Hibernate - many to many relationship update

Hi,

We are using spring and hibernate with mysql, we have 3 tables:
---------------------------------------------------
CREATE TABLE PRODUCT_LINE (
  product_line_id INTEGER UNSIGNED NOT NULL,
  product_line_name VARCHAR(64) NULL,
  PRIMARY KEY(product_line_id)
)
TYPE=InnoDB;

CREATE TABLE PRODUCT_FAMILY (
  product_family_id INTEGER UNSIGNED NOT NULL AUTO_INCREMENT,
  product_family_name VARCHAR(64) NULL,
  PRIMARY KEY(product_family_id)
)
TYPE=InnoDB;

CREATE TABLE PRODUCTFAMILY_PRODUCTLINE_LINK (
  product_family_id INTEGER UNSIGNED NOT NULL,
  product_line_id INTEGER UNSIGNED NOT NULL,
  FOREIGN KEY(product_line_id)
    REFERENCES PRODUCT_LINE(product_line_id)
      ON DELETE NO ACTION
      ON UPDATE NO ACTION,
  FOREIGN KEY(product_family_id)
    REFERENCES PRODUCT_FAMILY(product_family_id)
      ON DELETE NO ACTION
      ON UPDATE NO ACTION
)
TYPE=InnoDB;
---------------------------------------------------
In my .hbm files, we have:

    <class name="com.pcn.persistence.domain.ProductFamily" table="PRODUCT_FAMILY">
      ...          
        <set name="productLines" inverse="true" table="PRODUCTFAMILY_PRODUCTLINE_LINK">      
            <key>
                <column name="product_family_id" not-null="true" />
            </key>
            <many-to-many entity-name="com.pcn.persistence.domain.ProductLine">
                <column name="product_line_id" not-null="true" />
            </many-to-many>
        </set>
    </class>
-----------------
    <class name="com.pcn.persistence.domain.ProductLine" table="PRODUCT_LINE">

        <set name="productFamilies" inverse="true" table="PRODUCTFAMILY_PRODUCTLINE_LINK" cascade="merge,save-update,delete" lazy="true">
            <key>
                <column name="product_line_id" not-null="true" />
            </key>
            <many-to-many entity-name="com.sun.pcn.persistence.domain.ProductFamily" >
                <column name="product_family_id" not-null="true" />
            </many-to-many>
        </set>
</class>
---------------------------------------------------
so when I try to delete the link between and product_family and product_line, i do this:

ProductLine pl = session1.get(ProductLine .class, ProductLineId);
ProductFamily pf = getFromSomeWhere();
if (pl.getProductFamilies().contains(pf))
   pl.ProductFamilies.remove(pf);

session1.merge(pl);
-----------------------------------------------------

However, the link won't remove, the row in table "PRODUCTFAMILY_PRODUCTLINE_LINK" still exist.
I want to keep the product_family and product_line and just remove the link, how can i do it??

Please help.
Joe
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