEY
, Name varchar2(50)
, WaterBodyType char(2)
);


CREATE TABLE Exceedance
( ExceedanceId number PRIMARY KEY
, waterBodyId char(6) REFERENCES WaterBody(waterBodyId)
, parameter varchar2(30)
, min number
, max number
, UNIQUE ( waterBodyId , parameter )
);

----------------------------------------------------------------------------------------------------------
For now, I am able to retrieve any records from both the tables but when I try to add a exceedance record, it give me this error

I would like to have an auto-increment value of my ExceedanceId.

"Invalid index 5 for this OracleParameterCollection with Count=5."

Please help. Thank you

I think i manage to solve this error. It seems like the error is caused by the many-to-one relationship over at the mapping file. i inserted this,

<many-to-one name="WaterBody"
class="WaterBody"
column="WaterBodyId"
insert ="false"
update="false"/>

insert = false and update = false. (A solution I found somewhere).

Can anyone care to explain why do I have to put those 2 lines of codes? Will it affect my future development?