doc/book/devrepo/entityclasses/data-as-objects.rst
author Philippe Pepiot <philippe.pepiot@logilab.fr>
Mon, 29 Oct 2018 10:04:31 +0100
branch3.26
changeset 12432 2fcb53ee5178
parent 10491 c67bcee93248
child 12556 d1c659d70368
permissions -rw-r--r--
Fix flake8 issues since release 3.6.0 Flake8 had a new release which raise new issues, namely: W504: line break after binary operator F841: local variable 'ex' is assigned to but never used W605: invalid escape sequence F821: undefined name 'buffer' (noqa seems the only way to avoid this false positive) Also pin flake8>=3.6 in our tests and make explicit that we use python3 to run flake8 tests.
Ignore whitespace changes - Everywhere: Within whitespace: At end of lines:
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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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.. XXX this part is not clear. refactor it.
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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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* the attributes defined in the schema appear as attributes of these classes
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* the relations defined in the schema appear as attributes of these classes,
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  but are lists of instances
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`Formatting and output generation`:
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* :meth:`view(__vid, __registry='views', **kwargs)`, applies the given view to the entity
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  (and returns a unicode string)
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* :meth:`absolute_url(*args, **kwargs)`, returns an absolute URL including the base-url
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* :meth:`rest_path()`, returns a relative REST URL to get the entity
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* :meth:`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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* :meth:`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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* :meth:`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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* :meth:`get_value(name)`, returns the value associated to the attribute name given
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  in parameter
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* :meth:`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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* :meth:`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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* :meth:`cw_set(**kwargs)`, updates entity's attributes and/or relation with the
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  corresponding values given named parameters. To set a relation where this
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  entity is the object of the relation, use `reverse_<relation>` as argument
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  name.  Values may be an entity, a list of entities, or None (meaning that all
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  relations of the given type from or to this object should be deleted).
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* :meth:`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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* :meth:`cw_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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* :meth:`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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* :meth:`dc_long_title()`, same as dc_title but can return a more
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  detailed title
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* :meth:`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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* :meth:`dc_authors()`, returns a unicode string corresponding to the meta-data
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  `Authors` (owners by default)
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* :meth:`dc_creator()`, returns a unicode string corresponding to the
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  creator of the entity
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* :meth:`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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* :meth:`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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* :meth:`dc_language()`, returns the language used by the entity
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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`).