Chapter 6. Collection Mapping
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## Chapter 6. Collection Mapping

## 6.1. Persistent collections

Hibernate requires that persistent collection-valued fields be declared as an interface type, for example:

    public class Product {
        private String serialNumber;
        private Set parts = new HashSet();

        public Set getParts() { return parts; }
        void setParts(Set parts) { this.parts = parts; }
        public String getSerialNumber() { return serialNumber; }
        void setSerialNumber(String sn) { serialNumber = sn; }
    }

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! (Where "anything you like" means you will have to write an implementation of `org.hibernate.usertype.UserCollectionType`.)

Notice how we initialized the instance variable 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()`, for example - Hibernate will actually replace the `HashSet` with an instance of Hibernate's own implementation of `Set`. Watch out for errors like this:

    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 upon the interface type.

Collections instances have the usual behavior of value types. They are automatically persisted when referenced by a persistent object and 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 may not 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.

You shouldn't have to worry much about any of this. Use persistent collections the same way you use ordinary Java collections. Just make sure you understand the semantics of bidirectional associations (discussed later).

## 6.2. Collection mappings

The Hibernate mapping element used for mapping a collection depends upon the type of the interface. For example, a `<set>` element is used for mapping properties of type `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>

Apart from `<set>`, there is also `<list>`, `<map>`, `<bag>`, `<array>` and `<primitive-array>` mapping elements. The `<map>` element is representative:

    <map
        name="propertyName"                                         (1)
        table="table_name"                                          (2)
        schema="schema_name"                                        (3)
        lazy="true|extra|false"                                     (4)
        inverse="true|false"                                        (5)
        cascade="all|none|save-update|delete|all-delete-orphan|delet(6)e-orphan"
        sort="unsorted|natural|comparatorClass"                     (7)
        order-by="column_name asc|desc"                             (8)
        where="arbitrary sql where condition"                       (9)
        fetch="join|select|subselect"                               (10)
        batch-size="N"                                              (11)
        access="field|property|ClassName"                           (12)
        optimistic-lock="true|false"                                (13)
        mutable="true|false"                                        (14)
        node="element-name|."
        embed-xml="true|false"
    >

        <key .... />
        <map-key .... />
        <element .... />
    </map>

(1) | `name` the collection property name
(2) | `table` (optional - defaults to property name) the name of the collection table (not used for one-to-many associations)
(3) | `schema` (optional) the name of a table schema to override the schema declared on the root element
(4) | `lazy` (optional - defaults to `true`) may be used to disable lazy fetching and specify that the association is always eagerly fetched, or to enable "extra-lazy" fetching where most operations do not initialize the collection (suitable for very large collections)
(5) | `inverse` (optional - defaults to `false`) mark this collection as the "inverse" end of a bidirectional association
(6) | `cascade` (optional - defaults to `none`) enable operations to cascade to child entities
(7) | `sort` (optional) specify a sorted collection with `natural` sort order, or a given comparator class
(8) | `order-by` (optional, JDK1.4 only) specify a table column (or columns) that define the iteration order of the `Map`, `Set` or bag, together with an optional `asc` or `desc`
(9) | `where` (optional) specify an arbitrary SQL `WHERE` condition to be used when retrieving or removing the collection (useful if the collection should contain only a subset of the available data)
(10) | `fetch` (optional, defaults to `select`) Choose between outer-join fetching, fetching by sequential select, and fetching by sequential subselect.
(11) | `batch-size` (optional, defaults to `1`) specify a "batch size" for lazily fetching instances of this collection.
(12) | `access` (optional - defaults to `property`): The strategy Hibernate should use for accessing the collection property value.
(13) | `optimistic-lock` (optional - defaults to `true`): Species that changes to the state of the collection results in increment of the owning entity's version. (For one to many associations, it is often reasonable to disable this setting.)
(14) | `mutable` (optional - defaults to `true`): A value of `false` specifies that the elements of the collection never change (a minor performance optimization in some cases).

### 6.2.1. Collection foreign keys

Collection instances are distinguished in the database 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 `<key>` element.

There may 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 might need to specify `not-null="true"`.

    <key column="productSerialNumber" not-null="true"/>

The foreign key constraint may use `ON DELETE CASCADE`.

    <key column="productSerialNumber" on-delete="cascade"/>

See the previous chapter for a full definition of the `<key>` element.

### 6.2.2. Collection elements

Collections may contain almost any other Hibernate type, including all basic types, custom types, components, and of course, references to other entities. This is an important distinction: an object in a collection might be handled with "value" semantics (its lifecycle fully depends on the collection owner) or it might be a reference to another entity, with its own lifecycle. In the latter case, only the "link" between the two objects is considered to be state held by the collection.

The contained type is referred to as the collection element type. Collection elements are mapped by `<element>` or `<composite-element>`, or in the case of entity references, with `<one-to-many>` or `<many-to-many>`. The first two map elements with value semantics, the next two are used to map entity associations.

### 6.2.3. Indexed collections

All collection mappings, except those with set and bag semantics, need an index column in the collection table - a column that maps to an array index, or `List` index, or `Map` key. The index of a `Map` may be of any basic type, mapped with `<map-key>`, it may be an entity reference mapped with `<map-key-many-to-many>`, or it may be a composite type, mapped with `<composite-map-key>`. 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 (numbered from zero, by default).

    <list-index
            column="column_name"                (1)
            base="0|1|..."/>

(1) | `column_name` (required): The name of the column holding the collection index values.
(1) | `base` (optional, defaults to `0`): The value of the index column that corresponds to the first element of the list or array.

    <map-key
            column="column_name"                (1)
            formula="any SQL expression"        (2)
            type="type_name"                    (3)
            node="@attribute-name"
            length="N"/>

(1) | `column` (optional): The name of the column holding the collection index values.
(2) | `formula` (optional): A SQL formula used to evaluate the key of the map.
(3) | `type` (reguired): The type of the map keys.

    <map-key-many-to-many
            column="column_name"                (1)
            formula="any SQL expression"        (2)(3)
            class="ClassName"
    />

(1) | `column` (optional): The name of the foreign key column for the collection index values.
(2) | `formula` (optional): A SQL formula used to evaluate the foreign key of the map key.
(3) | `class` (required): The entity class used as the map key.

If your table doesn't have an index column, and you still wish to use `List` as the property type, you should map the property as a Hibernate <bag>. A bag does not retain its order when it is retrieved from the database, but it may be optionally sorted or ordered.

There are quite a range of mappings that can be generated for collections, covering many common relational models. We suggest you experiment with the schema generation tool to get a feeling for how various mapping declarations translate to database tables.

### 6.2.4. Collections of values and many-to-many associations

Any collection of values or many-to-many association requires a dedicated collection table with a foreign key column or columns, collection element column or columns and possibly an index column or columns.

For a collection of values, we use the `<element>` tag.

    <element
            column="column_name"                     (1)
            formula="any SQL expression"             (2)
            type="typename"                          (3)
            length="L"
            precision="P"
            scale="S"
            not-null="true|false"
            unique="true|false"
            node="element-name"
    />

(1) | `column` (optional): The name of the column holding the collection element values.
(2) | `formula` (optional): An SQL formula used to evaluate the element.
(3) | `type` (required): The type of the collection element.

A many-to-many association is specified using the `<many-to-many>` element.

    <many-to-many
            column="column_name"                               (1)
            formula="any SQL expression"                       (2)
            class="ClassName"                                  (3)
            fetch="select|join"                                (4)
            unique="true|false"                                (5)
            not-found="ignore|exception"                       (6)
            entity-name="EntityName"                           (7)
            property-ref="propertyNameFromAssociatedClass"     (8)
            node="element-name"
            embed-xml="true|false"
        />

(1) | `column` (optional): The name of the element foreign key column.
(2) | `formula` (optional): An SQL formula used to evaluate the element foreign key value.
(3) | `class` (required): The name of the associated class.
(4) | `fetch` (optional - defaults to `join`): enables outer-join or sequential select fetching for this association. This is a special case; for full eager fetching (in a single `SELECT`) of an entity and its many-to-many relationships to other entities, you would enable `join` fetching not only of the collection itself, but also with this attribute on the `<many-to-many>` nested element.
(5) | `unique` (optional): Enable the DDL generation of a unique constraint for the foreign-key column. This makes the association multiplicity effectively one to many.
(6) | `not-found` (optional - defaults to `exception`): Specifies how foreign keys that reference missing rows will be handled: `ignore` will treat a missing row as a null association.
(7) | `entity-name` (optional): The entity name of the associated class, as an alternative to `class`.
(8) | `property-ref`: (optional) The name of a property of the associated class that is joined to this foreign key. If not specified, the primary key of the associated class is used.

Some examples, first, a set of strings:

    <set name="names" table="person_names">
        <key column="person_id"/>
        <element column="person_name" type="string"/>
    </set>

A bag containing integers (with an iteration order determined by the `order-by` attribute):

    <bag name="sizes"
            table="item_sizes"
            order-by="size asc">
        <key column="item_id"/>
        <element column="size" type="integer"/>
    </bag>

An array of entities - in this case, a many to many association:

    <array name="addresses"
            table="PersonAddress"
            cascade="persist">
        <key column="personId"/>
        <list-index column="sortOrder"/>
        <many-to-many column="addressId" class="Address"/>
    </array>

A map from string indices to dates:

    <map name="holidays"
            table="holidays"
            schema="dbo"
            order-by="hol_name asc">
        <key column="id"/>
        <map-key column="hol_name" type="string"/>
        <element column="hol_date" type="date"/>
    </map>

A list of components (discussed in the next chapter):

    <list name="carComponents"
            table="CarComponents">
        <key column="carId"/>
        <list-index column="sortOrder"/>
        <composite-element class="CarComponent">
            <property name="price"/>
            <property name="type"/>
            <property name="serialNumber" column="serialNum"/>
        </composite-element>
    </list>

### 6.2.5. One-to-many associations

A one to many association links the tables of two classes via a foreign key, with no intervening collection table. This mapping loses certain semantics of normal Java collections:

- An instance of the contained entity class may not belong to more than one instance of the collection

- An instance of the contained entity class may not appear at more than one value of the collection index

An association from `Product` to `Part` requires existence of a foreign key column and possibly an index column to the `Part` table. A `<one-to-many>` tag indicates that this is a one to many association.

    <one-to-many
            class="ClassName"                                  (1)
            not-found="ignore|exception"                       (2)
            entity-name="EntityName"                           (3)
            node="element-name"
            embed-xml="true|false"
        />

(1) | `class` (required): The name of the associated class.
(2) | `not-found` (optional - defaults to `exception`): Specifies how cached identifiers that reference missing rows will be handled: `ignore` will treat a missing row as a null association.
(3) | `entity-name` (optional): The entity name of the associated class, as an alternative to `class`.

Notice that the `<one-to-many>` element does not need to declare any columns. Nor is it necessary to specify the `table` name anywhere.

Very important note: 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 the discussion of bidirectional associations later in this chapter.

This example shows a map of `Part` entities by name (where `partName` is a persistent property of `Part`). Notice the use of a formula-based index.

    <map name="parts"
            cascade="all">
        <key column="productId" not-null="true"/>
        <map-key formula="partName"/>
        <one-to-many class="Part"/>
    </map>

## 6.3. Advanced collection mappings

### 6.3.1. Sorted collections

Hibernate supports collections implementing `java.util.SortedMap` and `java.util.SortedSet`. You must specify a comparator in the mapping file:

    <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`.

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 only available under JDK 1.4 or higher (it is implemented using `LinkedHashSet` or `LinkedHashMap`). This performs the ordering in the SQL query, not in memory.

    <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 that the value of the `order-by` attribute is an SQL ordering, not a HQL ordering!

Associations may even be sorted by some arbitrary criteria at runtime using a collection `filter()`.

    sortedUsers = s.createFilter( group.getUsers(), "order by this.name" ).list();

### 6.3.2. Bidirectional associations

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, single-valued at the other

many-to-many

  set or bag valued at both ends

You may specify a bidirectional many-to-many association simply by mapping two many-to-many associations to the same database table and declaring one end as inverse (which one is your choice, but it can not be an indexed collection).

Here's an example of a bidirectional many-to-many association; each category can have many items and each item can be in many categories:

    <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="CATEGORY_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 we create a many-to-many relationship in Java:

    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.

You may 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"`.

    <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"` doesn't affect the operation of cascades, these are orthogonal concepts!

### 6.3.3. Bidirectional associations with indexed collections

A bidirectional association where one end is represented as a `<list>` or `<map>` requires special consideration. If there is a property of the child class which maps to the index column, no problem, we can continue using `inverse="true"` on the collection mapping:

    <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>

But, if there is no such property on the child class, we can't think of the association as truly bidirectional (there is information available at one end of the association that is not available at the other end). In this case, we can't map the collection `inverse="true"`. Instead, we could use the following mapping:

    <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. TODO: Does this really result in some unnecessary update statements?

### 6.3.4. Ternary associations

There are three possible approaches to mapping a ternary association. One is to use a `Map` with an association as its index:

    <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>

    <map name="connections">
        <key column="incoming_node_id"/>
        <map-key-many-to-many column="outgoing_node_id" class="Node"/>
        <many-to-many column="connection_id" class="Connection"/>
    </map>

A second approach is to simply remodel the association as an entity class. This is the approach we use most commonly.

A final alternative is to use composite elements, which we will discuss later.

### 6.3.5. `Using an <idbag>`

If you've fully embraced our view that composite keys are a bad thing and that entities should have synthetic identifiers (surrogate keys), then you might find it a bit odd that the many to many associations and collections of values that we've shown so far all map to tables with composite keys! Now, this point is quite arguable; a pure association table doesn't seem to benefit much from a surrogate key (though a collection of composite values might). Nevertheless, 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.

    <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>

As you can see, 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 provide any mechanism to discover the surrogate key value of a particular row, however.

Note that the update performance of an `<idbag>` is much better than a regular `<bag>`! Hibernate can locate individual rows efficiently and update or delete them individually, just like a list, map or set.

In the current implementation, the `native` identifier generation strategy is not supported for `<idbag>` collection identifiers.

## 6.4. Collection examples

The previous sections are pretty confusing. So lets look at an example. This class:

    package eg;
    import java.util.Set;

    public class Parent {
        private long id;
        private Set children;

        public long getId() { return id; }
        private void setId(long id) { this.id=id; }

        private Set getChildren() { return children; }
        private void setChildren(Set children) { this.children=children; }

        ....
        ....
    }

has a collection of `Child` instances. If each child has at most one parent, the most natural mapping is a one-to-many association:

    <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:

    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:

    <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:

    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 you absolutely insist that this association should be unidirectional, you can declare the `NOT NULL` constraint on the `<key>` mapping:

    <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 might have multiple parents, a many-to-many association is appropriate:

    <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:

    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 child

For more examples and a complete walk-through a parent/child relationship mapping, see Chapter 21, Example: Parent/Child.

Even more exotic association mappings are possible, we will catalog all possibilities in the next chapter.

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