--- /dev/null Thu Jan 01 00:00:00 1970 +0000
+++ b/doc/book/devrepo/datamodel/definition.rst Thu Jan 08 22:11:06 2015 +0100
@@ -0,0 +1,912 @@
+.. -*- coding: utf-8 -*-
+
+.. _datamodel_definition:
+
+Yams *schema*
+-------------
+
+The **schema** is the core piece of a *CubicWeb* instance as it
+defines and handles the data model. It is based on entity types that
+are either already defined in `Yams`_ and the *CubicWeb* standard
+library; or more specific types defined in cubes. The schema for a
+cube is defined in a `schema` python module or package.
+
+.. _`Yams`: http://www.logilab.org/project/yams
+
+.. _datamodel_overview:
+
+Overview
+~~~~~~~~
+
+The core idea of the yams schema is not far from the classical
+`Entity-relationship`_ model. But while an E/R model (or `logical
+model`) traditionally has to be manually translated to a lower-level
+data description language (such as the SQL `create table`
+sublanguage), also often described as the `physical model`, no such
+step is required with |yams| and |cubicweb|.
+
+.. _`Entity-relationship`: http://en.wikipedia.org/wiki/Entity-relationship_model
+
+This is because in addition to high-level, logical |yams| models, one
+uses the |rql| data manipulation language to query, insert, update and
+delete data. |rql| abstracts as much of the underlying SQL database as
+a |yams| schema abstracts from the physical layout. The vagaries of
+SQL are avoided.
+
+As a bonus point, such abstraction make it quite comfortable to build
+or use different backends to which |rql| queries apply.
+
+So, as in the E/R formalism, the building blocks are ``entities``
+(:ref:`EntityType`), ``relationships`` (:ref:`RelationType`,
+:ref:`RelationDefinition`) and ``attributes`` (handled like relation
+with |yams|).
+
+Let us detail a little the divergences between E/R and |yams|:
+
+* all relationship are binary which means that to represent a
+ non-binary relationship, one has to use an entity,
+* relationships do not support attributes (yet, see:
+ http://www.cubicweb.org/ticket/341318), hence the need to reify it
+ as an entity if need arises,
+* all entities have an `eid` attribute (an integer) that is its
+ primary key (but it is possible to declare uniqueness on other
+ attributes)
+
+Also |yams| supports the notions of:
+
+* entity inheritance (quite experimental yet, and completely
+ undocumented),
+* relation type: that is, relationships can be established over a set
+ of couple of entity types (henre the distinction made between
+ `RelationType` and `RelationDefinition` below)
+
+Finally |yams| has a few concepts of its own:
+
+* relationships being oriented and binary, we call the left hand
+ entity type the `subject` and the right hand entity type the
+ `object`
+
+.. note::
+
+ The |yams| schema is available at run time through the .schema
+ attribute of the `vregistry`. It's an instance of
+ :class:`cubicweb.schema.Schema`, which extends
+ :class:`yams.schema.Schema`.
+
+.. _EntityType:
+
+Entity type
+~~~~~~~~~~~
+
+An entity type is an instance of :class:`yams.schema.EntitySchema`. Each entity type has
+a set of attributes and relations, and some permissions which define who can add, read,
+update or delete entities of this type.
+
+The following built-in types are available: ``String``,
+``Int``, ``BigInt``, ``Float``, ``Decimal``, ``Boolean``,
+``Date``, ``Datetime``, ``Time``, ``Interval``, ``Byte`` and
+``Password``. They can only be used as attributes of an other entity
+type.
+
+There is also a `RichString` kindof type:
+
+ .. autoclass:: yams.buildobjs.RichString
+
+The ``__unique_together__`` class attribute is a list of tuples of names of
+attributes or inlined relations. For each tuple, CubicWeb ensures the unicity
+of the combination. For example:
+
+.. sourcecode:: python
+
+ class State(EntityType):
+ __unique_together__ = [('name', 'state_of')]
+
+ name = String(required=True)
+ state_of = SubjectRelation('Workflow', cardinality='1*',
+ composite='object', inlined=True)
+
+
+You can find more base entity types in
+:ref:`pre_defined_entity_types`.
+
+.. XXX yams inheritance
+
+.. _RelationType:
+
+Relation type
+~~~~~~~~~~~~~
+
+A relation type is an instance of
+:class:`yams.schema.RelationSchema`. A relation type is simply a
+semantic definition of a kind of relationship that may occur in an
+application.
+
+It may be referenced by zero, one or more relation definitions.
+
+It is important to choose a good name, at least to avoid conflicts
+with some semantically different relation defined in other cubes
+(since there's only a shared name space for these names).
+
+A relation type holds the following properties (which are hence shared
+between all relation definitions of that type):
+
+* `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 relations where cardinality
+ of subject->relation->object is 0..1 (`?`) or 1..1 (`1`) for *all* its relation
+ definitions.
+
+* `symmetric`: boolean indicating that the relation is symmetrical, which
+ means that `X relation Y` implies `Y relation X`.
+
+.. _RelationDefinition:
+
+Relation definition
+~~~~~~~~~~~~~~~~~~~
+
+A relation definition is an instance of
+:class:`yams.schema.RelationDefinition`. It is a complete triplet
+"<subject entity type> <relation type> <object entity type>".
+
+When creating a new instance of that class, the corresponding
+:class:`RelationType` instance is created on the fly if necessary.
+
+Properties
+``````````
+
+The available properties for relation definitions are enumerated
+here. There are several kind of properties, as some relation
+definitions are actually attribute definitions, and other are not.
+
+Some properties may be completely optional, other may have a default
+value.
+
+Common properties for attributes and relations:
+
+* `description`: a unicode 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. `Constraints`_)
+
+* `cardinality`: a two character string specifying 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...). Default value is '**'. The possible values are
+ inspired from regular expression syntax:
+
+ * `1`: 1..1
+ * `?`: 0..1
+ * `+`: 1..n
+ * `*`: 0..n
+
+Attributes properties:
+
+* `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`.
+
+* `metadata`: Is also accepted as an argument of the attribute contructor. It is
+ not really an attribute property. see `Metadata`_ for details.
+
+Properties for `String` attributes:
+
+* `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)
+
+Relation properties:
+
+* `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,
+ at least), the composed are also deleted.
+
+* `fulltext_container`: string indicating if the value if the full
+ text indexation of the entity on one end of the relation should be
+ used to find the entity on the other end. The possible values are
+ 'subject' or 'object'. For instance the use_email relation has that
+ property set to 'subject', since when performing a full text search
+ people want to find the entity using an email address, and not the
+ entity representing the email address.
+
+Constraints
+```````````
+
+By default, the available constraint types are:
+
+General Constraints
+......................
+
+* `SizeConstraint`: allows to specify a minimum and/or maximum size on
+ string (generic case of `maxsize`)
+
+* `BoundaryConstraint`: allows to specify a minimum and/or maximum value
+ on numeric types and date
+
+.. sourcecode:: python
+
+ from yams.constraints import BoundaryConstraint, TODAY, NOW, Attribute
+
+ class DatedEntity(EntityType):
+ start = Date(constraints=[BoundaryConstraint('>=', TODAY())])
+ end = Date(constraints=[BoundaryConstraint('>=', Attribute('start'))])
+
+ class Before(EntityType);
+ last_time = DateTime(constraints=[BoundaryConstraint('<=', NOW())])
+
+* `IntervalBoundConstraint`: allows to specify an interval with
+ included values
+
+.. sourcecode:: python
+
+ class Node(EntityType):
+ latitude = Float(constraints=[IntervalBoundConstraint(-90, +90)])
+
+* `UniqueConstraint`: identical to "unique=True"
+
+* `StaticVocabularyConstraint`: identical to "vocabulary=(...)"
+
+Constraints can be dependent on a fixed value (90, Date(2015,3,23)) or a variable.
+In this second case, yams can handle :
+
+* `Attribute`: compare to the value of another attribute.
+* `TODAY`: compare to the current Date.
+* `NOW`: compare to the current Datetime.
+
+RQL Based Constraints
+......................
+
+RQL based constraints may take three arguments. The first one is the ``WHERE``
+clause of a RQL query used by the constraint. The second argument ``mainvars``
+is the ``Any`` clause of the query. By default this include `S` reserved for the
+subject of the relation and `O` for the object. Additional variables could be
+specified using ``mainvars``. The argument expects a single string with all
+variable's name separated by spaces. The last one, ``msg``, is the error message
+displayed when the constraint fails. As RQLVocabularyConstraint never fails the
+third argument is not available.
+
+* `RQLConstraint`: allows to specify a RQL query that has to be satisfied
+ by the subject and/or the object of relation. In this query the variables
+ `S` and `O` are reserved for the relation subject and object entities.
+
+* `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.
+
+* `RQLUniqueConstraint`: allows to the specify a RQL query that ensure that an
+ attribute is unique in a specific context. The Query must **never** return more
+ than a single result to be satisfied. In this query the variables `S` is
+ reserved for the relation subject entity. The other variables should be
+ specified with the second constructor argument (mainvars). This constraint type
+ should be used when __unique_together__ doesn't fit.
+
+.. XXX note about how to add new constraint
+
+.. _securitymodel:
+
+The security model
+~~~~~~~~~~~~~~~~~~
+
+The security model of `CubicWeb` is based on `Access Control List`.
+The main principles are:
+
+* users and groups of users
+* a user belongs to at least one group of user
+* permissions (`read`, `update`, `create`, `delete`)
+* permissions are assigned to groups (and not to users)
+
+For *CubicWeb* in particular:
+
+* we associate rights at the entities/relations schema level
+
+* the default groups are: `managers`, `users` and `guests`
+
+* users belong to the `users` group
+
+* there is a virtual group called `owners` to which we can associate only
+ `delete` and `update` permissions
+
+ * we can not add users to the `owners` group, they are implicitly added to it
+ according to the context of the objects they own
+
+ * the permissions of this group are only checked on `update`/`delete` actions
+ if all the other groups the user belongs to do not provide those permissions
+
+Setting permissions is done with the class attribute `__permissions__`
+of entity types and relation definitions. The value of this attribute
+is a dictionary where the keys are the access types (action), and the
+values are the authorized groups or rql expressions.
+
+For an entity type, the possible actions are `read`, `add`, `update` and
+`delete`.
+
+For a relation, the possible actions are `read`, `add`, and `delete`.
+
+For an attribute, the possible actions are `read`, `add` and `update`,
+and they are a refinement of an entity type permission.
+
+.. note::
+
+ By default, the permissions of an entity type attributes are
+ equivalent to the permissions of the entity type itself.
+
+ It is possible to provide custom attribute permissions which are
+ stronger than, or are more lenient than the entity type
+ permissions.
+
+ In a situation where all attributes were given custom permissions,
+ the entity type permissions would not be checked, except for the
+ `delete` action.
+
+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 if the user is in one of the listed groups or if one of the RQL condition
+is satisfied.
+
+Default permissions
+```````````````````
+
+The default permissions for ``EntityType`` are:
+
+.. sourcecode:: python
+
+ __permissions__ = {
+ 'read': ('managers', 'users', 'guests',),
+ 'update': ('managers', 'owners',),
+ 'delete': ('managers', 'owners'),
+ 'add': ('managers', 'users',)
+ }
+
+The default permissions for relations are:
+
+.. sourcecode:: python
+
+ __permissions__ = {'read': ('managers', 'users', 'guests',),
+ 'delete': ('managers', 'users'),
+ 'add': ('managers', 'users',)}
+
+The default permissions for attributes are:
+
+.. sourcecode:: python
+
+ __permissions__ = {'read': ('managers', 'users', 'guests',),
+ 'add': ('managers', ERQLExpression('U has_add_permission X'),
+ 'update': ('managers', ERQLExpression('U has_update_permission X')),}
+
+.. note::
+
+ The default permissions for attributes are not syntactically
+ equivalent to the default permissions of the entity types, but the
+ rql expressions work by delegating to the entity type permissions.
+
+
+The standard user groups
+````````````````````````
+
+* `guests`
+
+* `users`
+
+* `managers`
+
+* `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 defined in the precreate
+script of the cube (``migration/precreate.py``). Defining groups in postcreate
+script or later makes them unavailable for security purposes (in this case, an
+`sync_schema_props_perms` command has to be issued in a CubicWeb shell).
+
+
+Use of RQL expression for write permissions
+```````````````````````````````````````````
+
+It is possible to define RQL expression to provide update permission (`add`,
+`delete` and `update`) on entity type / relation definitions. An rql expression
+is a piece of query (corresponds to the WHERE statement of an RQL query), and the
+expression will be considered as satisfied if it returns some results. They can
+not be used in `read` permission.
+
+To use RQL expression in entity type permission:
+
+* you have to use the class :class:`~cubicweb.schema.ERQLExpression`
+
+* 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
+
+For RQL expressions on a relation type, the principles are the same except for
+the following:
+
+* you have to use the class :class:`~cubicweb.schema.RRQLExpression` instead of
+ :class:`~cubicweb.schema.ERQLExpression`
+
+* 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
+
+To define security for attributes of an entity (non-final relation), you have to
+use the class :class:`~cubicweb.schema.ERQLExpression` in which `X` represents
+the entity the attribute belongs to.
+
+It is possible to use in those expression a special relation
+`has_<ACTION>_permission` where the subject is the user (eg 'U') and the object
+is any variable representing an entity (usually 'X' in
+:class:`~cubicweb.schema.ERQLExpression`, 'S' or 'O' in
+:class:`~cubicweb.schema.RRQLExpression`), meaning that the user needs to have
+permission to execute the action <ACTION> on the entities represented by this
+variable. It's recommanded to use this feature whenever possible since it
+simplify greatly complex security definition and upgrade.
+
+
+.. sourcecode:: python
+
+ class my_relation(RelationDefinition):
+ __permissions__ = {'read': ('managers', 'users'),
+ 'add': ('managers', RRQLExpression('U has_update_permission S')),
+ 'delete': ('managers', RRQLExpression('U has_update_permission S'))
+ }
+
+In the above example, user will be allowed to add/delete `my_relation` if he has
+the `update` permission on the subject of the relation.
+
+.. note::
+
+ 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, we are not able to verify the permissions
+ before we actually added 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
+ (CubicWebEntitySchema.check_perm and has_perm) can indicate that the user is
+ not allowed to create this entity while it would obtain the permission. To
+ compensate this problem, it is usually necessary in such case to use an action
+ that reflects the schema permissions but which check properly the permissions
+ so that it would show up only if possible.
+
+
+Use of RQL expression for reading rights
+````````````````````````````````````````
+
+The principles are the same but with the following restrictions:
+
+* you can not use rql expression for the `read` permission of relations and
+ attributes,
+
+* you can not use special `has_<ACTION>_permission` relation in the rql
+ expression.
+
+
+Important notes about write permissions checking
+````````````````````````````````````````````````
+
+Write permissions (e.g. 'add', 'update', 'delete') are checked in core hooks.
+
+When a permission is checked slightly vary according to if it's an entity or
+relation, and if the relation is an attribute relation or not). It's important to
+understand that since according to when a permission is checked, values returned
+by rql expressions may changes, hence the permission being granted or not.
+
+Here are the current rules:
+
+1. permission to add/update entity and its attributes are checked on
+ commit
+
+2. permission to delete an entity is checked in 'before_delete_entity' hook
+
+3. permission to add a relation is checked either:
+
+ - in 'before_add_relation' hook if the relation type is in the
+ `BEFORE_ADD_RELATIONS` set
+
+ - else at commit time if the relation type is in the `ON_COMMIT_ADD_RELATIONS`
+ set
+
+ - else in 'after_add_relation' hook (the default)
+
+4. permission to delete a relation is checked in 'before_delete_relation' hook
+
+Last but not least, remember queries issued from hooks and operation are by
+default 'unsafe', eg there are no read or write security checks.
+
+See :mod:`cubicweb.hooks.security` for more details.
+
+
+.. _yams_example:
+
+
+Derived attributes and relations
+--------------------------------
+
+.. note:: **TODO** Check organisation of the whole chapter of the documentation
+
+Cubicweb offers the possibility to *query* data using so called
+*computed* relations and attributes. Those are *seen* by RQL requests
+as normal attributes and relations but are actually derived from other
+attributes and relations. In a first section we'll informally review
+two typical use cases. Then we see how to use computed attributes and
+relations in your schema. Last we will consider various significant
+aspects of their implementation and the impact on their usage.
+
+Motivating use cases
+~~~~~~~~~~~~~~~~~~~~
+
+Computed (or reified) relations
+```````````````````````````````
+
+It often arises that one must represent a ternary relation, or a
+family of relations. For example, in the context of an exhibition
+catalog you might want to link all *contributors* to the *work* they
+contributed to, but this contribution can be as *illustrator*,
+*author*, *performer*, ...
+
+The classical way to describe this kind of information within an
+entity-relationship schema is to *reify* the relation, that is turn
+the relation into a entity. In our example the schema will have a
+*Contribution* entity type used to represent the family of the
+contribution relations.
+
+
+.. sourcecode:: python
+
+ class ArtWork(EntityType):
+ name = String()
+ ...
+
+ class Person(EntityType):
+ name = String()
+ ...
+
+ class Contribution(EntityType):
+ contributor = SubjectRelation('Person', cardinality='1*', inlined=True)
+ manifestation = SubjectRelation('ArtWork')
+ role = SubjectRelation('Role')
+
+ class Role(EntityType):
+ name = String()
+
+But then, in order to query the illustrator(s) ``I`` of a work ``W``,
+one has to write::
+
+ Any I, W WHERE C is Contribution, C contributor I, C manifestation W,
+ C role R, R name 'illustrator'
+
+whereas we would like to be able to simply write::
+
+ Any I, W WHERE I illustrator_of W
+
+This is precisely what the computed relations allow.
+
+
+Computed (or synthesized) attribute
+```````````````````````````````````
+
+Assuming a trivial schema for describing employees in companies, one
+can be interested in the total of salaries payed by a company for
+all its employees. One has to write::
+
+ Any C, SUM(SA) GROUPBY S WHERE E works_for C, E salary SA
+
+whereas it would be most convenient to simply write::
+
+ Any C, TS WHERE C total_salary TS
+
+And this is again what computed attributes provide.
+
+
+Using computed attributes and relations
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Computed (or reified) relations
+```````````````````````````````
+
+In the above case we would define the *computed relation*
+``illustrator_of`` in the schema by:
+
+.. sourcecode:: python
+
+ class illustrator_of(ComputedRelation):
+ rule = ('C is Contribution, C contributor S, C manifestation O,'
+ 'C role R, R name "illustrator"')
+
+You will note that:
+
+* the ``S`` and ``O`` RQL variables implicitly identify the subject and
+ object of the defined computed relation, akin to what happens in
+ RRQLExpression
+* the possible subject and object entity types are inferred from the rule;
+* computed relation definitions always have empty *add* and *delete* permissions
+* *read* permissions can be defined, permissions from the relations used in the
+ rewrite rule **are not considered** ;
+* nothing else may be defined on the `ComputedRelation` subclass beside
+ description, permissions and rule (e.g. no cardinality, composite, etc.,).
+ `BadSchemaDefinition` is raised on attempt to specify other attributes;
+* computed relations can not be used in 'SET' and 'DELETE' rql queries
+ (`BadQuery` exception raised).
+
+
+NB: The fact that the *add* and *delete* permissions are *empty* even
+for managers is expected to make the automatic UI not attempt to edit
+them.
+
+Computed (or synthesized) attributes
+````````````````````````````````````
+
+In the above case we would define the *computed attribute*
+``total_salary`` on the ``Company`` entity type in the schema by:
+
+.. sourcecode:: python
+
+ class Company(EntityType):
+ name = String()
+ total_salary = Int(formula='Any SUM(SA) GROUPBY E WHERE P works_for X, E salary SA')
+
+* the ``X`` RQL variable implicitly identifies the entity holding the
+ computed attribute, akin to what happens in ERQLExpression;
+* the type inferred from the formula is checked against the declared type, and
+ `BadSchemaDefinition` is raised if they don't match;
+* the computed attributes always have empty *update* permissions
+* `BadSchemaDefinition` is raised on attempt to set 'update' permissions;
+* 'read' permissions can be defined, permissions regarding the formula
+ **are not considered**;
+* other attribute's property (inlined, ...) can be defined as for normal attributes;
+* Similarly to computed relation, computed attribute can't be used in 'SET' and
+ 'DELETE' rql queries (`BadQuery` exception raised).
+
+
+API and implementation
+~~~~~~~~~~~~~~~~~~~~~~
+
+Representation in the data backend
+``````````````````````````````````
+
+Computed relations have no direct representation at the SQL table
+level. Instead, each time a query is issued the query is rewritten to
+replace the computed relation by its equivalent definition and the
+resulting rewritten query is performed in the usual way.
+
+On the contrary, computed attributes are represented as a column in the
+table for their host entity type, just like normal attributes. Their
+value is kept up-to-date with respect to their defintion by a system
+of hooks (also called triggers in most RDBMS) which recomputes them
+when the relations and attributes they depend on are modified.
+
+Yams API
+````````
+
+When accessing the schema through the *yams API* (not when defining a
+schema in a ``schema.py`` file) the computed attributes and relations
+are represented as follows:
+
+relations
+ The ``yams.RelationSchema`` class has a new ``rule`` attribute
+ holding the rule as a string. If this attribute is set all others
+ must not be set.
+attributes
+ A new property ``formula`` is added on class
+ ``yams.RelationDefinitionSchema`` alomng with a new keyword
+ argument ``formula`` on the initializer.
+
+Migration
+`````````
+
+The migrations are to be handled as summarized in the array below.
+
++------------+---------------------------------------------------+---------------------------------------+
+| | Computed rtype | Computed attribute |
++============+===================================================+=======================================+
+| add | * add_relation_type | * add_attribute |
+| | * add_relation_definition should trigger an error | * add_relation_definition |
++------------+---------------------------------------------------+---------------------------------------+
+| modify | * sync_schema_prop_perms: | * sync_schema_prop_perms: |
+| | checks the rule is | |
+| (rule or | synchronized with the database | - empty the cache, |
+| formula) | | - check formula, |
+| | | - make sure all the values get |
+| | | updated |
++------------+---------------------------------------------------+---------------------------------------+
+| del | * drop_relation_type | * drop_attribute |
+| | * drop_relation_definition should trigger an error| * drop_relation_definition |
++------------+---------------------------------------------------+---------------------------------------+
+
+
+Defining your schema using yams
+-------------------------------
+
+Entity type definition
+~~~~~~~~~~~~~~~~~~~~~~
+
+An entity type is defined by a Python class which inherits from
+:class:`yams.buildobjs.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 expected to
+be defined in the module ``mycube.schema``.
+
+:Note on schema definition:
+
+ The code in ``mycube.schema`` is not meant to be executed. The class
+ EntityType mentioned above is different from the EntitySchema class
+ described in the previous chapter. EntityType is a helper class to
+ make Entity definition easier. Yams will process EntityType classes
+ and create EntitySchema instances from these class definitions. Similar
+ manipulation happen for relations.
+
+When defining a schema using python files, you may use the following shortcuts:
+
+- `required`: boolean indicating if the attribute is required, ed subject cardinality is '1'
+
+- `vocabulary`: specify static possible values of an attribute
+
+- `maxsize`: integer providing the maximum size of a string (no limit by default)
+
+For example:
+
+.. sourcecode:: python
+
+ 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`.
+
+
+
+:Naming convention:
+
+ Entity class names must start with an uppercase letter. The common
+ usage is to use ``CamelCase`` names.
+
+ Attribute and relation names must start with a lowercase letter. The
+ common usage is to use ``underscore_separated_words``. Attribute and
+ relation names starting with a single underscore are permitted, to
+ denote a somewhat "protected" or "private" attribute.
+
+ In any case, identifiers starting with "CW" or "cw" are reserved for
+ internal use by the framework.
+
+ .. _Metadata:
+
+ Some attribute using the name of another attribute as prefix are considered
+ metadata. For example, if an EntityType have both a ``data`` and
+ ``data_format`` attribute, ``data_format`` is view as the ``format`` metadata
+ of ``data``. Later the :meth:`cw_attr_metadata` method will allow you to fetch
+ metadata related to an attribute. There are only three valid metadata names:
+ ``format``, ``encoding`` and ``name``.
+
+
+The name of the Python attribute corresponds to the name of the attribute
+or the relation in *CubicWeb* application.
+
+An attribute is defined in the schema as follows::
+
+ attr_name = AttrType(*properties, metadata={})
+
+where
+
+* `AttrType`: is one of the type listed in EntityType_,
+
+* `properties`: is a list of the attribute needs to satisfy (see `Properties`_
+ for more details),
+
+* `metadata`: is a dictionary of meta attributes related to ``attr_name``.
+ Dictionary keys are the name of the meta attribute. Dictionary values
+ attributes objects (like the content of ``AttrType``). For each entry of the
+ metadata dictionary a ``<attr_name>_<key> = <value>`` attribute is
+ automaticaly added to the EntityType. see `Metadata`_ section for details
+ about valid key.
+
+
+ ---
+
+While building your schema
+
+* it is possible to use the attribute `meta` to flag an entity type as a `meta`
+ (e.g. used to describe/categorize other entities)
+
+.. XXX the paragraph below needs clarification and / or moving out in
+.. another place
+
+*Note*: if you end up with an `if` in the definition of your entity, this probably
+means that you need two separate entities that implement the `ITree` interface and
+get the result from `.children()` which ever entity is concerned.
+
+.. Inheritance
+.. ```````````
+.. XXX feed me
+
+
+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'
+
+If provided, the `subject` and `object` attributes denote the subject
+and object of the various relation definitions related to the relation
+type. Allowed values for these attributes are:
+
+* a string corresponding to an entity type
+* a tuple of string corresponding to multiple entity types
+* the '*' special string, meaning all types of entities
+
+When a relation is not inlined and not symmetrical, and it does not require
+specific permissions, it can be defined using a `SubjectRelation`
+attribute in the EntityType class. The first argument of `SubjectRelation` gives
+the entity type for the object of the relation.
+
+:Naming convention:
+
+ Although this way of defining relations uses a Python class, the
+ naming convention defined earlier prevails over the PEP8 conventions
+ used in the framework: relation type class names use
+ ``underscore_separated_words``.
+
+:Historical note:
+
+ It has been historically possible to use `ObjectRelation` which
+ defines a relation in the opposite direction. This feature is
+ deprecated and therefore should not be used in newly written code.
+
+:Future deprecation note:
+
+ In an even more remote future, it is quite possible that the
+ SubjectRelation shortcut will become deprecated, in favor of the
+ RelationType declaration which offers some advantages in the context
+ of reusable cubes.
+
+
+
+
+Handling schema changes
+~~~~~~~~~~~~~~~~~~~~~~~
+
+Also, it should be clear that to properly handle data migration, an
+instance's schema is stored in the database, so the python schema file
+used to defined it is only read when the instance is created or
+upgraded.
+
+.. XXX complete me