Difference between @OneToMany and @ElementCollection?

ElementCollection is a standard JPA annotation, which is now preferred over the proprietary Hibernate annotation CollectionOfElements.

It means that the collection is not a collection of entities, but a collection of simple types (Strings, etc.) or a collection of embeddable elements (class annotated with @Embeddable).

It also means that the elements are completely owned by the containing entities: they're modified when the entity is modified, deleted when the entity is deleted, etc. They can't have their own lifecycle.

---

I believe @ElementCollection is mainly for mapping non-entities (embeddable or basic) while @OneToMany is used to map entities. So which one to use depend on what you want to achieve.

---

@ElementCollection allows you to simplify code when you want to implement one-to-many relationship with simple or embedded type. For instance in JPA 1.0 when you wanted to have a one-to-many relationship to a list of Strings, you had to create a simple entity POJO (StringWrapper) containing only primary key and the String in question:

@OneToMany
private Collection<StringWrapper> strings;

//...

public class StringWrapper {
  @Id
  private int id;

  private String string;
}

With JPA 2.0 you can simply write:

@ElementCollection
private Collection<String> strings;

Simpler, isn't it? Note that you can still control the table and column names using @CollectionTable annotation.

See also:

Java Persistence/ElementCollection

---

Basic or Embedded: @ElementCollection Entities: @OneToMany or @ManyToMany

@ElementCollection:

the relation is managed (only) by the entity in which the relation is defined

table contains id reference to the owning entity plus basic or embedded attributes

@OneToMany / @ManyToMany:

can also be managed by the other entity

join table or column(s) typically contains id references only

---

@ElementCollection marks a collection. This does not necessarily mean that this collection references a 1-n join.

---

ElementCollection can override the mappings, or table for their collection, so you can have multiple entities reference the same Embeddable class, but have each store their dependent objects in a separate table.

---

@ElementCollection

This annotation will be applied when there is a relation with non-entity and these associations relation was HAS-A. Every collection is created with a table and gets relation by Foreign Key.

There are two types of element collections

Index (List, Map)

Non-Index (Set)

Index: The index type collection has a table with 3 columns they are

Key Column (Foriegn Key)

Index Column (Position of data in collection)

Element Column (Data)

Non-Index: The Non-Index type collection has a table with 2 columns they are

Key Column

Element Column

Note: Here it won't have any index column because, Since a SET doesn’t retain the insertion order.

Multiplicity

---

You can use ElementCollection replace for you use @OneToMany. Example you can have one Project in many versions.

@ElementCollection
@CollectionTable(name="versions",
joinColumns = @JoinColumn(name="projectID"))
@LazyCollection(LazyCollectionOption.FALSE)
@JoinColumn(name="version",nullable = false)
private Set<String> versions;

You also can use @ElementCollection in mapping OGM for array in one collection.

@ElementCollection(fetch = FetchType.EAGER)
private Set<String> researchAreas;

---