Defining Entity Relationships

Relationships between database entities or Java objects are necessary for most applications. Take Liferay’s Message Boards application as an example. Each Message Board message belongs to a specific Message Board thread. A Message Board thread might also belong to a specific Message Board category.

You can see how the relationship is defined in the application’s service.xml file:

<entity external-reference-code="group" human-name="message-boards message" local-service="true" name="MBMessage" remote-service="true" trash-enabled="true" uuid="true">

	<!-- PK fields -->

	<column name="messageId" primary="true" type="long" />

	<!-- Group instance -->

	<column name="groupId" type="long" />

	<!-- Audit fields -->

	<column name="companyId" type="long" />
	<column name="userId" type="long" />
	<column name="userName" type="String" uad-anonymize-field-name="fullName" />
	<column name="createDate" type="Date" />
	<column name="modifiedDate" type="Date" />

	<!-- Other fields -->

	<column name="classNameId" type="long" />
	<column name="classPK" type="long" />
	<column name="categoryId" type="long" />
	<column name="threadId" type="long" />
	...

The threadId field referenced in this MBMessage object has the same name as the primary key in the MBThread object (not shown above). This creates the relationship between the two objects. A similar relationship can be seen with categoryId and the MBCategory object.

Congratulations, now you know how to relate two entities.