update discriminator column in Nhibernate

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ASP.NET

NHibernate

I have a discriminator column in the which i need to use for select insert and update. I need to update and select the Employee type. I have following code in the mapping xml file.

<discriminator column="EmployeeType" type="String"/>
<property name="EmployeeType" column="EmployeeType" update="false" insert="false"   type="String" />

my question is that EmployeeType column should be used for update,insert and select...if i use update="false" insert="false" then it is working fine for Employeetype in the select statement but not for the update or insert . but when i use update="true" or insert="true"...it is not working for even select statement...I need to have EmployeeType discriminator column for select/insert/update