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Collection table

As explained in Java Persistence with Spring Data and Hibernate epub

A collection table is a relational table that stores the elements of a persistent collection together with a key referencing its owning entity.

A collection table stores the elements of a persistent collection separately from its owning entity. Each row includes a foreign-key column referencing the owner, plus columns for the element, map key, or embedded value. section 8.2.6 section 8.2.2

In JPA, @ElementCollection identifies the persistent collection. @CollectionTable customizes the table name and join column; otherwise, provider defaults apply. A collection of strings can therefore map to CONTACT(USER_ID, NAME), while an image collection can map to IMAGE with an ITEM_ID owner key. section 8.2.2 section 8.2.6

The collection type determines additional columns and key constraints:

  • A Set uses the owner key and non-nullable element columns as a composite primary key, preventing duplicate values. section 8.2.2 section 8.2.6
  • A Map adds a key column; its primary key combines the owner key with that map key. @MapKeyColumn names the key column. section 8.2.4 section 8.1.7
  • A list, bag, or map can replace a Set for basic values, while embeddable values can provide multiple mapped columns. section 8.2.6

Schema constraints must agree with Java equality. For a Set of embeddables, non-nullable columns participate in the composite key, so value types should implement equals() and hashCode() by value. Use @Column(nullable = false) when a column must receive a database NOT NULL constraint; @NotNull alone may not generate it. section 8.2.1

Mapping a set of embeddable images

An Item can declare @ElementCollection, @CollectionTable(name = "IMAGE"), and @AttributeOverride to map Set<Image> to a named table and column. @OrderBy("filename DESC, width DESC") provides load-time ordering when a LinkedHashSet is used. section 8.2.2

Listing 8.14 A Set of embeddable components with an override
Path: Ch08/mapping-collections/src/main/java/com/manning/javapersistence, 
 /ch08/setofembeddables/Item.java
 
@Entity
public class Item {
    // . . .
 
    @ElementCollection                   #1
    @CollectionTable(name = "IMAGE")     #2
    @AttributeOverride(
            name = "filename",
            column = @Column(name = "FNAME", nullable = false)
    )
    private Set<Image> images = new HashSet<>();
#1 As before, the <kbd>@ElementCollection</kbd> annotation is required. Hibernate automatically knows that the target of the collection is an <kbd>@Embeddable</kbd> type from the declaration of a generic collection.
#2 The <kbd>@CollectionTable</kbd> annotation overrides the default name for the collection table, which would have been <kbd>ITEM_IMAGES</kbd>.

Shows the complete entity-side mapping for a Set of embeddable components, including table and column overrides. section 8.2.2

How Java Persistence with Spring Data and Hibernate epub covers it

How deep it goes
Explains annotations, schemas, composite keys, equality, nullability, ordering, and map-key variants without covering transaction behavior.
Examples
Uses contact strings, image sets, string maps, component-valued maps, overridden columns, and ordered image collections.
What it assumes
Assumes core Java collections and JPA annotations while teaching collection-table schema relationships and embeddable equality implications.
What you can do after
Build it from scratch by selecting a collection type and configuring its table, owner key, element columns, map keys, and constraints.
How it connects
Connects element collections with embeddable types, sets, maps, composite primary keys, foreign keys, and JPA annotations.
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