在休眠状态下使用复合键

Question

Suppose i want to have a composite key as street, city for purchase order entity.

Below is how i identify doing it,

    @Embeddable
    public class BillingAddress implements Serializable {

        private String street;
        private String city;

        public BillingAddress(){

        }

        public BillingAddress(String street, String city) {
            this.street = street;
            this.city = city;
        }
        //with getters and setters
}



@Entity
@IdClass(BillingAddress.class)
public class PurchaseOrder {

    public PurchaseOrder(BillingAddress billingAddress) {
        street = billingAddress.getStreet();
        city = billingAddress.getCity();

    }

    @Id
    @AttributeOverrides({
            @AttributeOverride(name = "street", column = @Column(name = "STREET")),
            @AttributeOverride(name = "city", column = @Column(name = "CITY")) })
    private String street;
    private String city;
    private String itemName;

    public String getItemName() {
        return itemName;
    }

    public void setItemName(String itemName) {
        this.itemName = itemName;
    }

}

I want to understand what is really @AttributeOverrides annotation do? Even i change to colomn name to something STREET1 i still see the table created with column name STREET. So what is column = @Column(name = "STREET")) doing here.

Also instead of constructore taking the BillingAddress i can have it like a field of PurchaseOrder class right like,

 public class PurchaseOrder {
     BillingAddress billingAddress;

}

In this case how this going to change? Do i still need to have private String street; private String city; in PurchaseOrder?

Finally i read that using composite keys should be avoided in new data base system design which using composite primary key is applicable a situation where in order to map the legacy data base tables with out changing the data base table structure right? Is that statement a valid one?

//Edit question

Saving purchase order which billing address is in the field,

PurchaseOrder purchaseOrder = new PurchaseOrder();
purchaseOrder.setItemName("name");

BillingAddress billingAddress = new BillingAddress();
billingAddress.setCity("c1"); billingAddress.setStreet("s1");                                                              purchaseOrder.setBillingAddress(billingAddress); 
session.save(purchaseOrder);

Answer 1

There's are few question you asked, I tried to go through all of them and answer each one:

What does @AnnotationOverride do?

answer here: What does @AttributeOverride mean?

The second question is a bit unclear to me but I presume you're asking whether you have to include all the fields from the composite key in the PurchaseOrder class.

No, I don't think so. Here's an example I've put together real fast:

@Entity
@Table(name = "PURCHASE_ORDER")
public class PurchaseOrder{

    @Id
    private BillingAddress billingAddress;

    //getters & setters

    @Embeddable
    public static class BillingAddress implements Serializable {
        @Column(name = "street")
        private String street;
        @Column(name = "city")
        private String city;
        @Column(name = "itemName")
        private String itemName;

        //getters & setters

    }
}

Don't worry about the syntax, just the structure. You can even add extra field into PurchaseOrder which isn't an id.

Should I use composite keys or not?

answer here: Should I use composite primary keys or not?

Answer 2

Well, your PurchaseOrder class does not extend from a mapped entity of any kind, and neither do the properties that you are (currently) applying the @AttributeOverride s to. So, there is nothing to actually override and your JPA provider is simply ignoring the annotations. What I think you are trying to do is define an embedded id for an entity, while overriding some of the column mappings for that id. You can do this with some modifications to your current code:

@Entity
public class PurchaseOrder {
    @EmbeddedId
    @AttributeOverrides({
            @AttributeOverride(name = "street", column = @Column(name = "BILLING_STREET")),
            @AttributeOverride(name = "city", column = @Column(name = "BILLING_CITY")) })
    private BillingAddress billingAddress;

    private String itemName;

    // Constructors, Getters/Setters
}

Note that I've changed the names of the overridden attributes, since with your current example, the embedded id name and overridden names are the same.