# HG changeset patch # User Sylvain Thenault # Date 1227195742 -3600 # Node ID d3005cdc968f6f9379650a757068df746d5db5c2 # Parent 10b63bb96e70b912e4ca35cc3680072f9e6600f4 add detailled notes about objects selection (to be translated / proprified...) diff -r 10b63bb96e70 -r d3005cdc968f doc/book/en/02-foundation.en.txt --- a/doc/book/en/02-foundation.en.txt Thu Nov 20 15:04:33 2008 +0100 +++ b/doc/book/en/02-foundation.en.txt Thu Nov 20 16:42:22 2008 +0100 @@ -52,7 +52,7 @@ stored in the database at the time an instance is created. `CubicWeb` provides a certain number of system entities included automatically as it is necessarry for the core of `CubicWeb` and a library of - cubes that can be explicitely included if necessarry. + cubes that can be explicitely included if necessary. *entity type* @@ -74,6 +74,10 @@ a relation is said final if its `object` is a final type. This is equivalent to an entity attribute. +*relation definition* + a relation definition is a 3-uple (subject entity type, relation type, object entity type), + with an associated set of property such as cardinality, constraints... + *repository* this is the RQL server side of `CubicWeb`. Be carefull not to get confused with a Mercurial repository or a debian repository. @@ -122,15 +126,15 @@ side of the framework and the RQL repository. .. _`Python Remote Object`: http://pyro.sourceforge.net/ -.. _`yams`: http://www.logilab.org/project/name/yams/ +.. _`yams`: http://www.logilab.org/project/yams/ `CubicWeb` engine ----------------- -The web engine in `CubicWeb` is a set of classes managing a set of objects loaded -dynamically at the startup of `CubicWeb`. Those dynamics objects, based on the schema -or the library, are building the final web site. The differents dymanic components are +The engine in `CubicWeb` is a set of classes managing a set of objects loaded +dynamically at the startup of `CubicWeb` (*appobjects*). Those dynamics objects, based on the schema +or the library, are building the final application. The differents dymanic components are by example: * client and server side @@ -158,19 +162,144 @@ * a frontal web (only twisted is available so far), transparent for dynamic objects * an object that encapsulates the configuration -* a `vregistry` (`cubicweb.cwvreg`) containing the dynamic objects loaded automatically +* a `registry` (`cubicweb.cwvreg`) containing the dynamic objects loaded automatically +Every *appobject* may access to the instance configuration using its *config* attribute +and to the registry using its *vreg* attribute. -Details of a recording process ------------------------------- +Details of the recording process +-------------------------------- -At startup, the `vregistry` or registers base, inspects a number of directories +At startup, the `registry` or registers base, inspects a number of directories looking for compatible classes definition. After a recording process, the objects are assigned to registers so that they can be selected dynamically while the application is running. The base class of those objects is `AppRsetObject` (module `cubicweb.common.appobject`). +XXX registers example +XXX actual details of the recording process! + +Runtime objects selection +------------------------- +XXX tell why it's a cw foundation! + +Application objects are stored in the registry using a two level hierarchy : + + object's `__registry__` : object's `id` : [list of app objects] + +The following rules are applied to select an object given a register and an id and an input context: +* each object has a selector + - its selector may be derivated from a set of basic (or not :) + selectors using `chainall` or `chainfirst` combinators +* a selector return a score >= 0 +* a score of 0 means the objects can't be applied to the input context +* the object with the greatest score is selected. If multiple objects have an + identical score, one of them is selected randomly (this is usually a bug) + +The object's selector is the `__selector__` class method on the object's class. + +The score is used to choose the most pertinent objects where there are more than +one selectable object. For instance, if you're selecting the primary +(eg `id = 'primary'`) view (eg `__registry__ = 'view'`) for a result set containing +a `Card` entity, 2 objects will probably be selectable: + +* the default primary view (`accepts = 'Any'`) +* the specific `Card` primary view (`accepts = 'Card'`) + +This is because primary views are using the `accept_selector` which is considering the +`accepts` class attribute of the object's class. Other primary views specific to other +entity types won't be selectable in this case. And among selectable objects, the +accept selector will return a higher score the the second view since it's more +specific, so it will be selected as expected. + +Usually, you won't define it directly but by defining the `__selectors__` tuple +on the class, with :: + + __selectors__ = (sel1, sel2) + +which is equivalent to :: + + __selector__ = chainall(sel1, sel2) + +The former is prefered since it's shorter and it's ease overriding in +subclasses (you have access to sub-selectors instead of the wrapping function). + +:chainall(selectors...): if one selector return 0, return 0, else return the sum of scores + +:chainfirst(selectors...): return the score of the first selector which has a non zero score + +XXX describe standard selector (link to generated api doc!) + +Example +~~~~~~~ +Le but final : quand on est sur un Blog, on veut que le lien rss de celui-ci pointe +vers les entrées de ce blog, non vers l'entité blog elle-même. + +L'idée générale pour résoudre ça : on définit une méthode sur les classes d'entité +qui renvoie l'url du flux rss pour l'entité en question. Avec une implémentation +par défaut sur AnyEntity et une implémentation particulière sur Blog qui fera ce +qu'on veut. + +La limitation : on est embêté dans le cas ou par ex. on a un result set qui contient +plusieurs entités Blog (ou autre chose), car on ne sait pas sur quelle entité appeler +la méthode sus-citée. Dans ce cas, on va conserver le comportement actuel (eg appel +à limited_rql) + +Donc : on veut deux cas ici, l'un pour un rset qui contient une et une seule entité, +l'autre pour un rset qui contient plusieurs entité. + +Donc... On a déja dans web/views/boxes.py la classe RSSIconBox qui fonctionne. Son +sélecteur :: + + class RSSIconBox(ExtResourcesBoxTemplate): + """just display the RSS icon on uniform result set""" + __selectors__ = ExtResourcesBoxTemplate.__selectors__ + (nfentity_selector,) + + +indique qu'il prend en compte : + +* les conditions d'apparition de la boite (faut remonter dans les classes parentes + pour voir le détail) +* nfentity_selector, qui filtre sur des rset contenant une liste d'entité non finale + +ça correspond donc à notre 2eme cas. Reste à fournir un composant plus spécifique +pour le 1er cas :: + + class EntityRSSIconBox(RSSIconBox): + """just display the RSS icon on uniform result set for a single entity""" + __selectors__ = RSSIconBox.__selectors__ + (onelinerset_selector,) + + +Ici, on ajoute onelinerset_selector, qui filtre sur des rset de taille 1. Il faut +savoir que quand on chaine des selecteurs, le score final est la somme des scores +renvoyés par chaque sélecteur (sauf si l'un renvoie zéro, auquel cas l'objet est +non sélectionnable). Donc ici, sur un rset avec plusieurs entités, onelinerset_selector +rendra la classe EntityRSSIconBox non sélectionnable, et on obtiendra bien la +classe RSSIconBox. Pour un rset avec une entité, la classe EntityRSSIconBox aura un +score supérieur à RSSIconBox et c'est donc bien elle qui sera sélectionnée. + +Voili voilou, il reste donc pour finir tout ça : + +* à définir le contenu de la méthode call de EntityRSSIconBox +* fournir l'implémentation par défaut de la méthode renvoyant l'url du flux rss sur + AnyEntity +* surcharger cette methode dans blog.Blog + +When to use selectors? +~~~~~~~~~~~~~~~~~~~~~~ +Il faut utiliser les sélecteurs pour faire des choses différentes en +fonction de ce qu'on a en entrée. Dès qu'on a un "if" qui teste la +nature de `self.rset` dans un objet, il faut très sérieusement se +poser la question s'il ne vaut pas mieux avoir deux objets différent +avec des sélecteurs approprié. + +If this is so fundamental, why don't I see them more often? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Because you're usually using base classes which are hiding the plumbing +of __registry__ (almost always), id (often when using "standard" object), +register and selector. + API Python/RQL -------------- @@ -409,3 +538,4 @@ * the file ``__pkginfo__.py`` * the schema definition + XXX false, we may want to have cubes which are only adding a service, no persistent data (eg embeding for instance)