CREATE TABLE users (
    ID INT AUTO_INCREMENT,
    FIRSTNAME VARCHAR(100) NOT NULL,
    LASTNAME VARCHAR(100) NOT NULL,
    AGE INT NOT NULL,
    ID_ROLE INT NOT NULL,
    ID_LEVEL INT NOT NULL,
    PRIMARY KEY (ID)
);
CREATE TABLE roles (
    ID INT AUTO_INCREMENT,
    ROLE VARCHAR(100) NOT NULL,
);
CREATE TABLE levels (
    ID INT AUTO_INCREMENT,
    LEVEL VARCHAR(100) NOT NULL,
);

public class User {
    int id, age;
    private String firstname, lastname;
    private Role ref_role;
    private Level ref_level;

    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    public String getFirstName() {
        return firstname;
    }
    public void setFirstName(String firstname) {
        this.firstname= firstname;
    }
    public String getLastName() {
        return lastname;
    }
    public void setLastName(String lastname) {
        this.lastname= lastname;
    }
    public Ruolo getRef_role() {
        return ref_role;
    }
    public void setRef_role(Role ref_role) {
        this.ref_ruolo = ref_ruolo;
    }
    public Livello getRef_level() {
        return ref_level;
    }
    public void setRef_level(Level ref_level) {
        this.ref_level= ref_level;
    }
    public User() {}
}
public class Role {
    private int id;
    private String role;

    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getRole() {
        return role;
    }
    public void setRole(String role) {
        this.role = role;
    }
    public Role() {
    }
}
public class Level {
    private int id;
    private String level;

    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getLevel() {
        return level;
    }
    public void setLevel(String level) {
        this.level = level;
    }
    public Level() {}
}

<hibernate-configuration>
    <session-factory>
        <property name="connection.driver_class">com.mysql.jdbc.Driver</property>
        <property name="connection.url">jdbc:mysql://localhost:3306/mydatabase</property>
        <property name="connection.username">root</property>
        <property name="connection.password"></property>
        <property name="hibernate.c3p0.min_size">5</property>
        <property name="hibernate.c3p0.max_size">20</property>
        <property name="hibernate.c3p0.timeout">300</property>
        <property name="hibernate.c3p0.max_statements">50</property>
        <property name="hibernate.c3p0.idle_test_period">3000</property>

        <mapping resource="user.hbm.xml"/>
        <mapping resource="role.hbm.xml"/>
        <mapping resource="level.hbm.xml"/>
    </session-factory>
</hibernate-configuration>

<hibernate-mapping>
    <class name="User" table="users">
        <id name="id" column="ID" type="integer">
            <generator class="native"/>
        </id>
        <property name="firstname" column="FIRSTNAME" type="string"/>
        <property name="lastname" column="LASTNAME" type="string"/>
        <property name="age" column="AGE" type="integer"/>

        <many-to-one name="ref_role" class="Role" column="ID_ROLE" not-null="true"/>
        <many-to-one name="ref_level" class="Level" column="ID_LEVEL" not-null="true"/>
    </class>
</hibernate-mapping>

<hibernate-mapping>
    <class name="Role" table="roles">
        <id name="id" column="ID" type="integer">
            <generator class="native"/>
        </id>
        <property name="role" column="ROLE" type="string"/>
    </class>
</hibernate-mapping>

<hibernate-mapping>
    <class name="Level" table="levels">
        <id name="id" column="ID" type="integer">
            <generator class="native"/>
        </id>
        <property name="level" column="LEVEL" type="string"/>
    </class>
</hibernate-mapping>

org.hibernate.PropertyValueException: not-null property references a null or transient value : model.User.ref_level