On Mar 6, 2013, at 1:02 PM, Denis Gervalle <[email protected]> wrote: > Hi Vincent, > > On Wed, Mar 6, 2013 at 11:36 AM, Vincent Massol <[email protected]> wrote: > >> Resending since I've made mistakes (it's only about ObjectReference, not >> Properties), sorry about that. Here's the new version: >> >> ------------ >> >> Hi devs, >> >> ATM in the model module there's no ability to reference an xobject other >> than by using a free form name. > > > We made this on purpose.
I know, I worked on this… but it doesn't make it right… :) >> The problem is that this is not really usable. This is why we introduced >> the BaseObjectReference in oldcore. >> > > We have introduce the BaseObjectReference in oldcore simply to easily > support the current storage implementation. At that time, we had a > discussion about using the object number (not to be confuse with the index) > or the object UUID. And, since the object UUID was suspicious in some edge > case, you convince me to use the object number to create the free form name > of these old objects (I should have resist more since I still do not have > evidence of these edge case). What? That defeats one of the main goal of references which is that you can directly address an entity without having retrieved it first! The reason entities have names is to be able to easily reference them. Using a UUID makes it impossible to reference something without getting it first... If to reference an Object I have to load the Document first, then in practice it means we don't need Object references and they shouldn't be considered entities. For any entity, you should be able to construct a direct reference to it without needing to load anything else. In the 7-8 years of XWiki we've used Class reference and numbers/position to reference xobjects and it has worked quite well IMO. In most cases you have only 1 xobject of a type and it's very nice to be able to say: here's a reference to the first object of type N in document P. Exposing an internal id of a document as reference is a no go for me since it cannot be constructed without retrieving the entity first. >> However this is major PITA since we can't have clean code that create an >> object reference and that doesn't depend on oldcore. >> > > Creating a reference to the third object of a given class in a document (or > object number 3, or even the third object of a document) has absolutely no > meaning unless you have already that object at hand Why doesn't it have a meaning? If in my app, my spec says: - each doc has 3 objects of type P and the first one means this, the second one that and the 3rd one that other thing, then the 3rd one has a meaning. > , and so you already > have a source for its reference (solved not nicely by the > BaseObjectReference actually, but this was another story). > > >> I'd like to propose the following: >> * Modify ObjectReference to add 2 named parameters: Class reference and >> position >> > > Since position is really fragile without any real meaning, > having a > reference using a position will be a source for spurious issues, I am > definitely -1 for any positional reference. > > >> * Make the name optional in EntityReference >> > > This sounds like a sign of bad design to me... what is a entity without a > name in general ? It means it's a reference to an entity by a mean other than a name :) We need to decide but we could very well decide that some entities don't have a name and that they can be found/addressed by some other means (as in the case of Object references when locating them through class reference and position). More below >> This means that when we use an EntityReferenceResolver to resolve >> "wiki:space.page^wiki2:space2.page2" we get an ObjetReference with: >> * name = null >> * param1: name = "classReference", value = EntityReference >> * param2: name = "objectPosition", value = 0 >> > >> Rationale: >> * This is exactly what we already do for Locale (and what we'll do for >> Version too probably) so it's logical to do it for Object References too >> > > I agree with your sample and your rationale, there is a need to create a > reference to the first (and probably the only for such use case) object of > a given class in a given document without having to compose weird names or > positional references. This has definitely a meaning, much more than the > second or any numbered objects... And this object is not always "[0]" in > base reference syntax ! ah good, was trying to despair :) If the first one has a meaning, then 2nd or 3rd can also have a meaning, see my example above. Obviously the majority of use cases will be the first one but that doesn't mean the other positions are not needed. 0 = first not null xobject 1 = second not null xobject ... > However, resolving "wiki:space.page^wiki2:space2.page2" to that object is > not valid, since you do not really know what you are doing here, are you > speaking about the first object of a given class or an object named " > wiki2:space2.page2" ? Exactly. Which is why in my proposal we would have to agree that the current notation (thomas calls it syntax) only describes class reference and NOT a name (name = null). If we ever want to also be able to write a string and specify a name in it then we would need to invent a syntax, in the same manner that we currently have no syntax to express a locale in a document reference when expressed as a string. > So if we need to resolve a string into the first (or, > even if I am against, any) object of a given class, we need another syntax > at least (BaseObjectReference had already cause some poor stuff in > LocalUidReferenceSerializer where we had to remove wiki in a very bad way > currently). Using a name is very very far in the future so we don't need to invent a syntax for that now IMO. We don't even know if it'll ever happen actually… >> Consequences: >> * We need to modify the Seralizers/Resolvers accordingly >> > > According we have a new syntax for your kind of object reference, we may do > so. I resolver/serializer (as well as setter of class reference) I suggest > like Thomas that we support relative reference to classes based on the > containing document, both simplifying creation and avoiding > useless repetition. That seems good for now, even though in some far future we may want to have a class defined in a wiki and an xobject located in another subwiki… or not… ;) >> * We need to modify EntityReference to support a null name >> > > Do we really need that, or the name would simply reflect our special syntax > ? It would be slightly misleading to use the serialized class ref as the name IMO but why not... One danger is in code that will use it instead of using the well-known parameters for class ref and position. >> * We deprecate BaseObjectReference >> > > Does it really true ? It does not have the same meaning (even for your > initial proposal) since number and position are not the same and the > BaseObjectReference is a special reference, not a general object reference, > since it does not have a real free form name. Deprecating means we had to > replace them everywhere, and I am not sure it is easy to use index in place > of number in some places, like storage where we use reference for IDs. Maybe, in any case what I meant is that we'll need to make our code use the new way and not use BaseObjectReference as much as possible. >> * Probably some other stuff to modify like modifying event listeners >> listening on objects since it's now going to be much easier, etc >> >> WDYT? >> > > So, to resume, I understand the need to have a way to create reference to > the first object of given class in a document. Even if using an object > reference with, let say a special syntax, will introduce some bad > consequences on the implementation of equals equals/hashcode shoudl already work since they already take into account parameters. > and related methods on entity > reference (like those we have for locale or version), we should probably > consider it (Note that it will also affect property reference). > > I am definitely -1 for any positional reference which are meaningless. I think this is a pity and you're closing the door to lots of use cases and you're going to make them really hard and not performant to implement as a consequence. As a user, if I **want** to access the 3rd xobject because it has a meaning for my app, I'm not going to be able to do so. I'm going to need to load the doc and do some queries to iterate over xobject and count to 3… (which obviously is both painful and not performant). BTW your counter-suggestion to drop the position means you're going to break everything that already exists since it's currently possible to reference an object with class and number. It means inventing a new syntax, which is always painful to do. Thanks -Vincent _______________________________________________ devs mailing list [email protected] http://lists.xwiki.org/mailman/listinfo/devs
