doc/book/en/devrepo/entityclasses/data-as-objects.rst
author Aurelien Campeas <aurelien.campeas@logilab.fr>
Mon, 05 Jul 2010 12:37:21 +0200
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Access to persistent data
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--------------------------
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Python-level access to persistent data is provided by the
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:class:`Entity <cubicweb.entity>` class.
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An entity class is bound to a schema entity type.  Descriptors are added when
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classes are registered in order to initialize the class according to its schema:
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* we can access the defined attributes in the schema thanks to the attributes of
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  the same name on instances (typed value)
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* we can access the defined relations in the schema thanks to the relations of
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  the same name on instances (entities instances list)
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`Formatting and output generation`:
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* `view(__vid, __registry='views', **kwargs)`, applies the given view to the entity
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  (and returns an unicode string)
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* `absolute_url(*args, **kwargs)`, returns an absolute URL to access the primary view
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  of an entity
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* `rest_path()`, returns a relative REST URL to get the entity
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* `printable_value(attr, value=_marker, attrtype=None, format='text/html', displaytime=True)`,
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  returns a string enabling the display of an attribute value in a given format
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  (the value is automatically recovered if necessary)
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`Data handling`:
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* `as_rset()`, converts the entity into an equivalent result set simulating the
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   request `Any X WHERE X eid _eid_`
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* `complete(skip_bytes=True)`, executes a request that recovers at
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  once all the missing attributes of an entity
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* `get_value(name)`, returns the value associated to the attribute name given
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  in parameter
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* `related(rtype, role='subject', limit=None, entities=False)`,
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  returns a list of entities related to the current entity by the
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  relation given in parameter
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* `unrelated(rtype, targettype, role='subject', limit=None)`,
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  returns a result set corresponding to the entities not (yet)
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  related to the current entity by the relation given in parameter
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  and satisfying its constraints
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* `set_attributes(**kwargs)`, updates the attributes list with the corresponding
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  values given named parameters
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* `set_relations(**kwargs)`, add relations to the given object. To
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   set a relation where this entity is the object of the relation,
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   use `reverse_<relation>` as argument name.  Values may be an
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   entity, a list of entities, or None (meaning that all relations of
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   the given type from or to this object should be deleted).
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* `copy_relations(ceid)`, copies the relations of the entities having the eid
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  given in the parameters on the current entity
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* `delete()` allows to delete the entity
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The :class:`AnyEntity` class
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----------------------------
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To provide a specific behavior for each entity, we can define a class
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inheriting from `cubicweb.entities.AnyEntity`. In general, we define this class
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in `mycube.entities` module (or in a submodule if we want to split code among
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multiple files) so that it will be available on both server and client side.
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The class `AnyEntity` is a sub-class of Entity that add methods to it,
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and helps specializing (by further subclassing) the handling of a
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given entity type.
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Most methods defined for `AnyEntity`, in addition to `Entity`, add
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support for the `Dublin Core`_ metadata.
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.. _`Dublin Core`: http://dublincore.org/
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`Standard meta-data (Dublin Core)`:
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* `dc_title()`, returns a unicode string corresponding to the
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  meta-data `Title` (used by default is the first non-meta attribute
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  of the entity schema)
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* `dc_long_title()`, same as dc_title but can return a more
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  detailed title
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* `dc_description(format='text/plain')`, returns a unicode string
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  corresponding to the meta-data `Description` (looks for a
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  description attribute by default)
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* `dc_authors()`, returns a unicode string corresponding to the meta-data
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  `Authors` (owners by default)
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* `dc_creator()`, returns a unicode string corresponding to the
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  creator of the entity
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* `dc_date(date_format=None)`, returns a unicode string corresponding to
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  the meta-data `Date` (update date by default)
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* `dc_type(form='')`, returns a string to display the entity type by
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  specifying the preferred form (`plural` for a plural form)
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* `dc_language()`, returns the language used by the entity
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`Misc methods`:
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* `after_deletion_path`, return (path, parameters) which should be
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   used as redirect information when this entity is being deleted
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* `pre_web_edit`, callback called by the web editcontroller when an
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  entity will be created/modified, to let a chance to do some entity
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  specific stuff (does nothing by default)
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Inheritance
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-----------
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When describing a data model, entities can inherit from other entities as is
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common in object-oriented programming.
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You have the possibility to redefine whatever pleases you, as follow:
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.. sourcecode:: python
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    from cubes.OTHER_CUBE import entities
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    class EntityExample(entities.EntityExample):
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        def dc_long_title(self):
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            return '%s (%s)' % (self.name, self.description)
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The most specific entity definition will always the one used by the
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ORM. For instance, the new EntityExample above in mycube replaces the
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one in OTHER_CUBE. These types are stored in the `etype` section of
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the `vregistry`.
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Notice this is different than yams schema inheritance, which is an
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experimental undocumented feature.
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Application logic
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-----------------
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While a lot of custom behaviour and application logic can be
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implemented using entity classes, the programmer must be aware that
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adding new attributes and method on an entity class adds may shadow
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schema-level attribute or relation definitions.
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To keep entities clean (mostly data structures plus a few universal
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methods such as listed above), one should use `adapters` (see
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:ref:`adapters`).