I have read around that this is because I'm trying to persist an entity that has been otherwise removed, but I guess I'm not sure what constitutes "removed" or what makes an entity "removed".
I'm trying to make the fairly basic OneToMany/ManyToOne with e.g. a shopping cart and items, items pointing back to parent cart. I want to be able to:
-delete an item from the cart
-delete the cart and all items

I have tried about 20 different examples, every time I google a new one I rush to try it. I am using annotations, something like:

Cart:
@OneToMany(mappedBy="cart", cascade={CascadeType.ALL})
private Set<Item> items = new HashSet<>();

Item:
@ManyToOne(fetch = FetchType.EAGER)
@JoinColumn(name = "cart", insertable = false, updatable = false, nullable = false)
private Cart cart;

I have tried @PreRemove on item to remove it from the cart's set, unlinking type methods on the entities to disconnect them before persisting etc. but it seems whatever I do I'm plagued by the "deleted entity passed to persist" error on committing the transaction.
I have read so much my head is now spinning and feel far less confident about it than I did a few days ago - please could someone point me to a good example for this?
Thanks,
Phil