How to apply a default-restriction on Entity-Bean @OneToMany Relationships

问题

I have two entity models, Customer and Order. Each customer could have thousands of orders. I have a OneToMany and ManyToOne relationship between these two entities.

How do I restrict this relationship's list to only top 10 orders?

Is it possible to apply 'WHERE' condition as an attribute on @OneToMany or not?

Like:

@OneToMany("Where Order.orderNo > 100")

My problem is when the object created by Entity Manager all Orders are created in memory. Lazy loading can not solve my consideration, because I need to get top 10 orders in default construction.

回答1:

I mean if it is possible to apply 'WHERE' condition as an attribute on @OneToMany or not?

Not with standard JPA. But some providers have extensions for this. For example, Hibernate does have a @Where annotation:

@OneToMany(cascade=CascadeType.ALL, fetch=FetchType.EAGER)
@Where(clause="1=1")
public Set<Ticket> getTickets() {
    return tickets;
}

References

• Hibernate Annotations Reference Guide
  • 2.4.6. Collection related annotations

回答2:

JPA does not support this. But in EclipseLink you can use an Expression to filter a relationship.

See, http://wiki.eclipse.org/EclipseLink/Examples/JPA/MappingSelectionCriteria

来源：https://stackoverflow.com/questions/3514672/how-to-apply-a-default-restriction-on-entity-bean-onetomany-relationships