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

[Yoann Rodiere]

Hi Sanne,


> 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

Ok, that's true for most collection interfaces, but I'm not sure I
understand why what Hibernate can guess is relevant here. Users are free to
use custom collection types, at least in ORM. Is it not the case with OGM?
Will it never be possible?

I thought we were talking about making the collection persister
persist/retrieve the elements in the same order as they were retrieved from
the collection. If it's not the case, then yes, I'm missing something.

However, even in the specific case of Collection, by default Hibernate
instantiates a List behind the scene for such properties (well, a
PersistentBag, but it's basically a List). Thus if the original collection
had some kind of "ordering memory", at least this would be preserved,
provided we persist elements in the same order we retrieved them from the
original collection.

 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)


Well, we don't want to frustrate OGM users with limitations from the
relational world, do we? Wouldn't other document datastores also allow to
persist the elements' order?

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...)


By exposing a Collection, you're not telling the order isn't important, and
certainly not that it can change for no reason (which is our concern here).
In fact, you're not telling anything about the order.
By exposing a List, you're telling more than just "the order is important".
What you're telling is "the order can be managed explicitly by an external
actor". Also, you're telling "random access is possible" (not necessarily
efficient, but possible).

So while I probably wouldn't bother myself, I can see why someone would
choose Collection over List: to avoid unnecessarily exposing promises to
users.
You could add documentation on a getter stating what the expected ordering
is (and, frankly, it may be relevant even with a List: how would you
translate "the invoices are ordered by issuing date" in terms of a Java
interface?). But what you cannot (or at least, shouldn't) do is
contradicting the interfaces you expose ("yeah, I'm exposing a List because
elements are ordered explicitly since we only ever add one at the end, but
please be nice and don't try random access, it's a very long linked list").
And no, I don't consider that "instanceof RandomAccess" on the client side
would be a clean solution...

I'm not saying "it's super important, a critical use case, we can't ship a
new release without this, let's spend 3 months working on it". I'm just
saying it may be relevant for other collection types, so if we do it for
lists, and if it's not much more difficult/inefficient to do it for other
collection types, why wouldn't we?

Cheers,

Yoann Rodière <yoann at hibernate.org>
Hibernate NoORM Team

On 13 January 2017 at 15:26, Sanne Grinovero <sanne at hibernate.org> wrote:

> 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
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