Here is the stackTrace :
net.sf.hibernate.PropertyValueException: not-null property references a null or transient value: persistent.Child.parent
   at net.sf.hibernate.impl.SessionImpl.checkNullability(SessionImpl.java:1277)
   at net.sf.hibernate.impl.SessionImpl.doSave(SessionImpl.java:928)
   at net.sf.hibernate.impl.SessionImpl.doSave(SessionImpl.java:857)

            Transaction tx= session.beginTransaction();
            Parent parent = new Parent();
            parent.setName("parent1");
            parent.setChild(new TreeSet());
            Child child = new Child();
            child.setName("child1");
            parent.addChild(child);
            session.save(parent);
            tx.commit();

<hibernate-mapping>
    <class name="com.flashannonces.website.persistent.Parent" table="PARENTS">
        <id name="id" type="java.lang.String" unsaved-value="null">
                <column name="PARENT_ID" not-null="true" sql-type="char(32)"/>
            <generator class="uuid.string"/>
        </id>
        <set name="child" table="CHILDS" lazy="true" inverse="true" cascade="all">
              <key><column name="PARENT_ID" not-null="true"/></key>
              <one-to-many class="com.flashannonces.website.persistent.Child"/>
        </set>
        <property name="name" type="string" update="true" insert="true" access="property" column="NAME" length="64" not-null="true"/>
    </class>
</hibernate-mapping>

<hibernate-mapping>
    <class name="persistent.Child" table="CHILDS">
        <id name="id" type="java.lang.String" unsaved-value="null">
            <column name="CHILD_ID" not-null="true" sql-type="char(32)"/>
            <generator class="uuid.string"/>
        </id>
        <property name="name" type="string" update="true" insert="true" access="property" column="NAME" length="64" not-null="true"/>
        <many-to-one name="parent" class="persistent.Parent" column="PARENT_ID" not-null="true"/>
    </class>
</hibernate-mapping>

public class Parent {
    private String id;
    private String name;
    private Set child;
    public Set getChild() {return child;}
    public void setChild(Set child) {this.child = child;}
    public void addChild(Child child) {this.child.add(child);}
    public String getId() {return id;}
    public void setId(String id) {this.id = id;}
    public String getName() {return name;}
    public void setName(String name) {this.name = name;}
}
public class Child {
    private String id;
    private String name;
    private Parent parent;
    public String getId() {return id;}
    public void setId(String id) {this.id = id;}
    public String getName() {return name;}
    public void setName(String name) {this.name = name;}
    public Parent getParent() {return parent;}
    public void setParent(Parent parent) {this.parent = parent;}
}

create table PARENTS (
   PARENT_ID char(32) not null,
   NAME varchar(64) not null,
   primary key (PARENT_ID)
);
create table CHILDS (
   CHILD_ID char(32) not null,
   NAME varchar(64) not null,
   PARENT_ID char(32) not null,
   primary key (CHILD_ID)
);