#Data crossstore between Mongo and JPA

Spring Data Mongo provides another attractive feature, when you mix to use Mongo and JPA in your projects, you can unite the models of Mongo and JPA.

Model

The Conference and Signup are still used as example models, Conference is JPA entity class and Signup is a Mongo Document.

<pre> @Entity public class Conference { @Id @GeneratedValue(strategy = GenerationType.AUTO) @Column(name = "id") private Long id; @RelatedDocument private Signup signup; } </pre>

The Signup Document is defined as the following.

<pre> @Document public class Signup { @Id private String id; } </pre>

@RelatedDocument annotation indicates the signup property of Conference is a reference of Signup Mongo Document. This annotation will be processed by a AspectJ aspect which provided in Spring Data Mongo.

You have to add some code fragments to Spring configuration.

<pre> &lt;!-- Mongo cross-store aspect config --> &lt;bean class="org.springframework.data.mongodb.crossstore.MongoDocumentBacking" factory-method="aspectOf"> &lt;property name="changeSetPersister" ref="mongoChangeSetPersister" /> &lt;/bean> &lt;bean id="mongoChangeSetPersister" class="org.springframework.data.mongodb.crossstore.MongoChangeSetPersister"> &lt;property name="mongoTemplate" ref="mongoTemplate" /> &lt;property name="entityManagerFactory" ref="entityManagerFactory" /> &lt;/bean> </pre>

Firstly, this aspect will mark this property as @Transient and prevent JPA recognize it as a persistent property.

Then register a transaction synchronization to entity lifecycle events of the host JPA Entity( Conference ) to synchronize the @RelatedDocument data into Mongo Document.

We also use Junit @Before to initialize the data.

<pre> @Before public void beforeTestCase() { log.debug("==================before test case========================="); conferenceRepository.save(newConference()); } </pre>

The newConference() method is use for create a new Conference object, we also create a new Singup as the value of signup property.

<pre> @Test public void retrieveConference() { log.debug("==================enter retrieveConference========================="); assertTrue(null != id); Conference conf = em.find(Conference.class, id); log.debug("conf@@@" + conf); assertTrue(null != conf); assertTrue(null == conf.getSignup()); } </pre>

When you run this Test, pass!

Everything is OK?

Spring Data Mongo provides a LoggingEventListener to tracking the Mongo events.

<pre> &lt;bean class="org.springframework.data.mongodb.core.mapping.event.LoggingEventListener" /> </pre>

After you enable this listener, you should see the Mongo log info about saving Conference. But we run this Test again, we can not see it at all.

Add another line code in the first line of the test method.

<pre> em.clear(); </pre>

Run the Test, failed.

No doubt the Signup was not stored in Mongo when saving Conference object.

As motioned before, the crosssotre synchronization is fired by the entity lifecycle events. If JPA event @PostPersist is not fired in a transaction, the Mongo event will not be fired.

<pre> @Before // @Transactional(propagation = Propagation.REQUIRED) public void beforeTestCase() { log.debug("==================before test case========================="); transactionTemplate = new TransactionTemplate(transactionManager); transactionTemplate.execute(new TransactionCallback&lt;Void>() { @Override public Void doInTransaction(TransactionStatus status) { } } } </pre>

Alternatively, we change the initial codes slightly, and use a TranscationTemplate to process the Conference saving action as an unit of work.

Run this test again, pass.

##More ...

If possible store a Collection of data into Mongo like this,

<pre> @RelatedDocument private List&lt;Signup> signups; </pre>

Unfortunately, it does not work.

But you wrap the List into standalone Document like.

<pre> @Document public class SignupInfo { @Id private String id; @DBRef private List&lt;Signup> signups = new ArrayList&lt;Signup>(); } </pre>

And change the relation to SignupInfo.

<pre> @RelatedDocument private SignupInfo signupinfo; </pre>

But you have to save the Signup objects in Mongo firstly, because Spring Data Mongo does not support Cascade saving feature like JPA.

I have some wishes for the crossstore.

1. Store reverse when store a Document.

Update to now, there is no way to do the work reverse. When you store a Mongo Document it does not support to store the related JPA entity into database.

1. Query cross different data storage. Obviously, it is impossible to execute query cross different stores.

Summary

Personally, I think the Mongo and JPA crossstore is only a toy, and the provided feature is too weak. I hope it can be improved in future, and provides seamless integration between different data stores as I expected.