Strange behavior on one-to-one and many-to-one (unique=true) fetching in hibernate

I'm trying to achieve lazy load on the following.

User.java

public class User {

    private int id;
    private String userName;
    private String password;
    private Employee employee;

    //getter and setters

}

User.hbm.xml

<hibernate-mapping>
    <class name="com.site.dto.User" table="user">
        <id name="id" type="java.lang.Integer">
            <column name="id" />
            <generator class="identity" />
        </id>
        <property name="userName" type="string" update="false">
            <column name="user_name" length="50" not-null="true" unique="true" />
        </property>
        <property name="password" type="string">
            <column name="password" length="50" not-null="true" unique="true" />
        </property>
        <one-to-one name="employee" class="com.site.dto.Employee" fetch="select" cascade="save-update" />
    </class>
</hibernate-mapping>

Employee.java

public class Employee implements Serializable{

    private int id;
    private String name;
    private String email;
    private User user;

    // getter and setters

}

Employee.hbm.xml

<hibernate-mapping>
    <class name="com.site.dto.Employee" table="employee">
        <id name="id" type="java.lang.Integer">
            <column name="id" />
            <generator class="identity" />
        </id>
        <property name="name" type="string">
            <column name="name" length="50" not-null="true" unique="true" />
        </property>
        <property name="email" type="string" update="false">
            <column name="email" length="50" not-null="true" unique="true" />
        </property>

        // enforcing many-to-one to one-to-one by putting unique="true"
        <many-to-one name="user" column="user_id" class="com.site.dto.User" unique="true" not-null="true" fetch="select" cascade="save-update" />
    </class>
</hibernate-mapping>

First I'm getting the User Object based on username. Now I'm trying to load the employee object which seems to be using a select statement with wrong where clause. Here is the hibernate debug

select employee0_.id as id1_1_0_, employee0_.name as name2_1_0_, employee0_.email as email3_1_0_,employee0_.user_id as user_id25_1_0_, from employee employee0_ where employee0_.id=?

Why is the where clause is based on employee_id and not user_id ? I think this is due to the reason on how one-to-one mapping works in hbm.xml configuration where one-to-one will be linked to child table's primary key id but not user_id. I'm forcing the many-to-one to one-to-one in employee by using unique="true". I can fetch the employee in Hibernate annotation's one-to-one by defining @Join-column but I can't figure out how to map the one-to-one in hbm.xml which should refer the child's user_id.