Naturally Hibernate also allows to persist collections. These persistent collections can contain almost any other Hibernate type, including: basic types, custom types, components and references to other entities. The distinction between value and reference semantics is in this context very important. An object in a collection might be handled with "value" semantics (its life cycle fully depends on the collection owner), or it might be a reference to another entity with its own life cycle. In the latter case, only the "link" between the two objects is considered to be a state held by the collection.
As a requirement persistent collection-valued fields must be
declared as an interface type (see Example 7.2, “Collection mapping using @OneToMany and @JoinColumn”). The actual interface
might be java.util.Set,
java.util.Collection,
java.util.List, java.util.Map,
java.util.SortedSet,
java.util.SortedMap or anything you like ("anything you
like" means you will have to write an implementation of
org.hibernate.usertype.UserCollectionType).
Notice how in Example 7.2, “Collection mapping using @OneToMany and @JoinColumn” the instance variable
parts was initialized with an instance of
HashSet. This is the best way to initialize collection
valued properties of newly instantiated (non-persistent) instances. When
you make the instance persistent, by calling persist(),
Hibernate will actually replace the HashSet with an
instance of Hibernate's own implementation of Set. Be
aware of the following error:
Example 7.1. Hibernate uses its own collection implementations
Cat cat = new DomesticCat();
Cat kitten = new DomesticCat();
....
Set kittens = new HashSet();
kittens.add(kitten);
cat.setKittens(kittens);
session.persist(cat);
kittens = cat.getKittens(); // Okay, kittens collection is a Set
(HashSet) cat.getKittens(); // Error!
The persistent collections injected by Hibernate behave like
HashMap, HashSet,
TreeMap, TreeSet or
ArrayList, depending on the interface type.
Collections instances have the usual behavior of value types. They are automatically persisted when referenced by a persistent object and are automatically deleted when unreferenced. If a collection is passed from one persistent object to another, its elements might be moved from one table to another. Two entities cannot share a reference to the same collection instance. Due to the underlying relational model, collection-valued properties do not support null value semantics. Hibernate does not distinguish between a null collection reference and an empty collection.
Note
Use persistent collections the same way you use ordinary Java collections. However, ensure you understand the semantics of bidirectional associations (see Section 7.3.2, “Bidirectional associations”).
Using annotations you can map Collections,
Lists, Maps and
Sets of associated entities using @OneToMany and
@ManyToMany. For collections of a basic or embeddable type use
@ElementCollection. In the simplest case a collection mapping looks like
this:
Example 7.2. Collection mapping using @OneToMany and @JoinColumn
@Entity
public class Product {
private String serialNumber;
private Set<Part> parts = new HashSet<Part>();
@Id
public String getSerialNumber() { return serialNumber; }
void setSerialNumber(String sn) { serialNumber = sn; }
@OneToMany
@JoinColumn(name="PART_ID")
public Set<Part> getParts() { return parts; }
void setParts(Set parts) { this.parts = parts; }
}
@Entity
public class Part {
...
}
Product describes a unidirectional relationship with Part using the join column PART_ID. In this unidirectional one to many scenario you can also use a join table as seen in Example 7.3, “Collection mapping using @OneToMany and @JoinTable”.
Example 7.3. Collection mapping using @OneToMany and @JoinTable
@Entity
public class Product {
private String serialNumber;
private Set<Part> parts = new HashSet<Part>();
@Id
public String getSerialNumber() { return serialNumber; }
void setSerialNumber(String sn) { serialNumber = sn; }
@OneToMany
@JoinTable(
name="PRODUCT_PARTS",
joinColumns = @JoinColumn( name="PRODUCT_ID"),
inverseJoinColumns = @JoinColumn( name="PART_ID")
)
public Set<Part> getParts() { return parts; }
void setParts(Set parts) { this.parts = parts; }
}
@Entity
public class Part {
...
}
Without describing any physical mapping (no
@JoinColumn or @JoinTable),
a unidirectional one to many with join table is used. The table name is
the concatenation of the owner table name, _, and the other side table
name. The foreign key name(s) referencing the owner table is the
concatenation of the owner table, _, and the owner primary key column(s)
name. The foreign key name(s) referencing the other side is the
concatenation of the owner property name, _, and the other side primary
key column(s) name. A unique constraint is added to the foreign key
referencing the other side table to reflect the one to many.
Lets have a look now how collections are mapped using Hibernate
mapping files. In this case the first step is to chose the right mapping
element. It depends on the type of interface. For example, a
<set> element is used for mapping properties of
type Set.
Example 7.4. Mapping a Set using <set>
<class name="Product">
<id name="serialNumber" column="productSerialNumber"/>
<set name="parts">
<key column="productSerialNumber" not-null="true"/>
<one-to-many class="Part"/>
</set>
</class>
In Example 7.4, “Mapping a Set using <set>” a
one-to-many association links the
Product and Part entities. This
association requires the existence of a foreign key column and possibly an
index column to the Part table. This mapping loses
certain semantics of normal Java collections:
An instance of the contained entity class cannot belong to more than one instance of the collection.
An instance of the contained entity class cannot appear at more than one value of the collection index.
Looking closer at the used <one-to-many>
tag we see that it has the following options.
Example 7.5. options of <one-to-many> element
<one-to-many
class=
"ClassName"
not-fo
und="ignore|exception"
entity
-name="EntityName"
node="element-name"
embed-xml="true|false"
/>
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The <one-to-many> element does not need to
declare any columns. Nor is it necessary to specify the
table name anywhere.
Warning
If the foreign key column of a
<one-to-many> association is declared
NOT NULL, you must declare the
<key> mapping
not-null="true" or use a bidirectional
association with the collection mapping marked
inverse="true". See Section 7.3.2, “Bidirectional associations”.
Apart from the <set> tag as shown in Example 7.4, “Mapping a Set using <set>”, there is also
<list>, <map>,
<bag>, <array> and
<primitive-array> mapping elements. The
<map> element is representative:
Example 7.6. Elements of the <map> mapping
<map
name="propertyName"
table="table_name"
schema="schema_name"
lazy="true
|extra|false"
inverse="t
rue|false"
cascade="a
ll|none|save-update|delete|all-delete-orphan|delete-orphan"
sort="unso
rted|natural|comparatorClass"
order-by="
column_name asc|desc"
where="arb
itrary sql where condition"
fetch="joi
n|select|subselect"
batch-size
="N"
access="fi
eld|property|ClassName"
optimistic
-lock="true|false"
mutable="t
rue|false"
node="element-name|."
embed-xml="true|false"
>
<key .... />
<map-key .... />
<element .... />
</map>
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After exploring the basic mapping of collections in the preceding paragraphs we will now focus details like physical mapping considerations, indexed collections and collections of value types.
On the database level collection instances are distinguished by
the foreign key of the entity that owns the collection. This foreign key
is referred to as the collection key column, or
columns, of the collection table. The collection key column is mapped by
the @JoinColumn annotation respectively the
<key> XML element.
There can be a nullability constraint on the foreign key column. For most collections, this is implied. For unidirectional one-to-many associations, the foreign key column is nullable by default, so you may need to specify
@JoinColumn(nullable=false)
or
<key column="productSerialNumber" not-null="true"/>
The foreign key constraint can use ON DELETE
CASCADE. In XML this can be expressed via:
<key column="productSerialNumber" on-delete="cascade"/>
In annotations the Hibernate specific annotation @OnDelete has to be used.
@OnDelete(action=OnDeleteAction.CASCADE)
See Section 5.1.11.3, “Key” for more information
about the <key> element.
In the following paragraphs we have a closer at the indexed
collections List and Map
how the their index can be mapped in Hibernate.
Lists can be mapped in two different ways:
as ordered lists, where the order is not materialized in the database
as indexed lists, where the order is materialized in the database
To order lists in memory, add
@javax.persistence.OrderBy to your property. This
annotation takes as parameter a list of comma separated properties (of
the target entity) and orders the collection accordingly (eg
firstname asc, age desc), if the string is empty, the
collection will be ordered by the primary key of the target
entity.
Example 7.7. Ordered lists using @OrderBy
@Entity
public class Customer {
@Id @GeneratedValue public Integer getId() { return id; }
public void setId(Integer id) { this.id = id; }
private Integer id;
@OneToMany(mappedBy="customer")
@OrderBy("number")
public List<Order> getOrders() { return orders; }
public void setOrders(List<Order> orders) { this.orders = orders; }
private List<Order> orders;
}
@Entity
public class Order {
@Id @GeneratedValue public Integer getId() { return id; }
public void setId(Integer id) { this.id = id; }
private Integer id;
public String getNumber() { return number; }
public void setNumber(String number) { this.number = number; }
private String number;
@ManyToOne
public Customer getCustomer() { return customer; }
public void setCustomer(Customer customer) { this.customer = customer; }
private Customer number;
}
-- Table schema
|-------------| |----------|
| Order | | Customer |
|-------------| |----------|
| id | | id |
| number | |----------|
| customer_id |
|-------------|
To store the index value in a dedicated column, use the
@javax.persistence.OrderColumn annotation on
your property. This annotations describes the column name and
attributes of the column keeping the index value. This column is
hosted on the table containing the association foreign key. If the
column name is not specified, the default is the name of the
referencing property, followed by underscore, followed by
ORDER (in the following example, it would be
orders_ORDER).
Example 7.8. Explicit index column using
@OrderColumn
@Entity
public class Customer {
@Id @GeneratedValue public Integer getId() { return id; }
public void setId(Integer id) { this.id = id; }
private Integer id;
@OneToMany(mappedBy="customer")
@OrderColumn(name="orders_index")
public List<Order> getOrders() { return orders; }
public void setOrders(List<Order> orders) { this.orders = orders; }
private List<Order> orders;
}
@Entity
public class Order {
@Id @GeneratedValue public Integer getId() { return id; }
public void setId(Integer id) { this.id = id; }
private Integer id;
public String getNumber() { return number; }
public void setNumber(String number) { this.number = number; }
private String number;
@ManyToOne
public Customer getCustomer() { return customer; }
public void setCustomer(Customer customer) { this.customer = customer; }
private Customer number;
}
-- Table schema
|--------------| |----------|
| Order | | Customer |
|--------------| |----------|
| id | | id |
| number | |----------|
| customer_id |
| orders_order |
|--------------|
Note
We recommend you to convert the legacy
@org.hibernate.annotations.IndexColumn usages to
@OrderColumn unless you are making use of the
base property. The base property lets you define
the index value of the first element (aka as base index). The usual
value is 0 or 1. The default
is 0 like in Java.
Looking again at the Hibernate mapping file equivalent, the
index of an array or list is always of type integer
and is mapped using the <list-index> element.
The mapped column contains sequential integers that are numbered from
zero by default.
Example 7.9. index-list element for indexed collections in xml mapping
<list-index
column="column_name"
base="0|1|..."/>
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If your table does not have an index column, and you still wish
to use List as the property type, you can map the
property as a Hibernate <bag>. A bag does
not retain its order when it is retrieved from the database, but it
can be optionally sorted or ordered.
The question with Maps is where the key
value is stored. There are everal options. Maps can borrow their keys
from one of the associated entity properties or have dedicated columns
to store an explicit key.
To use one of the target entity property as a key of the map,
use @MapKey(name="myProperty"), where
myProperty is a property name in the target entity.
When using @MapKey without the name attribuate, the
target entity primary key is used. The map key uses the same column as
the property pointed out. There is no additional column defined to
hold the map key, because the map key represent a target property. Be
aware that once loaded, the key is no longer kept in sync with the
property. In other words, if you change the property value, the key
will not change automatically in your Java model.
Example 7.10. Use of target entity property as map key via
@MapKey
@Entity
public class Customer {
@Id @GeneratedValue public Integer getId() { return id; }
public void setId(Integer id) { this.id = id; }
private Integer id;
@OneToMany(mappedBy="customer")
@MapKey(name="number")
public Map<String,Order> getOrders() { return orders; }
public void setOrders(Map<String,Order> order) { this.orders = orders; }
private Map<String,Order> orders;
}
@Entity
public class Order {
@Id @GeneratedValue public Integer getId() { return id; }
public void setId(Integer id) { this.id = id; }
private Integer id;
public String getNumber() { return number; }
public void setNumber(String number) { this.number = number; }
private String number;
@ManyToOne
public Customer getCustomer() { return customer; }
public void setCustomer(Customer customer) { this.customer = customer; }
private Customer number;
}
-- Table schema
|-------------| |----------|
| Order | | Customer |
|-------------| |----------|
| id | | id |
| number | |----------|
| customer_id |
|-------------|
Alternatively the map key is mapped to a dedicated column or columns. In order to customize the mapping use one of the following annotations:
@
MapKeyColumnif the map key is a basic type. If you don't specify the column name, the name of the property followed by underscore followed byKEYis used (for exampleorders_KEY).@MapKeyEnumerated/@MapKeyTemporalif the map key type is respectively an enum or aDate.@MapKeyJoinColumn/@MapKeyJoinColumnsif the map key type is another entity.@AttributeOverride/@AttributeOverrideswhen the map key is a embeddable object. Usekey.as a prefix for your embeddable object property names.
You can also use @MapKeyClass to define
the type of the key if you don't use generics.
Example 7.11. Map key as basic type using
@MapKeyColumn
@Entity
public class Customer {
@Id @GeneratedValue public Integer getId() { return id; }
public void setId(Integer id) { this.id = id; }
private Integer id;
@OneToMany @JoinTable(name="Cust_Order")
@MapKeyColumn(name="orders_number")
public Map<String,Order> getOrders() { return orders; }
public void setOrders(Map<String,Order> orders) { this.orders = orders; }
private Map<String,Order> orders;
}
@Entity
public class Order {
@Id @GeneratedValue public Integer getId() { return id; }
public void setId(Integer id) { this.id = id; }
private Integer id;
public String getNumber() { return number; }
public void setNumber(String number) { this.number = number; }
private String number;
@ManyToOne
public Customer getCustomer() { return customer; }
public void setCustomer(Customer customer) { this.customer = customer; }
private Customer number;
}
-- Table schema
|-------------| |----------| |---------------|
| Order | | Customer | | Cust_Order |
|-------------| |----------| |---------------|
| id | | id | | customer_id |
| number | |----------| | order_id |
| customer_id | | orders_number |
|-------------| |---------------|
Note
We recommend you to migrate from
@org.hibernate.annotations.MapKey /
@org.hibernate.annotation.MapKeyManyToMany to
the new standard approach described above
Using Hibernate mapping files there exists equivalent concepts
to the descibed annotations. You have to use
<map-key>,
<map-key-many-to-many> and
<composite-map-key>.
<map-key> is used for any basic type,
<map-key-many-to-many> for an entity
reference and <composite-map-key> for a
composite type.
Example 7.12. map-key xml mapping element
<map-key
column="column_name"
formula="any SQL expression"
type="type_name"
node="@attribute-name"
length="N"/>
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Example 7.13. map-key-many-to-many
<map-key-many-to-many
column="column_name"
formula="any SQL expression"
class="ClassName"
/>
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In some situations you don't need to associate two entities but
simply create a collection of basic types or embeddable objects. Use the
@ElementCollection for this case.
Example 7.14. Collection of basic types mapped via
@ElementCollection
@Entity
public class User {
[...]
public String getLastname() { ...}
@ElementCollection
@CollectionTable(name="Nicknames", joinColumns=@JoinColumn(name="user_id"))
@Column(name="nickname")
public Set<String> getNicknames() { ... }
}
The collection table holding the collection data is set using the
@CollectionTable annotation. If omitted the
collection table name defaults to the concatenation of the name of the
containing entity and the name of the collection attribute, separated by
an underscore. In our example, it would be
User_nicknames.
The column holding the basic type is set using the
@Column annotation. If omitted, the column name
defaults to the property name: in our example, it would be
nicknames.
But you are not limited to basic types, the collection type can be
any embeddable object. To override the columns of the embeddable object
in the collection table, use the
@AttributeOverride annotation.
Example 7.15. @ElementCollection for embeddable objects
@Entity
public class User {
[...]
public String getLastname() { ...}
@ElementCollection
@CollectionTable(name="Addresses", joinColumns=@JoinColumn(name="user_id"))
@AttributeOverrides({
@AttributeOverride(name="street1", column=@Column(name="fld_street"))
})
public Set<Address> getAddresses() { ... }
}
@Embeddable
public class Address {
public String getStreet1() {...}
[...]
}
Such an embeddable object cannot contains a collection itself.
Note
in @AttributeOverride, you must use the
value. prefix to override properties of the
embeddable object used in the map value and the
key. prefix to override properties of the
embeddable object used in the map key.
@Entity
public class User {
@ElementCollection
@AttributeOverrides({
@AttributeOverride(name="key.street1", column=@Column(name="fld_street")),
@AttributeOverride(name="value.stars", column=@Column(name="fld_note"))
})
public Map<Address,Rating> getFavHomes() { ... }
Note
We recommend you to migrate from
@org.hibernate.annotations.CollectionOfElements
to the new @ElementCollection
annotation.
Using the mapping file approach a collection of values is mapped
using the <element> tag. For example:
Example 7.16. <element> tag for collection values using mapping files
<element
column="column_name"
formula="any SQL expression"
type="typename"
length="L"
precision="P"
scale="S"
not-null="true|false"
unique="true|false"
node="element-name"
/>
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Hibernate supports collections implementing
java.util.SortedMap and
java.util.SortedSet. With annotations you declare a
sort comparator using @Sort. You chose between the
comparator types unsorted, natural or custom. If you want to use your
own comparator implementation, you'll also have to specify the
implementation class using the comparator attribute.
Note that you need to use either a SortedSet or a
SortedMap interface.
Example 7.17. Sorted collection with @Sort
@OneToMany(cascade=CascadeType.ALL, fetch=FetchType.EAGER)
@JoinColumn(name="CUST_ID")
@Sort(type = SortType.COMPARATOR, comparator = TicketComparator.class)
public SortedSet<Ticket> getTickets() {
return tickets;
}
Using Hibernate mapping files you specify a comparator in the
mapping file with <sort>:
Example 7.18. Sorted collection using xml mapping
<set name="aliases"
table="person_aliases"
sort="natural">
<key column="person"/>
<element column="name" type="string"/>
</set>
<map name="holidays" sort="my.custom.HolidayComparator">
<key column="year_id"/>
<map-key column="hol_name" type="string"/>
<element column="hol_date" type="date"/>
</map>
Allowed values of the sort attribute are
unsorted, natural and the name of
a class implementing java.util.Comparator.
Tip
Sorted collections actually behave like
java.util.TreeSet or
java.util.TreeMap.
If you want the database itself to order the collection elements,
use the order-by attribute of set,
bag or map mappings. This solution
is implemented using LinkedHashSet or
LinkedHashMap and performs the ordering in the SQL
query and not in the memory.
Example 7.19. Sorting in database using order-by
<set name="aliases" table="person_aliases" order-by="lower(name) asc">
<key column="person"/>
<element column="name" type="string"/>
</set>
<map name="holidays" order-by="hol_date, hol_name">
<key column="year_id"/>
<map-key column="hol_name" type="string"/>
<element column="hol_date type="date"/>
</map>
Note
The value of the order-by attribute is an SQL
ordering, not an HQL ordering.
Associations can even be sorted by arbitrary criteria at runtime
using a collection filter():
Example 7.20. Sorting via a query filter
sortedUsers = s.createFilter( group.getUsers(), "order by this.name" ).list();
A bidirectional association allows navigation from both "ends" of the association. Two kinds of bidirectional association are supported:
- one-to-many
set or bag valued at one end and single-valued at the other
- many-to-many
set or bag valued at both ends
Often there exists a many to one association which is the owner
side of a bidirectional relationship. The corresponding one to many
association is in this case annotated by
@OneToMany(mappedBy=...)
Example 7.21. Bidirectional one to many with many to one side as association owner
@Entity
public class Troop {
@OneToMany(mappedBy="troop")
public Set<Soldier> getSoldiers() {
...
}
@Entity
public class Soldier {
@ManyToOne
@JoinColumn(name="troop_fk")
public Troop getTroop() {
...
}
Troop has a bidirectional one to many
relationship with Soldier through the
troop property. You don't have to (must not) define
any physical mapping in the mappedBy side.
To map a bidirectional one to many, with the one-to-many side as
the owning side, you have to remove the mappedBy
element and set the many to one @JoinColumn as
insertable and updatable to false. This solution is not optimized and
will produce additional UPDATE statements.
Example 7.22. Bidirectional associtaion with one to many side as owner
@Entity
public class Troop {
@OneToMany
@JoinColumn(name="troop_fk") //we need to duplicate the physical information
public Set<Soldier> getSoldiers() {
...
}
@Entity
public class Soldier {
@ManyToOne
@JoinColumn(name="troop_fk", insertable=false, updatable=false)
public Troop getTroop() {
...
}
How does the mappping of a bidirectional mapping look like in
Hibernate mapping xml? There you define a bidirectional one-to-many
association by mapping a one-to-many association to the same table
column(s) as a many-to-one association and declaring the many-valued end
inverse="true".
Example 7.23. Bidirectional one to many via Hibernate mapping files
<class name="Parent">
<id name="id" column="parent_id"/>
....
<set name="children" inverse="true">
<key column="parent_id"/>
<one-to-many class="Child"/>
</set>
</class>
<class name="Child">
<id name="id" column="child_id"/>
....
<many-to-one name="parent"
class="Parent"
column="parent_id"
not-null="true"/>
</class>
Mapping one end of an association with
inverse="true" does not affect the operation of
cascades as these are orthogonal concepts.
A many-to-many association is defined logically using the
@ManyToMany annotation. You also have to describe the
association table and the join conditions using the
@JoinTable annotation. If the association is
bidirectional, one side has to be the owner and one side has to be the
inverse end (ie. it will be ignored when updating the relationship
values in the association table):
Example 7.24. Many to many association via @ManyToMany
@Entity
public class Employer implements Serializable {
@ManyToMany(
targetEntity=org.hibernate.test.metadata.manytomany.Employee.class,
cascade={CascadeType.PERSIST, CascadeType.MERGE}
)
@JoinTable(
name="EMPLOYER_EMPLOYEE",
joinColumns=@JoinColumn(name="EMPER_ID"),
inverseJoinColumns=@JoinColumn(name="EMPEE_ID")
)
public Collection getEmployees() {
return employees;
}
...
}
@Entity
public class Employee implements Serializable {
@ManyToMany(
cascade = {CascadeType.PERSIST, CascadeType.MERGE},
mappedBy = "employees",
targetEntity = Employer.class
)
public Collection getEmployers() {
return employers;
}
}
In this example @JoinTable defines a
name, an array of join columns, and an array of
inverse join columns. The latter ones are the columns of the association
table which refer to the Employee primary key
(the "other side"). As seen previously, the other side don't have to
(must not) describe the physical mapping: a simple
mappedBy argument containing the owner side property
name bind the two.
As any other annotations, most values are guessed in a many to many relationship. Without describing any physical mapping in a unidirectional many to many the following rules applied. The table name is the concatenation of the owner table name, _ and the other side table name. The foreign key name(s) referencing the owner table is the concatenation of the owner table name, _ and the owner primary key column(s). The foreign key name(s) referencing the other side is the concatenation of the owner property name, _, and the other side primary key column(s). These are the same rules used for a unidirectional one to many relationship.
Example 7.25. Default values for @ManyToMany
(uni-directional)
@Entity
public class Store {
@ManyToMany(cascade = CascadeType.PERSIST)
public Set<City> getImplantedIn() {
...
}
}
@Entity
public class City {
... //no bidirectional relationship
}
A Store_City is used as the join table. The
Store_id column is a foreign key to the
Store table. The implantedIn_id
column is a foreign key to the City table.
Without describing any physical mapping in a bidirectional many to many the following rules applied. The table name is the concatenation of the owner table name, _ and the other side table name. The foreign key name(s) referencing the owner table is the concatenation of the other side property name, _, and the owner primary key column(s). The foreign key name(s) referencing the other side is the concatenation of the owner property name, _, and the other side primary key column(s). These are the same rules used for a unidirectional one to many relationship.
Example 7.26. Default values for @ManyToMany
(bi-directional)
@Entity
public class Store {
@ManyToMany(cascade = {CascadeType.PERSIST, CascadeType.MERGE})
public Set<Customer> getCustomers() {
...
}
}
@Entity
public class Customer {
@ManyToMany(mappedBy="customers")
public Set<Store> getStores() {
...
}
}
A Store_Customer is used as the join table. The
stores_id column is a foreign key to the
Store table. The customers_id
column is a foreign key to the Customer table.
Using Hibernate mapping files you can map a bidirectional many-to-many association by mapping two many-to-many associations to the same database table and declaring one end as inverse.
Note
You cannot select an indexed collection.
Example 7.27, “Many to many association using Hibernate mapping files” shows a bidirectional many-to-many association that illustrates how each category can have many items and each item can be in many categories:
Example 7.27. Many to many association using Hibernate mapping files
<class name="Category">
<id name="id" column="CATEGORY_ID"/>
...
<bag name="items" table="CATEGORY_ITEM">
<key column="CATEGORY_ID"/>
<many-to-many class="Item" column="ITEM_ID"/>
</bag>
</class>
<class name="Item">
<id name="id" column="ITEM_ID"/>
...
<!-- inverse end -->
<bag name="categories" table="CATEGORY_ITEM" inverse="true">
<key column="ITEM_ID"/>
<many-to-many class="Category" column="CATEGORY_ID"/>
</bag>
</class>
Changes made only to the inverse end of the association are not persisted. This means that Hibernate has two representations in memory for every bidirectional association: one link from A to B and another link from B to A. This is easier to understand if you think about the Java object model and how a many-to-many relationship in Javais created:
Example 7.28. Effect of inverse vs. non-inverse side of many to many associations
category.getItems().add(item); // The category now "knows" about the relationship
item.getCategories().add(category); // The item now "knows" about the relationship
session.persist(item); // The relationship won't be saved!
session.persist(category); // The relationship will be saved
The non-inverse side is used to save the in-memory representation to the database.
There are some additional considerations for bidirectional
mappings with indexed collections (where one end is represented as a
<list> or <map>) when
using Hibernate mapping files. If there is a property of the child class
that maps to the index column you can use
inverse="true" on the collection mapping:
Example 7.29. Bidirectional association with indexed collection
<class name="Parent">
<id name="id" column="parent_id"/>
....
<map name="children" inverse="true">
<key column="parent_id"/>
<map-key column="name"
type="string"/>
<one-to-many class="Child"/>
</map>
</class>
<class name="Child">
<id name="id" column="child_id"/>
....
<property name="name"
not-null="true"/>
<many-to-one name="parent"
class="Parent"
column="parent_id"
not-null="true"/>
</class>
If there is no such property on the child class, the association
cannot be considered truly bidirectional. That is, there is information
available at one end of the association that is not available at the
other end. In this case, you cannot map the collection
inverse="true". Instead, you could use the following
mapping:
Example 7.30. Bidirectional association with indexed collection, but no index column
<class name="Parent">
<id name="id" column="parent_id"/>
....
<map name="children">
<key column="parent_id"
not-null="true"/>
<map-key column="name"
type="string"/>
<one-to-many class="Child"/>
</map>
</class>
<class name="Child">
<id name="id" column="child_id"/>
....
<many-to-one name="parent"
class="Parent"
column="parent_id"
insert="false"
update="false"
not-null="true"/>
</class>
Note that in this mapping, the collection-valued end of the association is responsible for updates to the foreign key.
There are three possible approaches to mapping a ternary
association. One approach is to use a Map with an
association as its index:
Example 7.31. Ternary association mapping
@Entity
public class Company {
@Id
int id;
...
@OneToMany // unidirectional
@MapKeyJoinColumn(name="employee_id")
Map<Employee, Contract> contracts;
}
// or
<map name="contracts">
<key column="employer_id" not-null="true"/>
<map-key-many-to-many column="employee_id" class="Employee"/>
<one-to-many class="Contract"/>
</map>
A second approach is to remodel the association as an entity class. This is the most common approach. A final alternative is to use composite elements, which will be discussed later.
The majority of the many-to-many associations and collections of values shown previously all map to tables with composite keys, even though it has been suggested that entities should have synthetic identifiers (surrogate keys). A pure association table does not seem to benefit much from a surrogate key, although a collection of composite values might. For this reason Hibernate provides a feature that allows you to map many-to-many associations and collections of values to a table with a surrogate key.
The <idbag> element lets you map a
List (or Collection) with bag
semantics. For example:
<idbag name="lovers" table="LOVERS">
<collection-id column="ID" type="long">
<generator class="sequence"/>
</collection-id>
<key column="PERSON1"/>
<many-to-many column="PERSON2" class="Person" fetch="join"/>
</idbag>
An <idbag> has a synthetic id generator,
just like an entity class. A different surrogate key is assigned to each
collection row. Hibernate does not, however, provide any mechanism for
discovering the surrogate key value of a particular row.
The update performance of an <idbag>
supersedes a regular <bag>. Hibernate can
locate individual rows efficiently and update or delete them
individually, similar to a list, map or set.
In the current implementation, the native
identifier generation strategy is not supported for
<idbag> collection identifiers.
This section covers collection examples.
The following class has a collection of Child
instances:
Example 7.32. Example classes Parent and
Child
public class Parent {
private long id;
private Set<Child> children;
// getter/setter
...
}
public class Child {
private long id;
private String name
// getter/setter
...
}
If each child has, at most, one parent, the most natural mapping is a one-to-many association:
Example 7.33. One to many unidirectional Parent-Child
relationship using annotations
public class Parent {
@Id
@GeneratedValue
private long id;
@OneToMany
private Set<Child> children;
// getter/setter
...
}
public class Child {
@Id
@GeneratedValue
private long id;
private String name;
// getter/setter
...
}
Example 7.34. One to many unidirectional Parent-Child
relationship using mapping files
<hibernate-mapping>
<class name="Parent">
<id name="id">
<generator class="sequence"/>
</id>
<set name="children">
<key column="parent_id"/>
<one-to-many class="Child"/>
</set>
</class>
<class name="Child">
<id name="id">
<generator class="sequence"/>
</id>
<property name="name"/>
</class>
</hibernate-mapping>
This maps to the following table definitions:
Example 7.35. Table definitions for unidirectional
Parent-Child
relationship
create table parent ( id bigint not null primary key )
create table child ( id bigint not null primary key, name varchar(255), parent_id bigint )
alter table child add constraint childfk0 (parent_id) references parent
If the parent is required, use a bidirectional one-to-many association:
Example 7.36. One to many bidirectional Parent-Child
relationship using annotations
public class Parent {
@Id
@GeneratedValue
private long id;
@OneToMany(mappedBy="parent")
private Set<Child> children;
// getter/setter
...
}
public class Child {
@Id
@GeneratedValue
private long id;
private String name;
@ManyToOne
private Parent parent;
// getter/setter
...
}
Example 7.37. One to many bidirectional Parent-Child
relationship using mapping files
<hibernate-mapping>
<class name="Parent">
<id name="id">
<generator class="sequence"/>
</id>
<set name="children" inverse="true">
<key column="parent_id"/>
<one-to-many class="Child"/>
</set>
</class>
<class name="Child">
<id name="id">
<generator class="sequence"/>
</id>
<property name="name"/>
<many-to-one name="parent" class="Parent" column="parent_id" not-null="true"/>
</class>
</hibernate-mapping>
Notice the NOT NULL constraint:
Example 7.38. Table definitions for bidirectional
Parent-Child
relationship
create table parent ( id bigint not null primary key )
create table child ( id bigint not null
primary key,
name varchar(255),
parent_id bigint not null )
alter table child add constraint childfk0 (parent_id) references parent
Alternatively, if this association must be unidirectional you can
enforce the NOT NULL constraint.
Example 7.39. Enforcing NOT NULL constraint in unidirectional relation using annotations
public class Parent {
@Id
@GeneratedValue
private long id;
@OneToMany(optional=false)
private Set<Child> children;
// getter/setter
...
}
public class Child {
@Id
@GeneratedValue
private long id;
private String name;
// getter/setter
...
}
Example 7.40. Enforcing NOT NULL constraint in unidirectional relation using mapping files
<hibernate-mapping>
<class name="Parent">
<id name="id">
<generator class="sequence"/>
</id>
<set name="children">
<key column="parent_id" not-null="true"/>
<one-to-many class="Child"/>
</set>
</class>
<class name="Child">
<id name="id">
<generator class="sequence"/>
</id>
<property name="name"/>
</class>
</hibernate-mapping>
On the other hand, if a child has multiple parents, a many-to-many association is appropriate.
Example 7.41. Many to many Parent-Child relationship
using annotations
public class Parent {
@Id
@GeneratedValue
private long id;
@ManyToMany
private Set<Child> children;
// getter/setter
...
}
public class Child {
@Id
@GeneratedValue
private long id;
private String name;
// getter/setter
...
}
Example 7.42. Many to many Parent-Child relationship
using mapping files
<hibernate-mapping>
<class name="Parent">
<id name="id">
<generator class="sequence"/>
</id>
<set name="children" table="childset">
<key column="parent_id"/>
<many-to-many class="Child" column="child_id"/>
</set>
</class>
<class name="Child">
<id name="id">
<generator class="sequence"/>
</id>
<property name="name"/>
</class>
</hibernate-mapping>
Table definitions:
Example 7.43. Table definitions for many to many releationship
create table parent ( id bigint not null primary key )
create table child ( id bigint not null primary key, name varchar(255) )
create table childset ( parent_id bigint not null,
child_id bigint not null,
primary key ( parent_id, child_id ) )
alter table childset add constraint childsetfk0 (parent_id) references parent
alter table childset add constraint childsetfk1 (child_id) references childFor more examples and a complete explanation of a parent/child relationship mapping, see Chapter 24, Example: Parent/Child for more information. Even more complex association mappings are covered in the next chapter.