How do you specify OrderBy clause on two columns

问题:

We would like to orderBy 2 columns in the Seam EntityQuery interface as well as the JPA model. How do we do this?

@Entity
public class A{

@OrderBy(???) // should this be hardcoded here, is it database agnostic
List<B> bobjects;
}

@Entity
public class B {

   public short startTimeHrs;
   public short startTimeMins;
}

@Name("bList")
public class B extends EntityQuery {
        setOrderColumn("startTimeHrs, startTimeMins"); // Is this correct?
        setOrderDirection("asc");
}

回答1:

If you are talking about javax.persistence.OrderBy, this annotation takes into parameter a list of comma separated properties (of the target entity) and order the collection accordingly. For example in your case, startTimeHrs asc, startTimeMins asc.

From the JPA 1.0 specification:

9.1.28 OrderBy Annotation

The OrderBy annotation specifies the ordering of the elements of a collection valued association at the point when the association is retrieved.

@Target({METHOD, FIELD}) @Retention(RUNTIME)
public @interface OrderBy {
String value() default "";
}

The syntax of the value ordering element is an orderby_list, as follows:

orderby_list::= orderby_item [,orderby_item]*
orderby_item::= property_or_field_name [ASC | DESC]

If ASC or DESC is not specified, ASC (ascending order) is assumed.

If the ordering element is not specified, ordering by the primary key of the associated entity is assumed.

The property or field name must correspond to that of a persistent property or field of the associated class. The properties or fields used in the ordering must correspond to columns for which comparison operators are supported.

回答2:

Yes you can use @OrderBy to order your queries.

@OrderBy("startTimeHrs, startTimeMins")
@OneToMany(...)
getBobjects() {
   return bobjects;
}

Now whenever you are saying A.getBobjects() they will be ordered. However, if you are using the EntityQuery way of retrieving the result you can override the getEjbql() and put the order by there.

@Name("bList")
public class B extends EntityQuery {
    @Override
    public String getEjbql() {
          return "select b from B b order by startTimeHrs, startTimeMins";
    }
}

Or you can @Override the getResultList() to manipulate the collection there in your entityLists