The VRegistry

--------------

The recording process on startup

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Details of the recording process

````````````````````````````````

* by default all objects are registered automatically

* if some objects have to replace other objects or be included only if a

.. note::

    Once the function `registration_callback(vreg)` is implemented, all the objects

    have to be explicitly registered as it disables the automatic object registering.

.. sourcecode:: python

   # web/views/basecomponents.py

   def registration_callback(vreg):

      # register everything in the module except SeeAlsoComponent

      vreg.register_all(globals().values(), __name__, (SeeAlsoVComponent,))

      # conditionally register SeeAlsoVComponent

      if 'see_also' in vreg.schema:

          vreg.register(SeeAlsoVComponent)

   # goa/appobjects/sessions.py

   def registration_callback(vreg):

      vreg.register(SessionsCleaner)

Runtime objects selection

~~~~~~~~~~~~~~~~~~~~~~~~~

The object's selector is defined by its `__select__` class attribute.

When two selectors are combined using the `&` operator (formerly `chainall`), it

means that both should return a positive score. On success, the sum of scores is returned.

When two selectors are combined using the `|` operator (former `chainfirst`), it

means that one of them should return a positive score. On success, the first

positive score is returned.

Of course you can use paren to balance expressions.

`__registry__ = 'view'`) for a result set containing a `Card` entity, 2 objects

will probably be selectable:

* the default primary view (`__select__ = implements('Any')`), meaning

  that the object is selectable for any kind of entity type

* the specific `Card` primary view (`__select__ = implements('Card')`,

  meaning that the object is selectable for Card entities

Other primary views specific to other entity types won't be selectable

in this case. Among selectable objects, the implements selector will

return a higher score than the second view since it's more specific,

so it will be selected as expected.

Example

````````

The goal: when on a Blog, one wants the RSS link to refer to blog

entries, not to the blog entity itself.

To do that, one defines a method on entity classes that returns the

RSS stream url for a given entity. The default implementation on

AnyEntity and a specific implementation on Blog will do what we want.

But when we have a result set containing several Blog entities (or

different entities), we don't know on which entity to call the

aforementioned method. In this case, we keep the current behaviour

(e.g : call to limited_rql).

Hence we have two cases here, one for a single-entity rsets, the other

for multi-entities rsets.

In web/views/boxes.py lies the RSSIconBox class. Look at its selector ::

  class RSSIconBox(ExtResourcesBoxTemplate):

    """just display the RSS icon on uniform result set"""

    __select__ = ExtResourcesBoxTemplate.__select__ & non_final_entity()

It takes into account:

* the inherited selection criteria (one has to look them up in the

  class hierarchy to know the details)

* non_final_entity, which filters on rsets containing non final

  entities (a 'final entity' being synonym for entity attribute)

This matches our second case. Hence we have to provide a specific

component for the first case::

  class EntityRSSIconBox(RSSIconBox):

    """just display the RSS icon on uniform result set for a single entity"""

    __select__ = RSSIconBox.__select__ & one_line_rset()

Here, one adds the one_line_rset selector, which filters result sets

of size 1. When one chains selectors, the final score is the sum of

the score of each individual selector (unless one of them returns 0,

in which case the object is non selectable). Thus, on a multiple

entities selector, one_line_rset makes the EntityRSSIconBox class non

selectable. For an rset with one entity, the EntityRSSIconBox class

will have a higher score then RSSIconBox, which is what we wanted.

Of course, once this is done, you have to:

* fill in the call method of EntityRSSIconBox

* provide the default implementation of the method returning the RSS

  stream url on AnyEntity

* redefine this method on Blog.

When to use selectors?

Debugging

`````````

Once in a while, one needs to understand why a view (or any AppObject)

is, or is not selected appropriately. Looking at which selectors fired

(or did not) is the way. There exists a traced_selection context

manager to help with that.

Here is an example:

.. sourcecode:: python

Don't forget the 'from __future__ import with_statement' at the module

This will yield additional WARNINGs in the logs, like this::

    2009-01-09 16:43:52 - (cubicweb.selectors) WARNING: selector one_line_rset returned 0 for <class 'cubicweb.web.views.basecomponents.WFHistoryVComponent'>