JPA 1:N relationship removing child does not remove it from parent

For 1:N, yours should work just fine with minor adjustment.

The reason why it fails: Upon doing "groupDao.save(group);" this group is now in the persistence context and calling "groupDao.findById(groupId).get().getProducts().size()" would return the copy which is from the persistence context.

To solve this: simply add: entityManager.flush(); and entityManager.clear(); before the Assert

I would like to demonstrate it with this Integration Test

    @Test
    @Transactional
    public void deleteProduct_groupShouldNowBeEmpty() {
        ProductGroup group = groupRepository.findById("0001").orElseThrow(() -> new IllegalArgumentException("id not found"));
        Assert.assertEquals(1, group.getProducts().size());

        Product product = productRepository.findById("0001").orElseThrow(() -> new IllegalArgumentException("id not found"));
        productRepository.delete(product);

        entityManager.flush();
        entityManager.clear();

        Assert.assertEquals(0, productRepository.findAll().size());
        Assert.assertEquals(0, groupRepository.findById("0001").get().getProducts().size());
    }

If we are to remove the first 2 lines, then we won't need to flush and clear. Like this.

    @Test
    @Transactional
    public void deleteProduct_groupShouldNowBeEmpty() {
        Product product = productRepository.findById("0001").orElseThrow(() -> new IllegalArgumentException("id not found"));
        productRepository.delete(product);

        Assert.assertEquals(0, productRepository.findAll().size());
        Assert.assertEquals(0, groupRepository.findById("0001").get().getProducts().size());
    }

For N:M, since there would be another table where product is being referenced, then we would need to delete the records from that table first before deleting the product.

N:M is a bit tricky so if I can suggest domain changes, here how I'll do it. (The integration test is at the bottom.)

I'll add a separate entity: CartItem which is associated to a Product and Cart

@Entity
public class CartItem {

    @Id
    @GeneratedValue(generator = "uuid")
    @GenericGenerator(name = "uuid", strategy = "uuid2")
    private String id;

    @ManyToOne
    private Product product;

    @ManyToOne
    private Cart cart;

    public String getId() {
        return id;
    }

    // Required by JPA
    protected CartItem() {}

}

And for the Product Entity: add a bidirectional relationship with CartItem

@Entity
public class Product {

    @Id
    @GeneratedValue(generator = "uuid")
    @GenericGenerator(name = "uuid", strategy = "uuid2")
    private String id;

    private String name;

    private String description;

    private BigDecimal price;

    @ManyToOne
    private ProductGroup group;

    @OneToMany(mappedBy = "product")
    private List<CartItem> cartItems;

    public List<CartItem> getCartItems() {
        return cartItems;
    }

    // Required by JPA
    protected Product() {}
}

Then, retrieve the product (using Join Fetch to avoid N+1, since later will be looping through each cartItem)

public interface ProductRepository extends JpaRepository<Product, String> {

    @Query("SELECT product FROM Product product JOIN FETCH product.cartItems")
    Optional<Product> findProduct(String Id);

}

create another query inside CartItemRepository to delete cartItems in bulk by ids

public interface CartItemRepository extends JpaRepository<CartItem, String> {

    @Modifying
    @Query("DELETE FROM CartItem cartItem WHERE cartItem.id IN :ids")
    void deleteByIds(@Param("ids") List<String> ids);

}

Lastly here's the integration test to wrap everthing up:

@Test
@Transactional
public void deleteProduct_associatedWithCart() {
    Cart cart = cartRepository.findById("0001").get();
    Assert.assertEquals(1, cart.getCartItems().size());

    Product product = productRepository.findProduct("0001").orElseThrow(() -> new IllegalArgumentException("id not found"));
    List<String> cartItemIds = product.getCartItems().stream()
            .map(CartItem::getId)
            .collect(Collectors.toList());

    cartItemRepository.deleteByIds(cartItemIds);
    productRepository.delete(product);

    entityManager.flush();
    entityManager.clear();

    Assert.assertEquals(0, productRepository.findAll().size());
    Assert.assertEquals(0, groupRepository.findById("0001").get().getProducts().size());

    Assert.assertEquals(0, cartItemRepository.findAll().size());
    Assert.assertEquals(0, cartRepository.findById("0001").get().getCartItems().size());
}

I've used DBUnit for this integration test so I think it would also be helpful to share the dataset.

    <?xml version="1.0" encoding="UTF-8" ?>
    <dataset>
        <product_group id="0001" name="product group with 1 product"/>
        <product id="0001" group_id="0001" />
    
        <cart id="0001" />
        <cart_item id="0001" product_id="0001" cart_id="0001" />
    </dataset>

When you remove an entity, this state transition should be propagated from parent to child, not the other way around.

In this case, you need to move that functionally to the Group entity, something like this:

@NoArgsConstructor
@AllArgsConstructor
@Getter
@Entity(name="Group")
public class Group {
    @Id
    @GeneratedValue
    @NotNull
    @Column(name = "GROUP_ID")
    private Long id;

    @Column(name="NAME")
    private String name;

    @OneToMany(
            targetEntity = Product.class,
            mappedBy = "groupId",
            cascade = CascadeType.ALL,
            fetch = FetchType.LAZY, // Always prefer LAZY initialized Collections to EAGER ones
            orphanRemoval = true
    )
    private List<Product> products = new ArrayList<>();

    public Group(String name) {
        this.name = name;
    }

    public void addProduct(Product product){
      product.setGroupId(this);
      this.products.add(product);
    }

    public void removeProduct(Product product){
      product.setGroupId(null);
      this.products.remove(product);
    }

If you want to remove a Product, you only need to invoke the removeProduct method and save the parent entity:

Group group = new Group("group1");
Product product = new Product("test", "testProduct", 100.0);

group.addProduct(product);

groupDao.save(group);

On the other hand, we have the many-to-many relation between Product and CartEntity.

First, if you configure the entity CartEntity with Cascade.ALL as in your example:

@ManyToMany(cascade = CascadeType.ALL, mappedBy = "carts")
private List<Product> productsList = new ArrayList<>();

It will have a probably undesired effect: if you remove the CartEntity, it will remove all the Products associated with the entity as well, even if other CartEntitys are still associated to them. Vlad Mihalcea explain it in great detail in this article.

To avoid that problem, the best option will be just define the relationship as follows:

@ManyToMany(cascade = {CascadeType.PERSIST, CascadeType.MERGE}, mappedBy = "carts")
private List<Product> productsList = new ArrayList<>();

This will give us a CartEntity like this:

@Getter
@NoArgsConstructor
@AllArgsConstructor
@Entity(name = "cart")
public class CartEntity {

    @Id
    @NotNull
    @GeneratedValue
    @Column(name = "CART_ID")
    private Long id;

    @ManyToMany(cascade = {CascadeType.PERSIST, CascadeType.MERGE}, mappedBy = "carts")
    private List<Product> productsList = new ArrayList<>();

    public void addProduct(Product product) {
        productsList.add(product);
        product.getCarts().add(this);
    }

    public void removeProduct(Product product) {
        productsList.remove(product);
        product.getCarts().remove(this);
    }

    public void removeProducts() {
        for(Product product : new ArrayList<>(products)) {
            removeProduct(product);
        }
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        CartEntity that = (CartEntity) o;
        return id.equals(that.id);
    }

    @Override
    public int hashCode() {
        return Objects.hash(id);
    }
}

Please, note the inclusion of the removeProduct and removeProducts methods.

With this code, if you need to remove a CartEntity, just do the following:

cart.removeProducts();
cartDao.remove(cart);

And if you need to remove a Product from the CartEntity (will only remove the relation):

cart.removeProduct(product);
cartDao.save(cart);

If you need to propagate the Product remove to the CartEntity, I think that the best option will be create a business method that takes care of the whole process. Think in something like:

public void removeProduct(Product product){
  Group group = product.getGroupId();
  group.removeProduct(product);

  final List<CartEntity> carts = product.getCarts();

  if (carts != null) {
    for(CartEntity cart : new ArrayList<>(carts)) {
      cart.removeProduct(product);
      cartDao.save(cart);
    }
  }

  groupDao.save(group);
}

It will remove the association, you just need to do small adjustments.

1. 1:N. When you remove Product, you don't have to do anything else in order to remove its association with Group, because the product itself holds the association (in DB column product.group_id). You just need to commit the transaction. And next time when you load a group from the DB it for sure will not contain this product.
2. N:M. There is no way to automatically remove the association because it is stored in a separate table and you don't have a separate entity for it. (YOU SHOULD NOT USE CascadeType.ALL for N:M relations). What you want to do is remove the association before you remove the product. Just add another helper method to Product.

public void removeFromCarts() {
        carts.forEach(c -> c.getProducts().remove(this));
        carts.clear();
}

So finally, in order to remove a product and all the associations with it. You will need to do the following:

        product.removeFromCarts();
        productDao.deleteById(productId); // not sure why you remove by id (not pass object)

*please note that you need to commit transaction and close the session. So you cannot rely on the test. In real app when you do what I described, it will work

**N:M is tricky. For instance, you should better use Set instead of List to avoid unexpected SQL under the hood. Also going down the road, I recommend you to consider splitting N:M into two N:1 and 1:M and have a dedicated Entity for a link table

Not sure I follow. Hibernate does not automatically maintain the inverse association for you. You can make it sensitive to changes on the owning side of the association, but that's as far as it goes.

As to why your test fails, cartDaoStub.findById(cartId) probably returns the same copy of the CartEntity that you already have loaded into the persistence context. Try calling entityManager.flush() followed by entityManager.clear() before making the assertion and the issue will probably go away.