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## Chapter 26. Example: Various Mappings

This chapters explores some more complex association mappings.

## 26.1. Employer/Employee

The following model of the relationship between `Employer` and `Employee` uses an entity class (`Employment`) to represent the association. You can do this when there might be more than one period of employment for the same two parties. Components are used to model monetary values and employee names.

Here is a possible mapping document:

    <hibernate-mapping>

        <class name="Employer" table="employers">
            <id name="id">
                <generator class="sequence">
                    <param name="sequence">employer_id_seq</param>
                </generator>
            </id>
            <property name="name"/>
        </class>

        <class name="Employment" table="employment_periods">

            <id name="id">
                <generator class="sequence">
                    <param name="sequence">employment_id_seq</param>
                </generator>
            </id>
            <property name="startDate" column="start_date"/>
            <property name="endDate" column="end_date"/>

            <component name="hourlyRate" class="MonetaryAmount">
                <property name="amount">
                    <column name="hourly_rate" sql-type="NUMERIC(12, 2)"/>
                </property>
                <property name="currency" length="12"/>
            </component>

            <many-to-one name="employer" column="employer_id" not-null="true"/>
            <many-to-one name="employee" column="employee_id" not-null="true"/>

        </class>

        <class name="Employee" table="employees">
            <id name="id">
                <generator class="sequence">
                    <param name="sequence">employee_id_seq</param>
                </generator>
            </id>
            <property name="taxfileNumber"/>
            <component name="name" class="Name">
                <property name="firstName"/>
                <property name="initial"/>
                <property name="lastName"/>
            </component>
        </class>

    </hibernate-mapping>

Here is the table schema generated by `SchemaExport`.

    create table employers (
        id BIGINT not null,
        name VARCHAR(255),
        primary key (id)
    )

    create table employment_periods (
        id BIGINT not null,
        hourly_rate NUMERIC(12, 2),
        currency VARCHAR(12),
        employee_id BIGINT not null,
        employer_id BIGINT not null,
        end_date TIMESTAMP,
        start_date TIMESTAMP,
        primary key (id)
    )

    create table employees (
        id BIGINT not null,
        firstName VARCHAR(255),
        initial CHAR(1),
        lastName VARCHAR(255),
        taxfileNumber VARCHAR(255),
        primary key (id)
    )

    alter table employment_periods
        add constraint employment_periodsFK0 foreign key (employer_id) references employers
    alter table employment_periods
        add constraint employment_periodsFK1 foreign key (employee_id) references employees
    create sequence employee_id_seq
    create sequence employment_id_seq
    create sequence employer_id_seq

## 26.2. Author/Work

Consider the following model of the relationships between `Work`, `Author` and `Person`. In the example, the relationship between `Work` and `Author` is represented as a many-to-many association and the relationship between `Author` and `Person` is represented as one-to-one association. Another possibility would be to have `Author` extend `Person`.

The following mapping document correctly represents these relationships:

    <hibernate-mapping>

        <class name="Work" table="works" discriminator-value="W">

            <id name="id" column="id">
                <generator class="native"/>
            </id>
            <discriminator column="type" type="character"/>

            <property name="title"/>
            <set name="authors" table="author_work">
                <key column name="work_id"/>
                <many-to-many class="Author" column name="author_id"/>
            </set>

            <subclass name="Book" discriminator-value="B">
                <property name="text"/>
            </subclass>

            <subclass name="Song" discriminator-value="S">
                <property name="tempo"/>
                <property name="genre"/>
            </subclass>

        </class>

        <class name="Author" table="authors">

            <id name="id" column="id">
                <!-- The Author must have the same identifier as the Person -->
                <generator class="assigned"/>
            </id>

            <property name="alias"/>
            <one-to-one name="person" constrained="true"/>

            <set name="works" table="author_work" inverse="true">
                <key column="author_id"/>
                <many-to-many class="Work" column="work_id"/>
            </set>

        </class>

        <class name="Person" table="persons">
            <id name="id" column="id">
                <generator class="native"/>
            </id>
            <property name="name"/>
        </class>

    </hibernate-mapping>

There are four tables in this mapping: `works`, `authors` and `persons` hold work, author and person data respectively. `author_work` is an association table linking authors to works. Here is the table schema, as generated by `SchemaExport`:

    create table works (
        id BIGINT not null generated by default as identity,
        tempo FLOAT,
        genre VARCHAR(255),
        text INTEGER,
        title VARCHAR(255),
        type CHAR(1) not null,
        primary key (id)
    )

    create table author_work (
        author_id BIGINT not null,
        work_id BIGINT not null,
        primary key (work_id, author_id)
    )

    create table authors (
        id BIGINT not null generated by default as identity,
        alias VARCHAR(255),
        primary key (id)
    )

    create table persons (
        id BIGINT not null generated by default as identity,
        name VARCHAR(255),
        primary key (id)
    )

    alter table authors
        add constraint authorsFK0 foreign key (id) references persons
    alter table author_work
        add constraint author_workFK0 foreign key (author_id) references authors
    alter table author_work
        add constraint author_workFK1 foreign key (work_id) references works

## 26.3. Customer/Order/Product

In this section we consider a model of the relationships between `Customer`, `Order`, `Line Item` and `Product`. There is a one-to-many association between `Customer` and `Order`, but how can you represent `Order` / `LineItem` / `Product`? In the example, `LineItem` is mapped as an association class representing the many-to-many association between `Order` and `Product`. In Hibernate this is called a composite element.

The mapping document will look like this:

    <hibernate-mapping>

        <class name="Customer" table="customers">
            <id name="id">
                <generator class="native"/>
            </id>
            <property name="name"/>
            <set name="orders" inverse="true">
                <key column="customer_id"/>
                <one-to-many class="Order"/>
            </set>
        </class>

        <class name="Order" table="orders">
            <id name="id">
                <generator class="native"/>
            </id>
            <property name="date"/>
            <many-to-one name="customer" column="customer_id"/>
            <list name="lineItems" table="line_items">
                <key column="order_id"/>
                <list-index column="line_number"/>
                <composite-element class="LineItem">
                    <property name="quantity"/>
                    <many-to-one name="product" column="product_id"/>
                </composite-element>
            </list>
        </class>

        <class name="Product" table="products">
            <id name="id">
                <generator class="native"/>
            </id>
            <property name="serialNumber"/>
        </class>

    </hibernate-mapping>

`customers`, `orders`, `line_items` and `products` hold customer, order, order line item and product data respectively. `line_items` also acts as an association table linking orders with products.

    create table customers (
        id BIGINT not null generated by default as identity,
        name VARCHAR(255),
        primary key (id)
    )

    create table orders (
        id BIGINT not null generated by default as identity,
        customer_id BIGINT,
        date TIMESTAMP,
        primary key (id)
    )

    create table line_items (
        line_number INTEGER not null,
        order_id BIGINT not null,
        product_id BIGINT,
        quantity INTEGER,
        primary key (order_id, line_number)
    )

    create table products (
        id BIGINT not null generated by default as identity,
        serialNumber VARCHAR(255),
        primary key (id)
    )

    alter table orders
        add constraint ordersFK0 foreign key (customer_id) references customers
    alter table line_items
        add constraint line_itemsFK0 foreign key (product_id) references products
    alter table line_items
        add constraint line_itemsFK1 foreign key (order_id) references orders

## 26.4. Miscellaneous example mappings

These examples are available from the Hibernate test suite. You will find many other useful example mappings there by searching in the `test` folder of the Hibernate distribution.

### 26.4.1. "Typed" one-to-one association

    <class name="Person">
        <id name="name"/>
        <one-to-one name="address"
                cascade="all">
            <formula>name</formula>
            <formula>'HOME'</formula>
        </one-to-one>
        <one-to-one name="mailingAddress"
                cascade="all">
            <formula>name</formula>
            <formula>'MAILING'</formula>
        </one-to-one>
    </class>

    <class name="Address" batch-size="2"
            check="addressType in ('MAILING', 'HOME', 'BUSINESS')">
        <composite-id>
            <key-many-to-one name="person"
                    column="personName"/>
            <key-property name="type"
                    column="addressType"/>
        </composite-id>
        <property name="street" type="text"/>
        <property name="state"/>
        <property name="zip"/>
    </class>

### 26.4.2. Composite key example

    <class name="Customer">

        <id name="customerId"
            length="10">
            <generator class="assigned"/>
        </id>

        <property name="name" not-null="true" length="100"/>
        <property name="address" not-null="true" length="200"/>

        <list name="orders"
                inverse="true"
                cascade="save-update">
            <key column="customerId"/>
            <index column="orderNumber"/>
            <one-to-many class="Order"/>
        </list>

    </class>

    <class name="Order" table="CustomerOrder" lazy="true">
        <synchronize table="LineItem"/>
        <synchronize table="Product"/>

        <composite-id name="id"
                class="Order$Id">
            <key-property name="customerId" length="10"/>
            <key-property name="orderNumber"/>
        </composite-id>

        <property name="orderDate"
                type="calendar_date"
                not-null="true"/>

        <property name="total">
            <formula>
                ( select sum(li.quantity*p.price)
                from LineItem li, Product p
                where li.productId = p.productId
                    and li.customerId = customerId
                    and li.orderNumber = orderNumber )
            </formula>
        </property>

        <many-to-one name="customer"
                column="customerId"
                insert="false"
                update="false"
                not-null="true"/>

        <bag name="lineItems"
                fetch="join"
                inverse="true"
                cascade="save-update">
            <key>
                <column name="customerId"/>
                <column name="orderNumber"/>
            </key>
            <one-to-many class="LineItem"/>
        </bag>

    </class>

    <class name="LineItem">

        <composite-id name="id"
                class="LineItem$Id">
            <key-property name="customerId" length="10"/>
            <key-property name="orderNumber"/>
            <key-property name="productId" length="10"/>
        </composite-id>

        <property name="quantity"/>

        <many-to-one name="order"
                insert="false"
                update="false"
                not-null="true">
            <column name="customerId"/>
            <column name="orderNumber"/>
        </many-to-one>

        <many-to-one name="product"
                insert="false"
                update="false"
                not-null="true"
                column="productId"/>

    </class>

    <class name="Product">
        <synchronize table="LineItem"/>

        <id name="productId"
            length="10">
            <generator class="assigned"/>
        </id>

        <property name="description"
            not-null="true"
            length="200"/>
        <property name="price" length="3"/>
        <property name="numberAvailable"/>

        <property name="numberOrdered">
            <formula>
                ( select sum(li.quantity)
                from LineItem li
                where li.productId = productId )
            </formula>
        </property>

    </class>

### 26.4.3. Many-to-many with shared composite key attribute

    <class name="User" table="`User`">
        <composite-id>
            <key-property name="name"/>
            <key-property name="org"/>
        </composite-id>
        <set name="groups" table="UserGroup">
            <key>
                <column name="userName"/>
                <column name="org"/>
            </key>
            <many-to-many class="Group">
                <column name="groupName"/>
                <formula>org</formula>
            </many-to-many>
        </set>
    </class>

    <class name="Group" table="`Group`">
        <composite-id>
            <key-property name="name"/>
            <key-property name="org"/>
        </composite-id>
        <property name="description"/>
        <set name="users" table="UserGroup" inverse="true">
            <key>
                <column name="groupName"/>
                <column name="org"/>
            </key>
            <many-to-many class="User">
                <column name="userName"/>
                <formula>org</formula>
            </many-to-many>
        </set>
    </class>

### 26.4.4. Content based discrimination

    <class name="Person"
        discriminator-value="P">

        <id name="id"
            column="person_id"
            unsaved-value="0">
            <generator class="native"/>
        </id>

        <discriminator
            type="character">
            <formula>
                case
                    when title is not null then 'E'
                    when salesperson is not null then 'C'
                    else 'P'
                end
            </formula>
        </discriminator>

        <property name="name"
            not-null="true"
            length="80"/>

        <property name="sex"
            not-null="true"
            update="false"/>

        <component name="address">
            <property name="address"/>
            <property name="zip"/>
            <property name="country"/>
        </component>

        <subclass name="Employee"
            discriminator-value="E">
                <property name="title"
                    length="20"/>
                <property name="salary"/>
                <many-to-one name="manager"/>
        </subclass>

        <subclass name="Customer"
            discriminator-value="C">
                <property name="comments"/>
                <many-to-one name="salesperson"/>
        </subclass>

    </class>

### 26.4.5. Associations on alternate keys

    <class name="Person">

        <id name="id">
            <generator class="hilo"/>
        </id>

        <property name="name" length="100"/>

        <one-to-one name="address"
            property-ref="person"
            cascade="all"
            fetch="join"/>

        <set name="accounts"
            inverse="true">
            <key column="userId"
                property-ref="userId"/>
            <one-to-many class="Account"/>
        </set>

        <property name="userId" length="8"/>

    </class>

    <class name="Address">

        <id name="id">
            <generator class="hilo"/>
        </id>

        <property name="address" length="300"/>
        <property name="zip" length="5"/>
        <property name="country" length="25"/>
        <many-to-one name="person" unique="true" not-null="true"/>

    </class>

    <class name="Account">
        <id name="accountId" length="32">
            <generator class="uuid"/>
        </id>

        <many-to-one name="user"
            column="userId"
            property-ref="userId"/>

        <property name="type" not-null="true"/>

    </class>

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