looking at the way the constructor is written for the CategorizedItem pojo, I think there could be some issues, setting the id properties if we just created a new Category and user objects that are nor persisted. because calling category.getId() will return null.

public CategorizedItem(String username, Category category, Item item) {
this.username = username;

this.category = category;
this.item = item;

// Set primary key
this.id = new Id(category.getId(), item.getId());

// Guarantee referential integrity
category.getCategorizedItems().add(this);
item.getCategorizedItems().add(this);
}

I would call that an unimplemented feature. The pattern gets more complex if you require linking in transient state. If you read on you will find an alternative implementation that is different, and maybe easier in many cases.