CODEWITHSUNDEEP
javagoogle-app-enginejpaentity
pgruetter
pgruetter

Reputation: 1214

Saving many entities in GAE with JPA

I'm using JPA with with Google App Engine. Let's say I have a very simple @Entity consisting of a Key and a String, nothing more. I now create 10000 of these entities, put them in a list and now want to store all of them.

If I try to use an EntityManager em and a for loop to go through the List of all my entities...

for(MyEntity entity : listOfAllEntities) {
 em.persist(entity);
}

.. I will get an IllegalArgumentException:

java.lang.IllegalArgumentException: can't operate on multiple entity groups in a single transaction.

As far as I understand it, I need to close and reopen the EntityManager for each persist() call. Of course, this is very time consuming. I'm trying to have a task running once a day that reloads all the entities. According to GAE policy, the task has a timeout of 30 seconds.

So an alternative is to only save 500 entities at a time and run the task multiple times which I think is more complicated than it has to be.

Is this the only way of achieving what I'm trying to do or am I missing something here?

Solution: All the answers point in the same direction. I simply created a One-To-Many relationship by creating a "dummy parent" entity. I don't really need a parent in my case and it doesn't make much sense in the real world so to speak. But after setting this dummy entity as parent of each of the child entities, I can save them exactly as I did before without caring too much about transactions. Thanks to all.

Upvotes: 6

Views: 2708

Answers (2)

Vladimir Prudnikov
Vladimir Prudnikov

Reputation: 7242

This error says that you use transaction and trying to operate on multiple entity groups.

From docs:

All datastore operations in a transaction must operate on entities in the same entity group. This includes querying for entities by ancestor, retrieving entities by key, updating entities, and deleting entities. Notice that each root entity belongs to a separate entity group, so a single transaction cannot create or operate on more than one root entity.

You can't just create more than one root (without parent) entity in transaction, but you can do it if each of them are child of another entity. If you want just to create entities — do it outside of transaction.

Upvotes: 1

Taylor Leese
Taylor Leese

Reputation: 52330

Perhaps this is oversimplified, but you could just add the X number of entities as children of a parent entity and this would cause them to be in the same entity group. This would allow you to persist them all in one transaction. Basically, create an owned one-to-many relationship between the parent entity and the children which are the objects you are trying to persist.

Upvotes: 4

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