Step 2. Generate O/R Mappings and Test with EJBQL

Time to complete this step: 15 minutes

Workshop provides a powerful and flexible object relational mapping interface for popular persistence services like Kodo JPA. Depending on the development scenario, O/R mappings can be generated through different mechanisms:

The tasks you will complete in this step are:

• To Generate Annotated POJOs from a Schema using the JPA ORM Generation Wizard
• To Review Generated Classes with Annotations
• To Run an EJBQL Query to Test Your Mappings

To Generate Annotated POJOs from a Schema using the JPA ORM Generation Wizard

In this step, we will use Workshop to automatically generate JPA entity beans from an existing database schema by reverse engineering the schema. Workshop will generate Java classes with the appropriate accessors and JPA annotations.

1. In the DbXplorer view, right-click the schema SalesDBConnection and select Generate JPA Mapping.

   

2. Select the JPA-based Web application project, workshop-jpa-tutorial, from the Web Application dialog and click OK.
   
   Workshop considers a web application to be a JPA application if it has the persistence file persistence.xml under the web/classes/META-INF folder.

   

3. Select the database tables from the JPA ORM Generation dialog as shown below and click Next.
   
   Do not select the CONTACT database table. Later on in this tutorial you will use the CONTACT database table to manually create a POJO class Contact and annotate this class for JPA in order to better understand the top-down development approach.

   

   • Note the greyed-out checkbox Copy driver and add to the build path. It instructs the JPA ORM Generation Wizard to add the JDBC driver for the Hypersonic Database to the project classpath. Currently the driver already exists in the project build path.
4. The Table Associations dialog displays entity relationships as observed in the database schema from the foreign key definitions. The dialog allows you to edit a table association by selecting each association and modifying its options in the editing panel.
   
   Select the association between LINEITEM and ORDER_DATA.
   Change the value of the first Property field from orderData to order.
   



5. Select the association between LINEITEM and PRODUCT.
   
   We want to define a many-to-one unidirectional relationship between LINEITEM and PRODUCT. Uncheck the checkbox Generate a reference to a collection of LineItem in Product.

   

6. Select the association between ORDER_DATA and CUSTOMER.
   Change the value of the second Property field from orderData to orders.
   Click Next.
   



7. In the Default Table Association dialog, confirm that none is selected in the Key Generator field.
   
   For Collection Properties Type dropdown list, select java.util.List.
   
   A Java Package is required for generating the entity beans. Click Browse next to the Java Package field to view the Choose Java Package dialog. Select the com.bea.beans package and click OK.
   
   Click Next.
   



8. The Tables and Columns dialog allows you to customize the tables and columns mapping generation. We will not customize the tables in this tutorial.
   
   Click Next.
9. In the JPA ORM Generation dialog,ensure that the Update JPA configuration file checkbox is selected, as it is by default. This will ensure that the persistence.xml file is updated to include the mapping information you've just defined.
   
   Click Finish.
   

Workshop will generate all of the annotated Entity Java Beans based on the properties supplied to the ORM Generation wizard.

1. You can access the entites through the AppXplorer view under the JPA Configuration node. To view the entities, in the AppXplorer view, open the nodes workshop-jpa-tutorial > web > JPA Configuration > Entities.
2. The object model resides in the nodes workshop-jpa-tutorial > web/WEB-INF/src/java > com.bea.beans.
3. To view the set of Persistence entities generated by Workshop, navigate to the persistence.xml file at workshop-jpa-tutorial > web/WEB-INF/src/java > META-INF > persistence.xml.

To Review Generated Classes with Annotations (Optional)

Each entity mapping is defined as having one of six property types:

• basic
• id
• many-to-one
• one-to-one
• one-to-many
• many-to-many

When you generate entities from a database, Workshop annotates the generated source code with JPA annotations corresponding to these properties. In this section we will review these basic JPA annotations.

Review the following annotations in the class Customer.

1. In the AppXplorer view, double-click the file workshop-jpa-tutorial > web/WEB-INF/src/java > com.bea.beans > Customer.java to view its source.
2. Basic Properties - A basic property handles a standard value that is persisted as-is to the database.
   
   The @Basic annotation is followed by the @Column annotation defining an attribute name which is the name of the column to which the property is bound; the attribute nullable is set to false to specify that the column cannot store null values; the column length attribute specifies the maximum length.

@Basic() @Column(name="NAME", nullable=false, length=255) public String getName() { return this.name; } public void setName(String name) { this.name = name; }

3. One-to-Many Properties - A one-to-many property designates a relationship in which one A entity references multiple B entities, and no two A entities reference the same B entity.
   
   Here a Customer entity references multiple OrderData entities, but an OrderData entity can have only one Customer entity reference.
   
   The one-to-many annotation defines an attribute mappedBy which is the name of the many-to-one field in OrderData entity that maps this bidirectional relation.

@OneToMany(mappedBy="customer", fetch=FetchType.EAGER) public java.util.Set<OrderData> getOrders() { return this.orders; } public void setOrders(java.util.Set<OrderData> orders) { this.orders = orders; }

4. Id properties - An Id property designates an identifier, such as a primary key. All entity beans must declare one or more fields which together form the persistent identity of an instance.
   
   An Id annotation is followed by a @Column annotation defining the attribute unique which is set to true to specify that the column is UNIQUE in the SQL sense (can have only unique values).

@Id() @Column(name="CUSTOMERID", unique=true, nullable=false) public Integer getCustomerid() { return this.customerid; } public void setCustomerid(Integer customerid) { this.customerid = customerid; }

5. Double-click the class file OrderData.java.
   
   Many-to-one Properties - A many-to-one property designates a relationship in which an entity A references a single entity B, and other A's might also reference the same B; there is a many-to-one relation from A to B.
   
   The many-to-one annotation defines an attribute fetch which is an enum that specifies whether to load the field's persisted data before the entity object is returned by the persistence provider (FetchType.EAGER) or later, when the property is accessed (FetchType.LAZY).
   
   The many- to-one annotation is followed by an @JoinColumn annotation defining the column name attribute which is the name of the column to which the property is bound and a referencedColumnName attribute which is the name of the primary key column being joined to.

   @ManyToOne(fetch=FetchType.EAGER) @JoinColumn(name="CUSTOMERID", referencedColumnName="CUSTOMERID") public Customer getCustomer() { return this.customer; } public void setCustomer(Customer customer) { this.customer = customer; }

To Run an EJBQL Query to Test Your Mappings (Optional)

In this step, you will become familiar with the EJBQL Editor. The EJBQL Editor lets you define and execute an EJBQL query.

1. On the DbXplorer view, click the Open DbXaminer button.

   

2. In the DbXaminer view, click the EJBQL Editor tab.

   

3. Browse to select the workshop-jpa-tutorial web application as the Current project:.
   



4. When defining an EJBQL query in the EJBQL Editor, Workshop provides code completion for EJBQL keywords and the persistent entities by pressing CTRL+SPACE. Here, we will define an EJBQL SELECT query using the code completion facility.
   
   In the EJBQL Editor, enter S and press CTRL+SPACE. It displays a list of EJBQL keywords starting with letter S.



5. Double-click the keyword select.
6. Enter a space and specify c as an alias to get the data for the properties of persistent object Customer.



7. Enter from (You can use code completion for the from keyword) and space.
8. Press CTRL+SPACE to get a list of persistent entities and select the Customer object.



9. Enter space and specify an alias c for Customer entity.
10. Now, we have defined the EJBQL SELECT query. You can execute them by either clicking the Execute Query button or by using the hotkey, CTRL+ENTER.



11. Similarly execute an EJBQL SELECT query on persistent object OrderData for a customer having a particular customerid.