I have a ShoppingCart object that maps to SHOPPING_CART table (primary key SHOPPING_CART_ID)
I have a Purchase object that maps to PURCHASE table (primary key PURCHASE_ID)
I have a table called SHOPPING_CART_PURCHASES (columns SHOPPING_CART_ID and PURCHASE_ID - both columns make a unique key) which is meant to house the associations between shopping carts and purchases.

My ShoppingCart object has a list of Purchase objects.
When I persist ShoppingCart, the Purchase objects that I have added to ShoppingCart purchases are persisted. However no rows are inserted into the association table.

Here is the annotation for ShoppingCart;
@ManyToMany(cascade=CascadeType.ALL, fetch=FetchType.LAZY)
@JoinTable(name="SHOPPING_CART_PURCHASES", joinColumns = {
@JoinColumn(name="SHOPPING_CART_ID", nullable=false, updatable=false) }, inverseJoinColumns = {
@JoinColumn(name="PURCHASE_ID", nullable=false, updatable=false) })
List<Purchase> purchases = this;

Any ideas how to get the associations to be inserted when I persist the ShoppingCart?

Found the issue, though I don't think it should be one;

My class extended ArrayList (public class ShoppingCart extends ArrayList...), consequently the purchases property was simply set to this. As;
List<Purchase> purchases = this;

The getter/setter still returned the property purchases.

When I modified my class to NOT subclass ArrayList and then initialized purchases with new ArrayList<Purchase>(), it worked fine.
List<Purchase> purchases = new ArrayList<Purchase>();

Not sure why this is necessary, but at least it works now.