workaround/get rid of multiple writable mappings in OneToMany and ManyToOne relationship?

i've been trying to solve a problem but got stuck now.

I have 2 entity classes/tables Order and OrderItem. OrderItem has 1 Order and Order has 1..* OrderItems.

My EJB works fine, it adds order, gets its' id and adds order's items

Order1 order = addOrder(project, name);
em.flush();
int orderId = order.getOrder();
addOrderItem(orderId, cart);

So everything works fine, the order id is inserted also in order item row as needed.

But now i want to perform JPQL with many mapped entities, Order and OrderItem included. As i know to perform a query i need to map my entities first, and so the problem is in my mappings.

Here is the mapping

public class Order1 implements Serializable {

    private static final long serialVersionUID = 1L;
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Basic(optional = false)
    @Column(name = "order_")
    private Integer order;
... 
    @OneToMany(mappedBy="orders",targetEntity=OrderItem.class, 
    fetch=FetchType.EAGER, cascade = CascadeType.ALL)
    private List<OrderItem> orderItems;
...
}

public class OrderItem implements Serializable {
    private static final long serialVersionUID = 1L;
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Basic(optional = false)
    @Column(name = "order_item")
    private Integer orderItem;
    @Column(name = "order_" /*, insertable=false, updatable=false*/)
    private BigInteger order;
...
    @ManyToOne(fetch=FetchType.LAZY)
    @JoinColumn(name="order_")
    private Order1 orders;
...
}

With this kind of mapping i get exception at compiletime

Exception Description: Multiple writable mappings exist for the field [order_item.order_]. Only one may be defined as writable, all others must be specified read-only.

I've googled and found out that i have to make the order column in OrderItem non-writable. As you can see in the code, i've done that and it compiles, but when i place an order in my application, then order id into order_item's order column is not inserted and is NULL. But i can't leave it so, there must be an order reference in order_item.

My JPQL query is createQuery("SELECT o.order FROM OrderItem oi JOIN oi.orders o"); It works when I make order column in order_item table uninsertable, but it can't be left that way. What are the suggestions? What is wrong in my mapping and why can't i have order_ in order_item insertable?

update about @toto answer:

My entities were generated by Netbeans wizard. If i won't define BigInteger order in OrderItem then i couldn't insert order id into my order_item table.

My EJB methods are following:

public Order1 addOrder(BigInteger project, BigInteger name) {

    Order1 order = new Order1();
...
    em.persist(order);
    return order;
}

private void addOrderItem(int orderId, ProjectCart cart) {

    // convert orderId int to BigInteger
    BigInteger oId = new BigInteger(String.valueOf(orderId));
    OrderItem orderItem = new OrderItem();
    orderItem.setOrder(oId);
....
    em.persi开发者_StackOverflow中文版st(orderItem);
}

So this means if i leave out from OrderItem

@Column(name = "order_")
private BigInteger order1;

my getters and setters for order1 inside OrderItem won't work and i can't orderItem.setOrder(oId) in addOrderItem() method simply cause my order1 is not defined in OrderItem. Other thing is that i don't have foreign keys in my datamodel, but i think it's not a concern, because in mappings i have set join columns.

You must not define a column BigInteger order of name order inside OrderItem. It will be done automatically (in the background) by JPA. The compiler complains because you are creating a column that has been created by JPA already.