Am 17.04.2014 um 15:52 schrieb Sabine Knöfel <sabine.knoe...@gmail.com>:
> Hi Norbert,
>
> for me the gain within smalltalk is huge because I want to navigate from
> person to trip and from trip to person by the attributes without asking mongo
> for the correspondent object via indexes.
>
> Don't you think that this is a question of design of the object model and
> should not be decided by database arguments?
>
I think there isn’t only one _or_ the other. Of course it is a lot more
beautiful to have set up proper objects and collections and do everything in a
smalltalkish way. That does only count as long as you stay on the island
(smalltalk image). As soon as you start to play with the outer world you have
to change your rules. If you agree it isn’t perfectly beautiful if you have a
proper object model that freezes your image, right? :)
IMHO the beauty comes from a mixture of both that fit them together without
restricting one or the other.
So I would try to avoid storing both directions. But which one to skip? If I
assume that a person can have a lot of trips than I would skip the collection
inside Person. The more trips a Person has the bigger the serialized document
becomes. This makes it first slower to operate on the database and will fail if
you reach a document size of 16MB because that is the maximum Mongo supports.
Most of the time this will be a non-issue but for now I take it into account.
So the data model will have a class Trip with an instVar pointing to Person.
And Person will have an instVar trips that is transient (see
VOMongoTransientDescription). The implementation for Person then goes
Person>>#trips
^ trips ifNil: [ trips := self fetchTrips ]
Person>>#fetchTrips
^ Trips selectMany: [: each| … ]
Person>>#addTrip: aTrip
aTrip person: self.
aTrip save
So the Trip will have a reference to Person in the database but Person will
have no reference to Trip. Person will retrieve its trips as soon as they are
used in a really smalltalkish way :). Person acts as a smalltalk object and as
a database object cache at the same time.
So this an implementation of Person that on the inside takes the database
nature in regard but on the outside the interface is pure smalltalk.
Still not good?
Norbert
> Regards
> Sabine
>
>
> On Thu, Apr 17, 2014 at 3:40 PM, NorbertHartl [via Smalltalk] <[hidden
> email]> wrote:
> Sabine,
>
> I read the case now the first time. Just one note. Having references in both
> directions is probably not the best idea if using a non-object store. For
> navigating the graph it might be good but for persisting it is not. The
> serialization is a lot more complex if you do and the gain is nothing. For
> most databases with proper indexes you can request both directions easily and
> fast. So having one reference pointing from A to B you get the opposite
> direction for free.
>
> Norbert
>
> Am 17.04.2014 um 15:22 schrieb Sabine Knöfel <[hidden email]>:
>
>> Hi Olivier,
>>
>> possibly the option
>> VOMongoContainer>>enableMissingContent;
>> is useful to avoid
>> http://forum.world.st/Voyage-image-freeze-VOMongoError-Lazy-reference-not-found-td4703739.html
>>
>> Here is an example:
>> http://esug.org/data/ESUG2013/4-Thu/03-ESUG2013%20-%20VoyageTutorial.pdf
>>
>> Regards
>> Sabine
>>
>>
>>
>>
>> On Thu, Apr 17, 2014 at 3:07 PM, Olivier Auverlot [via Smalltalk] <<a
>> href="x-msg://56/user/SendEmail.jtp?type=node&node=4755097&i=0"
>> target="_top" rel="nofollow" link="external">[hidden email]> wrote:
>> thanks Robert for the explications.
>>
>> I agree with you that's the best way to remove a book is to delete the
>> reference in ComicsCollection.
>>
>> But how to do that ? Must I simply remove the reference in the ordered
>> collection ? Voyage will syncronize automatically the data in memory with
>> the content of the database ? Must I force the save of the data after
>> removing the reference ?
>>
>> Olivier ;-)
>>
>>
>> 2014-04-17 14:10 GMT+02:00 Norbert Hartl <[hidden email]>:
>>
>>
>> Am 17.04.2014 um 13:53 schrieb olivier <[hidden email]>:
>>
>> > Hi,
>> >
>> > I'm using Voyage in a Pharo application.
>> >
>> > I have two MongoDB collections which are ComicsCollection and ComicsBook.
>> > Each book is attached to a instance of ComicsCollection. The reference of
>> > each book is stored in an ordered collection (in the instance of
>> > ComicsCollection). The problem is that if I remove a book, the reference
>> > to the book is not deleted from ComicsCollection.
>> >
>> > How can I remove properly a book and the reference to the book ?
>> >
>> From the smalltalk image view the problem is usually exactly the opposite:
>> You don’t delete objects but you remove references to them. As voyage maps
>> objects it is a good idea to stay in the object realm. So you should rather
>> remove the book from the collection. This way you won’t get errors just
>> garbage. The „real“ problem that arises then is that the book would still be
>> in the database. Just like it is in the image but there is no garbage
>> collector for mongo. To decide that from the image is not possible. You load
>> only a sub graph from the database into image memory. So you don’t know if
>> there are other objects referencing the book.
>>
>> If your records fit all in memory you could load all collections and books
>> and build the difference between all referenced books in the collections and
>> the total amount of books. The difference will be the set of objects not
>> being referenced by a collection and those can be deleted.
>>
>> If the records do not fit in memory an alternative strategy would be needed.
>> I started to research if it is possible to build a map/reduce based garbage
>> collector for common cases but got distracted. But I have the same problem
>> so I will need to pick it up some time.
>>
>> hope that helps,
>>
>> Norbert
>>
>>
>>
>>
>>
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>> NAML
>>
>>
>> View this message in context: Re: Voyage: how to delete references to
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>
>
>
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