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

Frequently Asked Questions

==========================

[XXX 'copy answer from forum' means reusing text from

http://groups.google.com/group/google-appengine/browse_frm/thread/c9476925f5f66ec6

and

http://groups.google.com/group/google-appengine/browse_frm/thread/d791ce17e2716147/eb078f8cfe8426e0

and

http://groups.google.com/group/google-appengine/browse_frm/thread/f48cf6099973aef5/c28cd6934dd72457

]

Why does not CubicWeb have a template language ?

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

  There are enough template languages out there. You can use your

  preferred template language if you want. [explain how to use a

  template language]

  *CubicWeb* does not define its own templating language as this was

  not our goal. Based on our experience, we realized that

  we could gain productivity by letting designers use design tools

  and developpers develop without the use of the templating language

  as an intermediary that could not be anyway efficient for both parties.

  Python is the templating language that we use in *CubicWeb*, but again,

  it does not prevent you from using a templating language.

  The reason template languages are not used in this book is that

  experience has proved us that using pure python was less cumbersome.

Why do you think using pure python is better than using a template language ?

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

  Python is an Object Oriented Programming language and as such it

  already provides a consistent and strong architecture and syntax

  a templating language would not reach.

  When doing development, you need a real language and template

  languages are not real languages.

  Using Python enables developing applications for which code is

  easier to maintain with real functions/classes/contexts

  without the need of learning a new dialect. By using Python,

  we use standard OOP techniques and this is a key factor in a

  robust application.

Why do you use the LGPL license to prevent me from doing X ?

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

  LGPL means that *if* you redistribute your application, you need to

  redistribute the changes you made to CubicWeb under the LGPL licence.

  Publishing a web site has nothing to do with redistributing

  source code. A fair amount of companies use modified LGPL code

  for internal use. And someone could publish a *CubicWeb* component

  under a BSD licence for others to plug into a LGPL framework without

  any problem. The only thing we are trying to prevent here is someone

  taking the framework and packaging it as closed source to his own

  clients.

CubicWeb looks pretty recent. Is it stable ?

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

  It is constantly evolving, piece by piece.  The framework has evolved since

  2001 and data has been migrated from one schema to the other ever since. There

  is a well-defined way to handle data and schema migration.

Why is the RQL query language looking similar to X ?

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

  It may remind you of SQL but it is higher level than SQL, more like

  SPARQL. Except that SPARQL did not exist when we started the project.

  Having SPARQL as a query language has been in our backlog for years.

  That RQL language is what is going to make a difference with django-

  like frameworks for several reasons.

  1. accessing data is *much* easier with it. One can write complex

     queries with RQL that would be tedious to define and hard to maintain

     using an object/filter suite of method calls.

  2. it offers an abstraction layer allowing your applications to run

     on multiple back-ends. That means not only various SQL backends

     (postgresql, sqlite, mysql), but also multiple databases at the

     same time, and also non-SQL data stores like LDAP directories and

     subversion/mercurial repositories (see the `vcsfile`

     component). Google App Engine is yet another supported target for

     RQL.

[copy answer from forum, explain why similar to sparql and why better

  than django and SQL]

which ajax library is CubicWeb using ?

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

[CubicWeb uses jQuery and adds a thin layer on top of that]

How is security implemented ?

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

  This is an example of how it works in our framework::

    class Version(EntityType):

        """a version is defining the content of a particular project's

        release"""

        # definition of attributes is voluntarily missing

        permissions = {'read': ('managers', 'users', 'guests',),

                       'update': ('managers', 'logilab', 'owners',),

                       'delete': ('managers', ),

                       'add': ('managers', 'logilab',

                           ERQLExpression('X version_of PROJ, U in_group G, '

                                          'PROJ require_permission P, '

                                          'P name "add_version", P require_group G'),)}

  The above means that permission to read a Version is granted to any

  user that is part of one of the groups 'managers', 'users', 'guests'.

  The 'add' permission is granted to users in group 'managers' or

  'logilab' and to users in group G, if G is linked by a permission

  entity named "add_version" to the version's project.

  ::

    class version_of(RelationType):

        """link a version to its project. A version is necessarily linked

        to one and only one project. """

        # some lines voluntarily missing

        permissions = {'read': ('managers', 'users', 'guests',),

                       'delete': ('managers', ),

                       'add': ('managers', 'logilab',

                            RRQLExpression('O require_permission P, P name "add_version",

                            'U in_group G, P require_group G'),) }

  You can find additional information in the section :ref:`security`.

  [XXX what does the second example means in addition to the first one?]

What is `Error while publishing rest text ...` ?

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

  While modifying the description of an entity, you get an error message in

  the application `Error while publishing ...` for Rest text and plain text.

  The server returns a traceback like as follows ::

      2008-10-06 15:05:08 - (cubicweb.rest) ERROR: error while publishing ReST text

      Traceback (most recent call last):

      File "/home/user/src/blogdemo/cubicweb/common/rest.py", line 217, in rest_publish

      File "/usr/lib/python2.5/codecs.py", line 817, in open

      file = __builtin__.open(filename, mode, buffering)

      TypeError: __init__() takes at most 3 arguments (4 given)

  This can be fixed by applying the patch described in :

  http://code.google.com/p/googleappengine/issues/detail?id=48

What are hooks used for ?

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

  Hooks are executed around (actually before or after) events.  The

  most common events are data creation, update and deletion.  They

  permit additional constraint checking (those not expressible at the

  schema level), pre and post computations depending on data

  movements.

  As such, they are a vital part of the framework.

  Other kinds of hooks, called Operations, are available

  for execution just before commit.

When should you define an HTML template rather than define a graphical component ?

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

  An HTML template cannot contain code, hence it is only about static

  content.  A component is made of code and operations that apply on a

  well defined context (request, result set). It enables much more

  dynamic views.

What is the difference between `AppRsetObject` and `AppObject` ?

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

  `AppRsetObject` instances are selected on a request and a result

  set. `AppObject` instances are directly selected by id.

How to update a database after a schema modification ?

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

  It depends on what has been modified in the schema.

  * Update of an attribute permissions and properties:

    ``synchronize_eschema('MyEntity')``.

  * Update of a relation permissions and properties:

    ``synchronize_rschema('MyRelation')``.

  * Add an attribute: ``add_attribute('MyEntityType', 'myattr')``.

  * Add a relation: ``add_relation_definition('SubjRelation', 'MyRelation', 'ObjRelation')``.

How to create an anonymous user ?

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

  This allows to bypass authentication for your site. In the

  ``all-in-one.conf`` file of your instance, define the anonymous user

  as follows ::

    # login of the CubicWeb user account to use for anonymous user (if you want to

    # allow anonymous)

    anonymous-user=anon

    # password of the CubicWeb user account matching login

    anonymous-password=anon

  You also must ensure that this `anon` user is a registered user of

  the DB backend. If not, you can create through the administation

  interface of your instance by adding a user with the role `guests`.

  This could be the admin account (for development

  purposes, of course).

.. note::

    While creating a new instance, you can decide to allow access

    to anonymous user, which will automatically execute what is

    decribed above.

How to change the application logo ?

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

  There are two ways of changing the logo.

  1. The easiest way to use a different logo is to replace the existing

     ``logo.png`` in ``myapp/data`` by your prefered icon and refresh.

     By default all application will look for a ``logo.png`` to be

     rendered in the logo section.

     .. image:: ../images/lax-book.06-main-template-logo.en.png

  2. In your cube directory, you can specify which file to use for the logo.

     This is configurable in ``mycube/data/external_resources``: ::

       LOGO = DATADIR/path/to/mylogo.gif

     where DATADIR is ``mycube/data``.

How to configure a LDAP source ?

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

  Your instance's sources are defined in ``/etc/cubicweb.d/myapp/sources``.

  Configuring an LDAP source is about declaring that source in your

  instance configuration file such as: ::

    [ldapuser]

    adapter=ldapuser

    # ldap host

    host=myhost

    # base DN to lookup for usres

    user-base-dn=ou=People,dc=mydomain,dc=fr

    # user search scope

    user-scope=ONELEVEL

    # classes of user

    user-classes=top,posixAccount

    # attribute used as login on authentication

    user-login-attr=uid

    # name of a group in which ldap users will be by default

    user-default-group=users

    # map from ldap user attributes to cubicweb attributes

    user-attrs-map=gecos:email,uid:login

  Any change applied to configuration file requires to restart your

  application.

I get NoSelectableObject exceptions, how do I debug selectors ?

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

  You just need to put the appropriate context manager around view/component

  selection (one standard place in in vreg.py) : ::

    def possible_objects(self, registry, *args, **kwargs):

        """return an iterator on possible objects in a registry for this result set

        actions returned are classes, not instances

        """

        from cubicweb.selectors import traced_selection

        with traced_selection():

            for vobjects in self.registry(registry).values():

                try:

                    yield self.select(vobjects, *args, **kwargs)

                except NoSelectableObject:

                    continue

  Don't forget the 'from __future__ improt with_statement' at the

  module top-level.

  This will yield additional WARNINGs, like this:

  ::

    2009-01-09 16:43:52 - (cubicweb.selectors) WARNING: selector one_line_rset returned 0 for <class 'cubicweb.web.views.basecomponents.WFHistoryVComponent'>

How to format an entity date attribute ?

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

  If your schema has an attribute of type Date or Datetime, you might

  want to format it. First, you should define your preferred format using

  the site configuration panel ``http://appurl/view?vid=systempropertiesform``

  and then set ``ui.date`` and/or ``ui.datetime``.

  Then in the view code, use::

    self.format_date(entity.date_attribute)

Can PostgreSQL and CubicWeb authentication work with kerberos ?

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

  If you have PostgreSQL set up to accept kerberos authentication, you can set

  the db-host, db-name and db-user parameters in the `sources` configuration

  file while leaving the password blank. It should be enough for your

  application to connect to postgresql with a kerberos ticket.

How to load data from a script ?

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

  The following script aims at loading data within a script assuming pyro-nsd is

  running and your application is configured with ``pyro-server=yes``, otherwise

  you would not be able to use dbapi. ::

    from cubicweb import dbapi

    cnx = dbapi.connection(database='instance-id', user='admin', password='admin')

    cur = cnx.cursor()

    for name in ('Personal', 'Professional', 'Computers'):

        cur.execute('INSERT Blog B: B name %s', name)

    cnx.commit()

What is the CubicWeb datatype corresponding to GAE datastore's UserProperty ?

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

  If you take a look at your application schema and

  click on "display detailed view of metadata" you will see that there

  is a Euser entity in there. That's the one that is modeling users. The

  thing that corresponds to a UserProperty is a relationship between

  your entity and the Euser entity. As in ::

    class TodoItem(EntityType):

       text = String()

       todo_by = SubjectRelation('Euser')

  [XXX check that cw handle users better by

  mapping Google Accounts to local Euser entities automatically]

How to reset the password for user joe ?

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

  You need to generate a new encrypted password::

    $ python

    >>> from cubicweb.server.utils import crypt_password

    >>> crypt_password('joepass')

    'qHO8282QN5Utg'

    >>>

  and paste it in the database::

    $ psql mydb

    mydb=> update cw_cwuser set cw_upassword='qHO8282QN5Utg' where cw_login='joe';

    UPDATE 1