.. _webform:

HTML form construction
----------------------

CubicWeb provides the somewhat usual form / field / widget / renderer abstraction
to provide generic building blocks which will greatly help you in building forms
properly integrated with CubicWeb (coherent display, error handling, etc...),
while keeping things as flexible as possible.

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
generation time) and 'decode' (fetch and give a proper Python type to) values
sent back by the browser.

The ``field`` should be used according to the type of what you want to edit.
E.g. if you want to edit some date, you'll have to use the
:class:`cubicweb.web.formfields.DateField`. Then you can choose among multiple
widgets to edit it, for instance :class:`cubicweb.web.formwidgets.TextInput` (a
bare text field), :class:`~cubicweb.web.formwidgets.DateTimePicker` (a simple
calendar) or even :class:`~cubicweb.web.formwidgets.JQueryDatePicker` (the JQuery
calendar).  You can of course also write your own widget.

Exploring the available forms
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

A small excursion into a |cubicweb| shell is the quickest way to
discover available forms (or application objects in general).

.. sourcecode:: python

 >>> from pprint import pprint
 >>> pprint( session.vreg['forms'] )
 {'base': [<class 'cubicweb.web.views.forms.FieldsForm'>,
           <class 'cubicweb.web.views.forms.EntityFieldsForm'>],
  'changestate': [<class 'cubicweb.web.views.workflow.ChangeStateForm'>,
                  <class 'cubes.tracker.views.forms.VersionChangeStateForm'>],
  'composite': [<class 'cubicweb.web.views.forms.CompositeForm'>,
                <class 'cubicweb.web.views.forms.CompositeEntityForm'>],
  'deleteconf': [<class 'cubicweb.web.views.editforms.DeleteConfForm'>],
  'edition': [<class 'cubicweb.web.views.autoform.AutomaticEntityForm'>,
              <class 'cubicweb.web.views.workflow.TransitionEditionForm'>,
              <class 'cubicweb.web.views.workflow.StateEditionForm'>],
  'logform': [<class 'cubicweb.web.views.basetemplates.LogForm'>],
  'massmailing': [<class 'cubicweb.web.views.massmailing.MassMailingForm'>],
  'muledit': [<class 'cubicweb.web.views.editforms.TableEditForm'>],
  'sparql': [<class 'cubicweb.web.views.sparql.SparqlForm'>]}


The two most important form families here (for all practical purposes) are `base`
and `edition`. Most of the time one wants alterations of the
:class:`AutomaticEntityForm` to generate custom forms to handle edition of an
entity.

The Automatic Entity Form
~~~~~~~~~~~~~~~~~~~~~~~~~

.. automodule:: cubicweb.web.views.autoform

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

.. sourcecode:: python

    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 = form.linked_to[('done_in', 'subject')]
        if linkedto:
            return linkedto
        # it isn't, get initial values
        vocab = field.relvoc_init(form)
        veid = None
        # try to fetch the (already or pending) related version and project
        if not entity.has_eid():
            peids = form.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
:class:`~cubicweb.web.formfields.RelationField` field class provides a
:meth:`~cubicweb.web.formfields.RelationField.relvoc_linkedto` method that gets a
list suitably filled with vocabulary values.

.. sourcecode:: python

        linkedto = field.relvoc_linkedto(form)
        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
:meth:`~cubicweb.web.formfields.RelationField.relvoc_init` method.

.. sourcecode:: python

        vocab = field.relvoc_init(form)

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.

.. sourcecode:: python

        veid = None
        if not entity.has_eid():
            peids = form.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.

.. sourcecode:: python

        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.


Building self-posted form with custom fields/widgets
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Sometimes you want a form that is not related to entity edition. For those,
you'll have to handle form posting by yourself. Here is a complete example on how
to achieve this (and more).

Imagine you want a form that selects a month period. There are no proper
field/widget to handle this in CubicWeb, so let's start by defining them:

.. sourcecode:: python

    # let's have the whole import list at the beginning, even those necessary for
    # subsequent snippets
    from logilab.common import date
    from logilab.mtconverter import xml_escape
    from cubicweb.view import View
    from cubicweb.predicates import match_kwargs
    from cubicweb.web import RequestError, ProcessFormError
    from cubicweb.web import formfields as fields, formwidgets as wdgs
    from cubicweb.web.views import forms, calendar

    class MonthSelect(wdgs.Select):
        """Custom widget to display month and year. Expect value to be given as a
        date instance.
        """

        def format_value(self, form, field, value):
            return u'%s/%s' % (value.year, value.month)

        def process_field_data(self, form, field):
            val = super(MonthSelect, self).process_field_data(form, field)
            try:
                year, month = val.split('/')
                year = int(year)
                month = int(month)
                return date.date(year, month, 1)
            except ValueError:
                raise ProcessFormError(
                    form._cw._('badly formated date string %s') % val)


    class MonthPeriodField(fields.CompoundField):
        """custom field composed of two subfields, 'begin_month' and 'end_month'.

        It expects to be used on form that has 'mindate' and 'maxdate' in its
        extra arguments, telling the range of month to display.
        """

        def __init__(self, *args, **kwargs):
            kwargs.setdefault('widget', wdgs.IntervalWidget())
            super(MonthPeriodField, self).__init__(
                [fields.StringField(name='begin_month',
                                    choices=self.get_range, sort=False,
                                    value=self.get_mindate,
                                    widget=MonthSelect()),
                 fields.StringField(name='end_month',
                                    choices=self.get_range, sort=False,
                                    value=self.get_maxdate,
                                    widget=MonthSelect())], *args, **kwargs)

        @staticmethod
        def get_range(form, field):
            mindate = date.todate(form.cw_extra_kwargs['mindate'])
            maxdate = date.todate(form.cw_extra_kwargs['maxdate'])
            assert mindate <= maxdate
            _ = form._cw._
            months = []
            while mindate <= maxdate:
                label = '%s %s' % (_(calendar.MONTHNAMES[mindate.month - 1]),
                                   mindate.year)
                value = field.widget.format_value(form, field, mindate)
                months.append( (label, value) )
                mindate = date.next_month(mindate)
            return months

        @staticmethod
        def get_mindate(form, field):
            return form.cw_extra_kwargs['mindate']

        @staticmethod
        def get_maxdate(form, field):
            return form.cw_extra_kwargs['maxdate']

        def process_posted(self, form):
            for field, value in super(MonthPeriodField, self).process_posted(form):
                if field.name == 'end_month':
                    value = date.last_day(value)
                yield field, value


Here we first define a widget that will be used to select the beginning and the
end of the period, displaying months like '<month> YYYY' but using 'YYYY/mm' as
actual value.

We then define a field that will actually hold two fields, one for the beginning
and another for the end of the period. Each subfield uses the widget we defined
earlier, and the outer field itself uses the standard
:class:`IntervalWidget`. The field adds some logic:

* a vocabulary generation function `get_range`, used to populate each sub-field

* two 'value' functions `get_mindate` and `get_maxdate`, used to tell to
  subfields which value they should consider on form initialization

* overriding of `process_posted`, called when the form is being posted, so that
  the end of the period is properly set to the last day of the month.

Now, we can define a very simple form:

.. sourcecode:: python

    class MonthPeriodSelectorForm(forms.FieldsForm):
        __regid__ = 'myform'
        __select__ = match_kwargs('mindate', 'maxdate')

        form_buttons = [wdgs.SubmitButton()]
        form_renderer_id = 'onerowtable'
        period = MonthPeriodField()


where we simply add our field, set a submit button and use a very simple renderer
(try others!). Also we specify a selector that ensures form will have arguments
necessary to our field.

Now, we need a view that will wrap the form and handle post when it occurs,
simply displaying posted values in the page:

.. sourcecode:: python

    class SelfPostingForm(View):
        __regid__ = 'myformview'

        def call(self):
            mindate, maxdate = date.date(2010, 1, 1), date.date(2012, 1, 1)
            form = self._cw.vreg['forms'].select(
                'myform', self._cw, mindate=mindate, maxdate=maxdate, action='')
            try:
                posted = form.process_posted()
                self.w(u'<p>posted values %s</p>' % xml_escape(repr(posted)))
            except RequestError: # no specified period asked
                pass
            form.render(w=self.w, formvalues=self._cw.form)


Notice usage of the :meth:`process_posted` method, that will return a dictionary
of typed values (because they have been processed by the field). In our case, when
the form is posted you should see a dictionary with 'begin_month' and 'end_month'
as keys with the selected dates as value (as a python `date` object).


APIs
~~~~

.. automodule:: cubicweb.web.formfields
.. automodule:: cubicweb.web.formwidgets
.. automodule:: cubicweb.web.views.forms
.. automodule:: cubicweb.web.views.formrenderers