[Heather]

bbq is right
El tag discriminator must be after the id. I send you the correct xml

Code:

<hibernate-mapping>
   <class name=" entity.Contact" table="contact" >
      <id name="id_contact"  column="id_contact" unsaved-value="0">
         <generator class="identity"/>
      </id>
<!-- HERE -->
        <discriminator column="contact"/>
<!--******** -->
        <property name="name" column="name" type="string"/>
        <property name="lastname" column="lastname" type="string"/>
        <property name="address" column="address" type="string"/>
        <property name="city" column="city" type="string"/>
            <subclass name="entity.Employee" discriminator-value="B">
   
    </subclass>
    </class>
</hibernate-mapping>