[hibernate-dev] [OGM] Keeping the order of the collection of elements in MongoDB

[Sanne Grinovero]

Hi Yoann,

while you might be using a specific implementation when persisting
your new entities, when you're loading the object back from the
database how is Hibernate supposed to know
 A) which implementation you want it to use
 B) which order to use, if you didn't map e.g. an order column to
store the ordering information (MongoDB not needing a strategy is an
exception)

The entity model expresses the intent of the end user, but also it
expresses its requirements which will be taken into account by ORM
when it needs to provide an implementation for the "Collection"
interface.

A List expresses the ordering requirement by contract; if you map a
relation as "Collection" I wage you're not interested in the order,
but you're rather more interested in accepting a wide range of
implementations, some of which might ignore the ordering requirement.
By doing so, you're explicitly telling both ORM and your other code
interacting with the model that ignoring the order is acceptable.

BTW I suspect mapping things as a generic Collection is a very unusual
(or lazy) choice; I don't remember ever using it as it's quite
ambiguous, and there's no way any specification diagram will lead you
to recommend using a Collection (other than cleanly and explicitly
wanting to express you don't care about ordering, nor other properties
like uniqueness...)

Thanks,
Sanne


On 13 January 2017 at 13:29, Yoann Rodiere <yrodiere at redhat.com> wrote:
>> I think it's a reasonable expectation, as long as we're talking
>> specifically about mapping a *List* and not just a generic Collection.
>
>
> Ah, this topic again :) I know I'll be all lone, but I'll try anyway!
>
> If we do it for List, and unless there are technical issues that prevent us
> from doing so, I would be in favor of doing it for any kind of collection.
>
> In Collections, the fact that iteration order is deterministic is mostly up
> to the implementation, which is different from saying it's not
> deterministic. From the javadoc for Collection#iterator():
>
>> There are no guarantees concerning the order in which the elements are
>> returned
>> (unless this collection is an instance of some class that provides a
>> guarantee).
>
>
> Deterministic, and even predictable order is not exclusive to List, either:
> for instance, LinkedHashSet is not a List, it has a specific iteration
> order, but there is nothing in its implemented interfaces (Collection, Set)
> that defines this order.
>
> My point is, we can't rely on the implemented interface to decide whether
> the iteration order is important or not, so we may as well decide it is
> always important. Unless there are annoying technical challenges, of course.
>
> Yoann Rodière <yrodiere at redhat.com>
> Software Engineer
> Red Hat / Hibernate NoORM Team
>
> On 13 January 2017 at 13:48, Sanne Grinovero <sanne at hibernate.org> wrote:
>>
>> I think it's a reasonable expectation, as long as we're talking
>> specifically about mapping a *List* and not just a generic Collection.
>>
>> On 13 January 2017 at 12:18, Davide D'Alto <davide at hibernate.org> wrote:
>> > Hi,
>> > it seems that when using Collection of elements in MongoDB, users
>> > expect to have the elements in the same order as in the db. You can
>> > see the question on the forum here:
>> >
>> > https://forum.hibernate.org/viewtopic.php?f=31&t=1043903&p=2491218#p2491218
>> >
>> > I also found this StackOverflow question:
>> >
>> > http://stackoverflow.com/questions/9013916/do-arrays-stored-in-mongodb-keep-their-order
>> >
>> > What do you think? Is it something that we should support?
>> >
>> > Cheers,
>> > Davide
>> > _______________________________________________
>> > hibernate-dev mailing list
>> > hibernate-dev at lists.jboss.org
>> > https://lists.jboss.org/mailman/listinfo/hibernate-dev
>> _______________________________________________
>> hibernate-dev mailing list
>> hibernate-dev at lists.jboss.org
>> https://lists.jboss.org/mailman/listinfo/hibernate-dev
>
>