Chapter 7. Mapping collections and entity associations

In this chapter

Mapping persistent collections
Collections of basic and embeddable type
Simple many-to-one and one-to-many entity associations

From our experience with the Hibernate user community, the first thing many developers try to do when they begin using Hibernate is to map a parent/children relationship. This is usually the first time they encounter collections. It’s also the first time they have to think about the differences between entities and value types, or get lost in the complexity of ORM.

Managing the associations between classes and the relationships between tables is at the heart of ORM. Most of the difficult problems involved in implementing an ORM solution relate to collections and entity association management. Feel free to come back to this chapter to grok this topic fully. We start this chapter with basic collection-mapping concepts and simple examples. After that, you’ll be prepared for the first collection in an entity association—although we’ll come back to more complicated entity association mappings in the next chapter. To get the full picture, we recommend you read both this chapter and the next.

Major new features in JPA 2

Support for collections and maps of basic and embeddable types.
Support for persistent lists where the index of each element is stored in an additional database column.
One-to-many associations now have an orphan-removal option.

7.1. Sets, bags, lists, and maps of value types

Path: /model/src/main/java/org/jpwh/model/collections/setofstrings/Item.java

Path: /model/src/main/java/org/jpwh/model/collections/bagofstrings/Item.java

Path: /model/src/main/java/org/jpwh/model/collections/listofstrings/Item.java

Path: /model/src/main/java/org/jpwh/model/collections/mapofstrings/Item.java

Path: /model/src/main/java/org/jpwh/model/collections/sortedmapofstrings/Item.java

@Entity
public class Item {
<enter/>
    @ElementCollection
    @CollectionTable(name = "IMAGE")
    @MapKeyColumn(name = "FILENAME")
    @Column(name = "IMAGENAME")
    @org.hibernate.annotations.SortComparator(ReverseStringComparator.class)
    protected SortedMap<String, String> images =
        new TreeMap<String, String>();
<enter/>
    // ...
}

Path: /model/src/main/java/org/jpwh/model/collections/sortedsetofstrings/Item.java

@Entity
public class Item {
<enter/>
    @ElementCollection
    @CollectionTable(name = "IMAGE")
    @Column(name = "FILENAME")
    @org.hibernate.annotations.SortNatural
    protected SortedSet<String> images = new TreeSet<String>();
<enter/>
    // ...
}

Path: /model/src/main/java/org/jpwh/model/collections/setofstringsorderby/Item.java

Path: /model/src/main/java/org/jpwh/model/collections/bagofstringsorderby/Item.java

Path: /model/src/main/java/org/jpwh/model/collections/mapofstringsorderby/Item.java

@Entity
public class Item {
<enter/>
    @ElementCollection
    @CollectionTable(name = "IMAGE")
    @MapKeyColumn(name = "FILENAME")
    @Column(name = "IMAGENAME")
    @org.hibernate.annotations.OrderBy(clause = "FILENAME desc")
    protected Map<String, String> images = new LinkedHashMap<String, String>();
<enter/>
    // ...
}