Entity type definition

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

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

class name correponds to the type name. Then the content of the class contains

the description of attributes and relations for the defined entity type,

by example ::

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

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

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

    title = String(vocabulary=('M', 'Mme', 'Mlle'))

    date_of_birth = Date()

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

* the name of the Python attribute corresponds to the name of the attribute

  or the relation in `CubicWeb` application.

* all built-in types are available : `String`, `Int`, `Float`,

* each entity type has at least the following meta-relations :

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

  - `owned_by` (`EUser`) (who does the entity belongs to, by default the 

     creator but not necessarry and it could have multiple owners)

* it is also possible to define relations of type object by using `ObjectRelation`

  instead of `SubjectRelation`

* the first argument of `SubjectRelation` and `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 correponding to multiple entities 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)

* optional properties for attributes and relations : 

  - `description` : 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` : list of conditions/constraints that the relation needs to

    satisfy (c.f. `Contraints`_)

  - `cardinality` : two characters string which specify the cardinality of the

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

    the subject, the second on the object of the relation. When a relation

    has multiple possible subjects or objects, the cardinality applies to all

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

    values are inspired from regular expressions syntax :

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

    default)

* optionnal 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 usefull 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 correpond to the RQL keywords `TODAY` and `NOW`.

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

* optionnal 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)

* optionnal 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 need

    to set 'object' as the 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 constraints 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 needs 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

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 

contains 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) by example ::

  class locked_by(RelationType):

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

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`)

* `symetric` : boolean indication that the relation is symetrical, 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 symetrical, and it does not require

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

`ObjectRelation`) is all we need.

Permissions definition

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

Define permissions is set through to the attribute `permissions` of entities and

relations types. It defines a dictionnary where the keys are the access types

(action), and the values are the authorized groups or expressions.

For an entity type, the possible actions are `read`, `add`, `update` and

For a relation type, the possible actions are `read`, `add`, and `delete`.

For each access type, a tuple indicates the name of the authorized groups and/or

one or multiple RQL expressions to satisfy to grant access. The access is

provided once the user is in the listed groups or one of the RQL condition is

satisfied.

The standard groups are :

* `owners` : virtual group corresponding to the entity's owner.

  This can only be used for the actions `update` and `delete` of an entity

  type.

It is also possible to use specific groups if they are define in the precreate 

of the application (``migration/precreate.py``).

Use of RQL expression for writing rights

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

It is possible to define RQL expression to provide update permission 

(`add`, `delete` and `update`) on relation and entity types.

RQL expression for entity type permission :

* you have to use the class `ERQLExpression`

* the used expression corresponds to the WHERE statement of an RQL query

* in this expression, the variables X and U are pre-defined references

  respectively on the current entity (on which the action is verified) and

  on the user who send the request

* it is possible to use, in this expression, a special relation 

  "has_<ACTION>_permission" where the subject is the user and the 

  object is a any variable, meaning that the user needs to have

  permission to execute the action <ACTION> on the entities related

  to this variable 

For RQL expressions on a relation type, the principles are the same except 

for the following :

* you have to use the class `RQLExpression` in the case of a non-final relation

* in the expression, the variables S, O and U are pre-defined references

  to respectively the subject and the object of the current relation (on

  which the action is being verified) and the user who executed the query

* we can also defined rights on attributes of an entity (non-final relation),

  knowing that : 

  - to defines RQL expression, we have to use the class `ERQLExpression`

    in which X represents the entity the attribute belongs to

  - the permissions `add` and `delete` are equivalent. Only `add`/`read`

    are actually taken in consideration.

In addition to that the entity type `EPermission` from the standard library

allow to build very complex and dynamic security architecture. The schema of

this entity type is as follow : ::

Example of configuration ::

This configuration indicates that an entity `EPermission` named

"add_version" can be associated to a project and provides rights to create

new versions on this project to specific groups. It is important to notice that :

* in such case, we have to protect both the entity type "Version" and the relation

  associating a version to a project ("version_of")

* because of the genricity of the entity type `EPermission`, we have to execute

  a unification with the groups and/or the states if necessary in the expression

  ("U in_group G, P require_group G" in the above example)

Use of RQL expression for reading rights

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

The principles are the same but with the following restrictions :

* we can not use `RRQLExpression` on relation types for reading

* special relations "has_<ACTION>_permission" can not be used

Note on the use of RQL expression for `add` permission

Potentially, the use of an RQL expression to add an entity or a relation

can cause problems for the user interface, because if the expression uses

the entity or the relation to create, then we are not able to verify the 

permissions before we actually add the entity (please note that this is

not a problem for the RQL server at all, because the permissions checks are

done after the creation). In such case, the permission check methods 

(check_perm, has_perm) can indicate that the user is not allowed to create 

this entity but can obtain the permission. 

To compensate this problem, it is usually necessary, for such case,

to use an action that reflects the schema permissions but which enables

to check properly the permissions so that it would show up if necessary.