Access to persistent data

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

Python-level access to persistent data is provided by the

:class:`Entity <cubicweb.entity>` class.

An entity class is bound to a schema entity type.  Descriptors are added when

classes are registered in order to initialize the class according to its schema:

* we can access the defined attributes in the schema thanks to the attributes of

  the same name on instances (typed value)

* we can access the defined relations in the schema thanks to the relations of

  the same name on instances (entities instances list)

`Formatting and output generation`:

* `view(__vid, __registry='views', **kwargs)`, applies the given view to the entity

  (and returns an unicode string)

* `absolute_url(*args, **kwargs)`, returns an absolute URL to access the primary view

  of an entity

* `rest_path()`, returns a relative REST URL to get the entity

* `printable_value(attr, value=_marker, attrtype=None, format='text/html', displaytime=True)`,

  returns a string enabling the display of an attribute value in a given format

  (the value is automatically recovered if necessary)

`Data handling`:

* `as_rset()`, converts the entity into an equivalent result set simulating the

   request `Any X WHERE X eid _eid_`

* `complete(skip_bytes=True)`, executes a request that recovers at

  once all the missing attributes of an entity

* `get_value(name)`, returns the value associated to the attribute name given

  in parameter

* `related(rtype, role='subject', limit=None, entities=False)`,

  returns a list of entities related to the current entity by the

  relation given in parameter

* `unrelated(rtype, targettype, role='subject', limit=None)`,

  returns a result set corresponding to the entities not (yet)

  related to the current entity by the relation given in parameter

  and satisfying its constraints

* `set_attributes(**kwargs)`, updates the attributes list with the corresponding

  values given named parameters

* `set_relations(**kwargs)`, add relations to the given object. To

   set a relation where this entity is the object of the relation,

   use `reverse_<relation>` as argument name.  Values may be an

   entity, a list of entities, or None (meaning that all relations of

   the given type from or to this object should be deleted).

* `copy_relations(ceid)`, copies the relations of the entities having the eid

  given in the parameters on the current entity

* `delete()` allows to delete the entity

The :class:`AnyEntity` class

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

inheriting from `cubicweb.entities.AnyEntity`. In general, we define this class

in `mycube.entities` module (or in a submodule if we want to split code among

multiple files) so that it will be available on both server and client side.

The class `AnyEntity` is a sub-class of Entity that add methods to it,

and helps specializing (by further subclassing) the handling of a

given entity type.

Most methods defined for `AnyEntity`, in addition to `Entity`, add

support for the `Dublin Core`_ metadata.

.. _`Dublin Core`: http://dublincore.org/

`Standard meta-data (Dublin Core)`:

* `dc_title()`, returns a unicode string corresponding to the

  meta-data `Title` (used by default is the first non-meta attribute

  of the entity schema)

* `dc_long_title()`, same as dc_title but can return a more

  detailed title

* `dc_description(format='text/plain')`, returns a unicode string

  corresponding to the meta-data `Description` (looks for a

  description attribute by default)

* `dc_authors()`, returns a unicode string corresponding to the meta-data

  `Authors` (owners by default)

* `dc_creator()`, returns a unicode string corresponding to the

  creator of the entity

* `dc_date(date_format=None)`, returns a unicode string corresponding to

  the meta-data `Date` (update date by default)

* `dc_type(form='')`, returns a string to display the entity type by

  specifying the preferred form (`plural` for a plural form)

* `dc_language()`, returns the language used by the entity

`Misc methods`:

* `after_deletion_path`, return (path, parameters) which should be

   used as redirect information when this entity is being deleted

* `pre_web_edit`, callback called by the web editcontroller when an

  entity will be created/modified, to let a chance to do some entity

  specific stuff (does nothing by default)

Inheritance

-----------

When describing a data model, entities can inherit from other entities as is

common in object-oriented programming.

You have the possibility to redefine whatever pleases you, as follow:

.. sourcecode:: python

    from cubes.OTHER_CUBE import entities

    class EntityExample(entities.EntityExample):

        def dc_long_title(self):

            return '%s (%s)' % (self.name, self.description)

The most specific entity definition will always the one used by the

ORM. For instance, the new EntityExample above in mycube replaces the

one in OTHER_CUBE. These types are stored in the `etype` section of

the `vregistry`.