.. -*- coding: utf-8 -*-

Entity type definition

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

An entity type is defined by a Python class which inherits from `EntityType`.

The class definition contains the description of attributes and relations

for the defined entity type.

The class name corresponds to the entity type name. It is exepected to be

defined in the module ``mycube.schema``.

For example ::

  class Person(EntityType):

    """A person with the properties and the relations necessary for my

    application"""

    last_name = String(required=True, fulltextindexed=True)

    first_name = String(required=True, fulltextindexed=True)

    title = String(vocabulary=('Mr', 'Mrs', 'Miss'))

    date_of_birth = Date()

    works_for = SubjectRelation('Company', cardinality='?*')

The entity described above defines three attributes of type String,

last_name, first_name and title, an attribute of type Date for the date of

birth and a relation that connects a `Person` to another entity of type

`Company` through the semantic `works_for`.

The name of the Python attribute corresponds to the name of the attribute

or the relation in `CubicWeb` application.

Built-in types for attributes

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

All `CubicWeb` built-in types are available : `String`, `Int`, `Float`,

`Decimal`, `Boolean`, `Date`, `Datetime`, `Time`, `Interval`, `Byte`

and `Password`.

They are implicitely imported (as well as the special the function "_"

for translation :ref:`internationalization`).

An attribute is defined in the schema as follows::

    attr_name = attr_type(properties*)

where `attr_type` is one of the type listed above and `properties` is

a list of  the attribute needs to statisfy (see :ref:`properties`

for more details).

Meta-data

`````````

Each entity type has at least the following meta-relations :

  - `eid` (`Int`)

  - `creation_date` (`Datetime`)

  - `modification_date` (`Datetime`)

  - `created_by` (`CWUser`) (which user created the entity)

  - `owned_by` (`CWUser`) (to whom the entity belongs; by default the

     creator but not necessary, and it could have multiple owners)

  - `is` (`CWEType`) (of which type the entity is)

* relations can be defined by using `ObjectRelation` or `SubjectRelation`.

  The first argument of `SubjectRelation` or `ObjectRelation` gives respectively

  the object/subject entity type of the relation. This could be :

  * a string corresponding to an entity type

  * a tuple of string corresponding to multiple entity types

  * special string such as follows :

    - "**" : all types of entities

    - "*" : all types of non-meta entities

    - "@" : all types of meta entities but not system entities (e.g. used for

      the basic schema description)

* it is possible to use the attribute `meta` to flag an entity type as a `meta`

  (e.g. used to describe/categorize other entities)

Optionnal properties

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

.. _properties:

* optional properties for attributes and relations :

  - `description` : a string describing an attribute or a relation. By default

    this string will be used in the editing form of the entity, which means

    that it is supposed to help the end-user and should be flagged by the

    function `_` to be properly internationalized.

  - `constraints` : a list of conditions/constraints that the relation has to

    satisfy (c.f. `Contraints`_)

  - `cardinality` : a two character string which specify the cardinality of the

    relation. The first character defines the cardinality of the relation on

    the subject, and the second on the object. When a relation can have

    multiple subjects or objects, the cardinality applies to all,

    not on a one-to-one basis (so it must be consistent...). The possible

    values are inspired from regular expression syntax :

    * `1`: 1..1

    * `?`: 0..1

    * `+`: 1..n

    * `*`: 0..n

  - `meta` : boolean indicating that the relation is a meta-relation (false by

    default)

* optional properties for attributes :

  - `required` : boolean indicating if the attribute is required (false by default)

  - `unique` : boolean indicating if the value of the attribute has to be unique

    or not within all entities of the same type (false by default)

  - `indexed` : boolean indicating if an index needs to be created for this

    attribute in the database (false by default). This is useful only if

    you know that you will have to run numerous searches on the value of this

    attribute.

  - `default` : default value of the attribute. In case of date types, the values

    which could be used correspond to the RQL keywords `TODAY` and `NOW`.

  - `vocabulary` : specify static possible values of an attribute

* optional properties of type `String` :

  - `fulltextindexed` : boolean indicating if the attribute is part of

    the full text index (false by default) (*applicable on the type `Byte`

    as well*)

  - `internationalizable` : boolean indicating if the value of the attribute

    is internationalizable (false by default)

  - `maxsize` : integer providing the maximum size of the string (no limit by default)

* optional properties for relations :

  - `composite` : string indicating that the subject (composite == 'subject')

    is composed of the objects of the relations. For the opposite case (when

    the object is composed of the subjects of the relation), we just set

    'object' as value. The composition implies that when the relation

    is deleted (so when the composite is deleted), the composed are also deleted.

Contraints

``````````

By default, the available constraint types are :

* `SizeConstraint` : allows to specify a minimum and/or maximum size on

  string (generic case of `maxsize`)

* `BoundConstraint` : allows to specify a minimum and/or maximum value on

  numeric types

* `UniqueConstraint` : identical to "unique=True"

* `StaticVocabularyConstraint` : identical to "vocabulary=(...)"

* `RQLConstraint` : allows to specify a RQL query that has to be satisfied

  by the subject and/or the object of the relation. In this query the variables

  `S` and `O` are reserved for the entities subject and object of the

  relation.

* `RQLVocabularyConstraint` : similar to the previous type of constraint except

  that it does not express a "strong" constraint, which means it is only used to

  restrict the values listed in the drop-down menu of editing form, but it does

  not prevent another entity to be selected.

Definition of relations

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

XXX add note about defining relation type / definition

A relation is defined by a Python class heriting `RelationType`. The name

of the class corresponds to the name of the type. The class then contains

a description of the properties of this type of relation, and could as well

contain a string for the subject and a string for the object. This allows to create

new definition of associated relations, (so that the class can have the

definition properties from the relation) for example ::

  class locked_by(RelationType):

    """relation on all entities indicating that they are locked"""

    inlined = True

    cardinality = '?*'

    subject = '*'

    object = 'CWUser'

In addition to the permissions, the properties of the relation types

(shared also by all definition of relation of this type) are :

* `inlined` : boolean handling the physical optimization for archiving

  the relation in the subject entity table, instead of creating a specific

  table for the relation. This applies to the relation when the cardinality

  of subject->relation->object is 0..1 (`?`) or 1..1 (`1`)

* `symmetric` : boolean indicating that the relation is symmetrical, which

  means `X relation Y` implies `Y relation X`

In the case of simultaneous relations definitions, `subject` and `object`

can both be equal to the value of the first argument of `SubjectRelation`

and `ObjectRelation`.

When a relation is not inlined and not symmetrical, and it does not require

specific permissions, its definition (by using `SubjectRelation` and

`ObjectRelation`) is all we need.

Updating your application with your new schema

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

If you modified your schema, the update is not automatic; indeed, this is

in general not a good idea.

Instead, you call a shell on your application, which is a

an interactive python shell, with an appropriate

cubicweb environment ::

   cubicweb-ctl shell myinstance

and type ::

   add_entity_type('Person')

And restart your application!