Hibernate insert causes update of another table - spring

Question

I have a model that looks like this:
#Entity
#Table(name = "A")
class A {
#ManyToMany(fetch = FetchType.LAZY, cascade = CascadeType.ALL)
#JoinTable(name = "A_categories", joinColumns = {
#JoinColumn(name = "A_id", nullable = false, updatable = false) },
inverseJoinColumns = { #JoinColumn(name = "category_id",
nullable = false, updatable = false) })
private List<Category> categories;
}
#Entity
#Table(name = "category")
class Category {
#Id
#Column(name="id")
#GeneratedValue
private Integer id;
#Column(name = "category_name")
private String categoryName;
#ManyToMany(fetch = FetchType.LAZY, mappedBy = "categories")
private List<A> a;
}
So there is a many-to-many relationship between A and Category. Now categories are static, and cannot be changed by a user. From the UI, the user will try to save an entity A, and each can have one or more categories. So the JSON that comes back looks a little like this:
{A: {categories: [{id: 1}, {id: 2}, {id: 3}]}}
Now when I try to save this A object (after jackson has unmarshalled to java), I just want entries to be made in the join table, A_categories, for each category the new entity has.
However, the Category entity itself also gets updated. So if you notice, the JSON does not have any category_name, and so the database entry for each Category will also get updated to a null entry for the name.
How can I prevent this from happening?

Answer 1

Two different approaches:
1) Set managed categories before merging.
a.setCategories(readAllByIds(a.getCategories()))
private Collection<Category> readAllByIds(Collection<Category> categories) {
Collection<Category> result = new ArrayList();
for (Category category : categories) {
result.add(entityManager.getReference(Category.class, category.getId()));
}
return result;
}
EntityManager.getReference returns proxy, so the additional benefit is that no database round trips are executed for reading the associated categories.
With this solution you are not merging the deserialized categories into the persistence context, thus Hibernate will not synchronize their state with the database.
2) Do not cascade any operations from A to categories (remove cascade attribute).
This way, neither PERSIST nor MERGE will be cascaded and Hibernate will just use ids of the detached Category instances to store the data into the relationship table.
Sidenote: Generally, cascading REMOVE or ALL in a many-to-many association makes no sense (if you remove an A you probably don't want to remove all the categories it belongs to).

Answer 2

#Column has the attributes insertable and updatable. You can set them to false:
#Entity
#Table(name = "category")
class Category {
#Id
#Column(name="id", insertable=false, updatable=false)
#GeneratedValue
private Integer id;
#Column(name = "category_name", insertable=false, updatable=false)
private String categoryName;
#ManyToMany(fetch = FetchType.LAZY, mappedBy = "categories")
private List<A> a;
}

Answer 3

You can try adding this
#Entity
#Table(name = "category")
class Category {
#Id
#Column(name="id")
#GeneratedValue
private Integer id;
#Column(name = "category_name")
private String categoryName;
#ManyToMany(fetch = FetchType.LAZY, mappedBy = "categories", cascade=CascadeType.DETACH)
private List<A> a;
}
with the cascade.DETACH should not save changes when you save A entity, but let me know if is not working to make an example modifying the ManyToMany relationship with this DETACH action