A ``form`` basically only holds a set of ``fields``, and has te be bound to a

``renderer`` which is responsible to layout them. Each field is bound to a

``widget`` that will be used to fill in value(s) for that field (at form

The ``field`` should be used according to the type of what you want to edit.

Anatomy of a choices function

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Let's have a look at the `ticket_done_in_choices` function given to

the `choices` parameter of the relation tag that is applied to the

('Ticket', 'done_in', '*') relation definition, as it is both typical

and sophisticated enough. This is a code snippet from the `tracker`_

cube.

.. _`tracker`: http://www.cubicweb.org/project/cubicweb-tracker

The ``Ticket`` entity type can be related to a ``Project`` and a

``Version``, respectively through the ``concerns`` and ``done_in``

relations. When a user is about to edit a ticket, we want to fill the

combo box for the ``done_in`` relation with values pertinent with

respect to the context. The important context here is:

* creation or modification (we cannot fetch values the same way in

  either case)

* ``__linkto`` url parameter given in a creation context

    from cubicweb.web import formfields

    def ticket_done_in_choices(form, field):

        entity = form.edited_entity

        # first see if its specified by __linkto form parameters

        linkedto = formfields.relvoc_linkedto(entity, 'done_in', 'subject')

        if linkedto:

            return linkedto

        # it isn't, get initial values

        vocab = formfields.relvoc_init(entity, 'done_in', 'subject')

        veid = None

        # try to fetch the (already or pending) related version and project

        if not entity.has_eid():

            peids = entity.linked_to('concerns', 'subject')

            peid = peids and peids[0]

        else:

            peid = entity.project.eid

            veid = entity.done_in and entity.done_in[0].eid

        if peid:

            # we can complete the vocabulary with relevant values

            rschema = form._cw.vreg.schema['done_in'].rdef('Ticket', 'Version')

            rset = form._cw.execute(

                'Any V, VN ORDERBY version_sort_value(VN) '

                'WHERE V version_of P, P eid %(p)s, V num VN, '

                'V in_state ST, NOT ST name "published"', {'p': peid}, 'p')

            vocab += [(v.view('combobox'), v.eid) for v in rset.entities()

                      if rschema.has_perm(form._cw, 'add', toeid=v.eid)

                      and v.eid != veid]

        return vocab

The first thing we have to do is fetch potential values from the

``__linkto`` url parameter that is often found in entity creation

contexts (the creation action provides such a parameter with a

predetermined value; for instance in this case, ticket creation could

occur in the context of a `Version` entity). The

:mod:`cubicweb.web.formfields` module provides a ``relvoc_linkedto``

utility function that gets a list suitably filled with vocabulary

values.

.. sourcecode:: python

        linkedto = formfields.relvoc_linkedto(entity, 'done_in', 'subject')

        if linkedto:

            return linkedto

Then, if no ``__linkto`` argument was given, we must prepare the

vocabulary with an initial empty value (because `done_in` is not

mandatory, we must allow the user to not select a verson) and already

linked values. This is done with the ``relvoc_init`` function.

        vocab = formfields.relvoc_init(entity, 'done_in', 'subject')

But then, we have to give more: if the ticket is related to a project,

we should provide all the non published versions of this project

(`Version` and `Project` can be related through the `version_of`

relation). Conversely, if we do not know yet the project, it would not

make sense to propose all existing versions as it could potentially

lead to incoherences. Even if these will be caught by some

RQLConstraint, it is wise not to tempt the user with error-inducing

candidate values.

The "ticket is related to a project" part must be decomposed as:

* this is a new ticket which is created is the context of a project

* this is an already existing ticket, linked to a project (through the

  `concerns` relation)

* there is no related project (quite unlikely given the cardinality of

  the `concerns` relation, so it can only mean that we are creating a

  new ticket, and a project is about to be selected but there is no

  ``__linkto`` argument)

.. note::

   the last situation could happen in several ways, but of course in a

   polished application, the paths to ticket creation should be

   controlled so as to avoid a suboptimal end-user experience

Hence, we try to fetch the related project.

        veid = None

        if not entity.has_eid():

            peids = entity.linked_to('concerns', 'subject')

            peid = peids and peids[0]

        else:

            peid = entity.project.eid

            veid = entity.done_in and entity.done_in[0].eid

We distinguish between entity creation and entity modification using

the ``Entity.has_eid()`` method, which returns `False` on creation. At

creation time the only way to get a project is through the

``__linkto`` parameter. Notice that we fetch the version in which the

ticket is `done_in` if any, for later.

.. note::

  the implementation above assumes that if there is a ``__linkto``

  parameter, it is only about a project. While it makes sense most of

  the time, it is not an absolute. Depending on how an entity creation

  action action url is built, several outcomes could be possible

  there

If the ticket is already linked to a project, fetching it is

trivial. Then we add the relevant version to the initial vocabulary.

        if peid:

            rschema = form._cw.vreg.schema['done_in'].rdef('Ticket', 'Version')

            rset = form._cw.execute(

                'Any V, VN ORDERBY version_sort_value(VN) '

                'WHERE V version_of P, P eid %(p)s, V num VN, '

                'V in_state ST, NOT ST name "published"', {'p': peid})

            vocab += [(v.view('combobox'), v.eid) for v in rset.entities()

                      if rschema.has_perm(form._cw, 'add', toeid=v.eid)

                      and v.eid != veid]

.. warning::

   we have to defend ourselves against lack of a project eid. Given

   the cardinality of the `concerns` relation, there *must* be a

   project, but this rule can only be enforced at validation time,

   which will happen of course only after form subsmission

Here, given a project eid, we complete the vocabulary with all

unpublished versions defined in the project (sorted by number) for

which the current user is allowed to establish the relation. Of

course, we take care *not* to provide a version the ticket is already

linked to (through ``done_in``).

APIs

~~~~