[FIXED] JPA/Hibernate How to add duplicate values to a List<> with a @OneToMany mapping

Issue

I am getting an error every time I try to add duplicate products to the Client class.

org.postgresql.util.PSQLException: ERROR: duplicate key value violates unique constraint "uk_8opk6otadh2pkvi9bghguwp89"
  Detail: Key (products_id)=(500010) already exists.

My entities

@Entity
public class Client {
    @Id
    private String id;

    @OneToMany
    private List<Product> products;
}

@Entity
public class Product {
    @Id
    private String id;

    private Boolean foreign;
    private Integer price;
}

When I call .save() hibernate is trying to re-save an already existing Product. I believe this is due to the how it cascades things.

public Client addNewProduct(Client client, Product newProduct) {
    List<Product> products = client.getProducts();
    products.add(newProduct);
    client.setProducts(products);
    return clientRepository.save(client);
}

I just want to be able to add duplicates to a List<>. I believe this is & should be a uni-directional relationship.

Solution

This does not work with your current relationship, I don't understand why you would want to add duplicates, but if you have to, then you'd have to create a new entity for that. One example would be something like this:

@Entity
public class ProductBatch {

  @Id
  private String id;

  @OneToOne
  private Product product;

  private Integer count;

  // getter & setter
}

and then you change your Client like this:

@Entity
public class Client {

    @Id
    private String id;

    @OneToMany
    private List<ProductBatch> products;
}

this makes something like this possible for your addNewProduct function:

public Client addNewProduct(Client client, Product newProduct) {
    List<ProductBatch> products = client.getProducts();
    boolean exists = false;
    for(ProductBatch product : products) {
        if(product.getProduct().equals(newProduct)) {
            product.setCount(product.getCount() + 1);
            exists = true;
        }
    }
    if(!exists) {
        BatchProduct product = new BatchProduct();
        product.setProduct(newProduct);
        product.setCount(0);
        products.add(product);
    }
    client.setProducts(products);
    return clientRepository.save(client);
}

Answered By - PaulD
Answer Checked By - Candace Johnson (JavaFixing Volunteer)