> On Jan 28, 2016, at 7:22 AM, Louise Elmose Hedegaard <louiseelm...@gmail.com>
> wrote:
>
> Hi guys,
>
> Thank you for your answers.
> I do not think I have explained my problem properly though when I read your
> answers.
>
> I have a table "order" where I save different information about an order. The
> unique id of the order comes from an external system, and I want to save this
> unique id for the order.
> When an order is updated in the external system, I want to update the GAE
> datastore accordingly.
> For this reason I first check whether the order that was updated in the
> external system already exists in my datastore. If it already exists I update
> it, if it does not already exist, I create the order in the datastore.
> My problem is that I use this:
>
> datastore.prepare(query).asSingleEntity();
>
> which fails, as there are more than one entity with the unique id.
> I do not understand how several entities can be created with the same unique
> id, when my logic says:
>
In order for this to work as you expect you need strong consistency, which
typically means a) have your entities be a part of the same entity group and b)
do the query inside of a transaction.
As others have pointed out it’s much, much easier to simply use the id as they
key and fetch by key. Though you still need to do the logic below inside of a
transaction.
Karl
> Entity entity = getEntity(uniqueId);
> if (entity == null){
> createEntity();
> }else{
> updateEntity();
> }
>
> It is as if the getEntity check does not detect that there already is an
> entity with the unique id.
> It does seem that this is caused by a timing/concurrency issue - the external
> system sends two updates approximately at the same time, but shouldn't the
> datastore be able to handle this?
>
> My first attempt of getEntity was:
>
> Query query = new Query(OrderDBFields.ORDER_TABLE_NAME).setFilter(new
> Query.FilterPredicate(OrderDBFields.ID, Query.FilterOperator.EQUAL, id));
> return datastore.prepare(query).asSingleEntity();
>
> My second (confused attempt) is:
>
>
> Transaction txn = datastore.beginTransaction();
> Key orderKey = KeyFactory.createKey(OrderDBFields.ORDER_TABLE_NAME,
> id);
> Entity orderEntity = null;
> try {
> orderEntity = datastore.get(orderKey);
> } catch (EntityNotFoundException e) {
> orderEntity = null;
> }
> txn.commit();
> return orderEntity;
>
> Any input is appreciated,
> Thanks
> -Louise
>
> Den onsdag den 27. januar 2016 kl. 12.39.54 UTC+1 skrev Louise Elmose
> Hedegaard:
>
> Hi,
>
> I have an issue with too many entities being created in my GAE datastore.
>
> I have an unique id on an entity (which is not the key in this case), and
> there should thus only be one entity in the datastore per unique id.
> When an entity is updated or created by the user, I check whether the entity
> already exists in the DB, and only create a new entity, if there is not
> already an existing entity with a matching id. If there is an existing
> entity, I update the parameters of the entity.
> This logic should ensure that there is only one entry/entity per unique id,
> but somehow several entities with the same id are created in the DB - I do
> not understand how/why!
>
> When I look at this page
> https://cloud.google.com/appengine/docs/java/datastore/entities#Java_Updating_an_entity
>
> <https://cloud.google.com/appengine/docs/java/datastore/entities#Java_Updating_an_entity>
> is says "You can use a transaction
> <https://cloud.google.com/appengine/docs/java/datastore/transactions> to test
> whether an entity with a given key exists before creating one.".
>
> Do I really need to use transactions to validate whether there is an existing
> entity with a given id, or does that only apply when you use the key as
> primary key??
>
> Thanks,
> -Louise
>
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