Chapter 2. Learning mapping basics

In IntelliJ IDEA select "File > New > Project..." and then select "Maven" and click "Next". In the dialog shown on the screenshot below, fill the "Group Id" and "Artifact Id" fields and click "Next".

On next dialog screen you can customize directory for your project and click "Finish". Now you should have a new empty project.

Although later in this tutorial we'll be using Maven to include Cayenne runtime jars in the project, you'll still need to download Cayenne to get access to the CayenneModeler tool.

Download the latest release. Unpack the distribution somewhere in the file system and start CayenneModeler, following platform-specific instructions. On most platforms it is done simply by doubleclicking the Modeler icon. The welcome screen of the Modeler looks like this:

Click on the "New Project" button on Welcome screen. A new mapping project will appear that contains a single DataDomain. The meaning of a DataDomain is explained elsewhere in the User Guide. For now it is sufficient to understand that DataDomain is the root of your mapping project.

The next project object you will create is a DataNode. DataNode is a descriptor of a single database your application will connect to. Cayenne mapping project can use more than one database, but for now, we'll only use one. With "project" selected on the left, click on "Create DataNode" button on the toolbar (or select "Project > Create DataNode" from the menu).

A new DataNode is displayed. Now you need to specify JDBC connection parameters. For an in-memory Derby database you can enter the following settings:

Also you will need to change "Schema Update Strategy". Select "org.apache.cayenne.access.dbsync.CreateIfNoSchemaStrategy" from the dropdown, so that Cayenne creates a new schema on Derby based on the ORM mapping when the application starts.

Now you will create a DataMap. DataMap is an object that holds all the mapping information. To create it, click on "Create DataMap" button (or select a corresponding menu item). Note that the newly created DataMap is automatically linked to the DataNode that you created in the previous step. If there is more than one DataNode, you may need to link a DataMap to the correct node manually. In other words a DataMap within DataDomain must point to a database described by the map.

You can leave all the DataMap defaults unchanged except for one - "Java Package". Enter "org.example.cayenne.persistent". This name will later be used for all persistent classes.

Before you proceed with the actual mapping, let's save the project. Click on "Save" button in the toolbar and navigate to the "tutorial" IDEA project folder that was created earlier in this section and its "src/main/resources" subfolder and save the project there. Now go back to IDEA and you will see two Cayenne XML files.

Note that the location of the XML files is not coincidental. Cayenne runtime looks for "cayenne-*.xml" file in the application CLASSPATH and "src/main/resources" folder should already be a "class folder" in IDEA for our project (and is also a standard location that Maven would copy to a jar file, if we were using Maven from command-line).

Lets go back to CayenneModeler where we have the newly created project open and start by adding the ARTIST table. Database tables are called "DbEntities" in Cayenne mapping (those can be actual tables or database views).

Select "datamap" on the left-hand side project tree and click "Create DbEntity" button (or use "Project > Create DbEntity" menu). A new DbEntity is created. In "DbEntity Name" field enter "ARTIST". Then click on "Create Attribute" button on the entity toolbar. This action changes the view to the "Attribute" tab and adds a new attribute (attribute means a "table column" in this case) called "untitledAttr". Let's rename it to ID, make it an INTEGER and make it a PK:

Similarly add NAME VARCHAR(200) and DATE_OF_BIRTH DATE attributes. After that repeat this procedure for PAINTING and GALLERY entities to match DB schema shown above.

Now we need to specify relationships between ARTIST, PAINTING and GALLERY tables. Start by creating a one-to-many ARTIST/PAINTING relationship:

Now that the database schema mapping is complete, CayenneModeler can create mappings of Java classes (aka "ObjEntities") by deriving everything from DbEntities. At present there is no way to do it for the entire DataMap in one click, so we'll do it for each table individually.

Now you need to synchronize relationships. Artist and Gallery entities were created when there was no related "Painting" entity, so their relationships were not set.

Unless you want to customize the Java class and property names (which you can do easily) the mapping is complete.

Now go back to IDEA - you should see pairs of classes generated for each mapped entity. You probably also see that there's a bunch of red squiggles next to the newly generated Java classes in IDEA. This is because our project does not include Cayenne as a Maven dependency yet. Let's fix it now by adding "cayenne-server" artifact in the bottom of the pom.xml file. Also we should tell Maven compile plugin that our project needs Java 8. The resulting POM should look like this:

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <groupId>org.example.cayenne</groupId>
    <artifactId>tutorial</artifactId>
    <version>0.0.1-SNAPSHOT</version>

    <dependencies>
        <dependency>
            <groupId>org.apache.cayenne</groupId>
            <artifactId>cayenne-server</artifactId>
            <!-- Here specify the version of Cayenne you are actually using -->
            <version>4.1.M2-SNAPSHOT</version>
        </dependency>
        <dependency>
            <groupId>org.slf4j</groupId>
            <artifactId>slf4j-simple</artifactId>
            <version>1.7.25</version>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <!-- Tell maven to support Java 8 -->
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>3.6.1</version>
                <configuration>
                    <source>1.8</source>
                    <target>1.8</target>
                </configuration>
            </plugin>
        </plugins>
    </build>
</project>

Your computer must be connected to the internet. Once you edit the pom.xml, IDEA will download the needed Cayenne jar file and add it to the project build path. As a result, all the errors should disappear. In tutorial for console output we use slf4j-simple logger implementation. Due to use SLF4J logger in Apache Cayenne, you can use your custom logger (e.g. log4j or commons-logging) through bridges.

Now let's check the entity class pairs. Each one is made of a superclass (e.g. _Artist) and a subclass (e.g. Artist). You should not modify the superclasses whose names start with "_" (underscore), as they will be replaced on subsequent generator runs. Instead all custom logic should be placed in the subclasses in "org.example.cayenne.persistent" package - those will never be overwritten by the class generator.