Added formfields/manipulators docs; added a few notes to the FAQ
git-svn-id: http://code.djangoproject.com/svn/django/trunk@303 bcc190cf-cafb-0310-a4f2-bffc1f526a37
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Jacob Kaplan-Moss
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docs/faq.txt
34
docs/faq.txt
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@ -101,10 +101,22 @@ any capital-M Methodologies; we do what "feels" right. If you squint the right
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way, you can call Django's ORM the "Model", the view functions the "View", and
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the dynamically-generated API the "Controller" -- but not really.
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In fact, you might say that Django is a "MTV" framework -- that is, Model,
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Template, and View make much more sense to us.
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So, although we've been strongly influenced by MVC -- especially in the
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separation-of-data-from-logic department -- we've also strayed from the path
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where it makes sense.
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<Framework X> does <feature Y> -- why doesn't Django?
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-----------------------------------------------------
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We're well aware that there are other awesome web frameworks out there, and
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we're not adverse to borrowing ideas where appropriate. However, Django was
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developed precisely because we were unhappy with the status quo, so please be
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aware that "because <Framework X>" does it is not going to be sufficient reason
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to add a given feature to Django.
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Do you have any of those nifty "screencast" things?
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---------------------------------------------------
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@ -224,6 +236,28 @@ but we recognize that choosing a template language runs close to religion.
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There's nothing about Django that requires using the template language, so
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if you're attached to ZPT, Cheetah, or whatever, feel free to use those.
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How do I use image and file fields?
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-----------------------------------
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Using a ``FileField`` or an ``ImageField`` in a model takes a few steps:
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#. In your settings file, define ``MEDIA_ROOT`` as the full path to
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a directory where you'd like Django to store uploaded files (for
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performance these files are not stored in the database). Define
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``MEDIA_URL`` as the base public URL of that directory. Make
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sure that this directory is writable by the web user.
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#. Add the ``FileField`` or ``ImageField`` to your model, making sure
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to define the ``upload_to`` option to tell Django what subdirectory
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of ``MEDIA_ROOT`` to upload files to.
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#. All that will be stored in your database is a path to the file
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(relative to ``MEDIA_ROOT``). You'll must likely want to use
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the convenience ``get_<fieldname>_url`` function provided by
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Django (that is, if your ``ImageField`` is called ``mug_shot``,
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you can get the absolute URL to your image in a template with
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``{{ object.get_mug_shot_url }}``.
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The database API
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================
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@ -0,0 +1,456 @@
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===============================
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Forms, fields, and manipulators
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===============================
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Once you've got a chance to play with Django's admin interface, you'll probably
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wonder if the fantastic form validation framework it uses is available to user
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code. It is, and this document explains how the framework works.
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.. admonition:: A note to the lazy
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If all you want to do is present forms for a user to create and/or
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update a given object, don't read any further but instead click thyself
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over to the `generic views`_ documentation. The following exercises are
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for those interested in how Django's form framework works and those
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needing to do more than simple create/update.
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We'll take a top-down approach to examining Django's form validation framework
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since much of the time you won't need to use the lower-level APIs. Throughout
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this document, we'll be working with the following model, a "place" object::
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PLACE_TYPES = (
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(1, 'Bar'),
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(2, 'Restaurant'),
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(3, 'Movie Theater'),
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(4, 'Secret Hideout'),
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)
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class Place(meta.Model):
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fields = (
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meta.CharField('name', 'name', maxlength=100),
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meta.CharField('address', 'address', maxlength=100, blank=True),
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meta.CharField('city', 'city', maxlength=50, blank=True),
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meta.USStateField('state', 'state'),
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meta.CharField('zip_code', 'zip code', maxlength=5, blank=True),
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meta.IntegerField('place_type', 'place type', choices=PLACE_TYPES)
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)
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def __repr__(self):
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return self.name
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Defining the above class is enough to create an admin interface to a ``place``,
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but what if you want to allow public users to submit places?
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Manipulators
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============
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The highest-level interface for object creation and modification is the
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**Manipulator** framework. A manipulator is a utility class tied to a given
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model that "knows" how to create or modify instances of that model and how to
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validate data for the object. Manipulators come in two flavors:
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``AddManipulators`` and ``ChangeManipulators``. Functionally they are quite
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similar, but the former knows how to create new instances of the model, while
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the later modifies existing instances. Both types of classes are automatically
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created when you define a new class::
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>>> from django.models.places import places
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>>> places.AddManipulator
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<class django.models.places.PlaceManipulatorAdd at 0x4c1540>
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>>> places.ChangeManipulator
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<class django.models.places.PlaceManipulatorChange at 0x4c1630>
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Using the ``AddManipulator``
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----------------------------
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We'll start with the ``AddManipulator``. Here's a very simple view that takes
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POSTed data from the browser and creates a new ``Place`` object::
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from django.core import template_loader
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from django.core.exceptions import Http404
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from django.core.extensions import DjangoContext as Context
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from django.utils.httpwrappers import HttpResponse, HttpResponseRedirect
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from django.models.places import places
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from django.core import formfields
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def naive_create_place(request):
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"""A naive approach to creating places; don't actually use this!"""
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# Create the AddManipulator
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manipulator = places.AddManipulator()
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# Make a copy of the POSTed data so that do_html2python can
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# modify it in place (request.POST is immutable)
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new_data = request.POST.copy()
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# Convert the request data (which will all be strings) into the
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# appropriate Python types for those fields
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manipulator.do_html2python(new_data)
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# Save the new object
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new_place = manipulator.save(new_data)
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# It worked!
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return HttpResponse("Place created: %s" % new_place)
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The ``naive_create_place`` example works (somewhat), but as you probably can
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tell, there's all sorts of problems (some more subtle than others) with this view:
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* No validation of any sort is performed; if, for example, the ``name`` field
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isn't given in ``request.POST``, the save step will cause a database error
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because that field is required. Ugly.
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* Even if you *do* perform validation, there's still no way to give that information
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to the user is any sort of useful way.
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* You'll have to separate create a form (and view) that submits to this page, which is
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a pain and is redundant.
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Let's dodge these problems momentarily to take a look at how you could create a
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view with a form that submits to this flawed creation view::
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def naive_create_place_form(request):
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"""Simplistic place form view; don't actually use anything like this!"""
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# Create a FormWrapper object which the template can use; more
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# on what the second two arguments to FormWrapper do later.
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form = formfields.FormWrapper(places.AddManipulator(), {}, {})
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# Create a template, context, and response
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t = template_loader.get_template('places/naive_create_form')
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c = Context(request, {'form' : form})
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return HttpResponse(t.render(c))
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(This view, as well as all the following ones, have the same imports as the
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first example above does.)
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The ``formfields.FormWrapper`` object is a wrapper that templates can
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easily deal with to create forms; here's the ``naive_create_form`` template::
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{% extends "base" %}
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{% block content %}
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<h1>Create a place:</h1>
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<form method="post" action="../do_new/">
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<p><label for="id_name">Name:</label> {{ form.name }}</p>
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<p><label for="id_address">Address:</label> {{ form.address }}</p>
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<p><label for="id_city">City:</label> {{ form.city }}</p>
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<p><label for="id_state">State:</label> {{ form.state }}</p>
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<p><label for="id_zip_code">Zip:</label> {{ form.zip_code }}</p>
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<p><label for="id_place_type">Place type:</label> {{ form.place_type }}</p>
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<input type="submit" />
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</form>
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{% endblock %}
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Before we get back to the problems with these naive set of views, let's go over
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some salient points of the above template::
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* Field "widgets" are handled for you: ``{{ form.field }}`` automatically
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creates the "right" type of widget for the form, as you can see with the
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``place_type`` field above.
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* There isn't a way just to spit out the form; you'll still need to define
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how the form gets laid out. This is a feature: every form needs to be
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designed differently; Django doesn't force you into any type of mould.
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If you must use tables, use tables; if you're a semantic purist you can
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probably find better HTML than the above template.
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* To avoid name conflicts, the ``id``s of form elements take the form
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"id_*fieldname*".
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By creating a creation form we've solved problem number 3 above, but we still don't
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have any validation; if you enter bad data into any of the . Let's revise the validation
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issue by writing a new creation view that takes into account validation::
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def create_place_with_validation(request):
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manipulator = places.AddManipulator()
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new_data = request.POST.copy()
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# Check for validation errors
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errors = manipulator.get_validation_errors(new_data)
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if errors:
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t = template_loader.get_template('places/errors')
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c = Context(request, {'errors' : errors}
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return HttpResponse(t.render(c))
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else:
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manipulator.do_html2python(request.POST)
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new_place = manipulator.save(request.POST)
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return HttpResponse("Place created: %s" % new_place)
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In this new version, errors will be found -- ``manipulator.get_validation_errors``
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handles all the validation for you -- and those errors can be nicely presented
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on an error page (templated, of course)::
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{% extends "base" %}
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{% block content %}
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<h1>Please go back and correct the following error{{ errors|pluralize }}:</h1>
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<ul>
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{% for e in errors.items %}
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<li>Field "{{ e.0 }}": {{ e.1|join:", " }}</li>
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{% endfor %}
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</ul>
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{% endblock %}
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Still, this now has its own problems:
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* There's still the issue of creating a seperate (redundant) view for the
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submission form.
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* Errors, though nicely presented are on a seperate page, so the user will have
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to use the "back" button to fix errors -- not exactly usable!
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The best way to deal with these issues is to collapse the two views -- the form and the
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submission -- into a single view. This view will be responsible for creating the
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form, validating POSTed data, and creating the new object (should it the data be
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valid). An added bonus of this approach is that errors and the form will both
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be available on the same page, so errors with fields can be presented in context.
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.. admonition:: Philosophy::
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Finally, for the HTTP purists in the audience (and the authorship), this
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nicely matches the "true" meanings of HTTP-GET and HTTP-POST: GET fetches
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the form, POST creates the new object.
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Below is the finished view::
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def create_place(request):
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manipulator = places.AddManipulator()
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if request.POST:
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# If data was POSTed, we're trying to create a new Place
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new_data = request.POST.copy()
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# Check for errors
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errors = manipulator.get_validation_errors(new_data)
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if not errors:
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# No errors -- this means we can save the data!
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manipulator.do_html2python(new_data)
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new_place = manipulator.save(new_data)
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# Redirect to the object's "edit" page (so that reloads
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# don't accidentally create duplicate entries)
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return HttpResponseRedirect("/places/edit/%i/" % new_place.id)
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else:
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# No POST, so we want a brand new form without any data or errors
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errors = new_data = {}
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# Create the FormWrapper, template, context, response
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form = formfields.FormWrapper(manipulator, new_data, errors)
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t = template_loader.get_template("places/create_form")
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c = Context(request, {
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'form' : form,
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})
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return HttpResponse(t.render(c))
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and here's the ``create_form`` template::
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{% extends "base" %}
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{% block content %}
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<h1>Create a place:</h1>
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{% if form.has_errors %}
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<h2>Please correct the following error{{ errors|pluralize }}:</h2>
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{% endif %}
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<form method="post" action=".">
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<p>
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<label for="id_name">Name:</label> {{ form.name }}
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{% if form.name.errors %}*** {{ form.name.errors|join:", " }}{% endif %}
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</p>
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<p>
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<label for="id_address">Address:</label> {{ form.address }}
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{% if form.address.errors %}*** {{ form.address.errors|join:", " }}{% endif %}
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</p>
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<p>
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<label for="id_city">City:</label> {{ form.city }}
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{% if form.city.errors %}*** {{ form.city.errors|join:", " }}{% endif %}
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</p>
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<p>
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<label for="id_state">State:</label> {{ form.state }}
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{% if form.state.errors %}*** {{ form.state.errors|join:", " }}{% endif %}
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</p>
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<p>
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<label for="id_zip_code">Zip:</label> {{ form.zip_code }}
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{% if form.zip_code.errors %}*** {{ form.zip_code.errors|join:", " }}{% endif %}
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</p>
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<p>
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<label for="id_place_type">Place type:</label> {{ form.place_type }}
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{% if form.place_type.errors %}*** {{ form.place_type.errors|join:", " }}{% endif %}
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</p>
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<input type="submit" />
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</form>
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{% endblock %}
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The second two arguments to ``FormWrapper`` (``new_data`` and ``errors``)
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deserve some mention.
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The first is any "default" data to be used as values for the fields; pulling the
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data from ``request.POST`` as is done above makes sure that if there are errors,
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the values the user put in aren't lost. If you try the above example, you'll see
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this in action.
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The second argument is the error list retrieved from
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``manipulator.get_validation_errors``. When passed into the ``FormWrapper``, this gives
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each field an ``errors`` item (which is a list of error messages associated with the
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field) as well as a ``html_error_list`` item which is a ``<ul>`` of error messages.
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The above template uses these error items to display a simple error message next
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to each field.
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Using the ``ChangeManipulator``
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-------------------------------
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So: the above has covered using the ``AddManipulator`` to create a new object;
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what about editing an existing one? It's rather shockingly similar to creating
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a new one::
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def edit_place(request, place_id):
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# Get the place in question from the database and create a ChangeManipulator
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# at the same time
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try:
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manipulator = places.ChangeManipulator(place_id)
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except places.PlaceDoesNotExist:
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raise Http404
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# Grab the Place object is question for future use
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place = manipulator.original_object
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if request.POST:
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new_data = request.POST.copy()
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errors = manipulator.get_validation_errors(new_data)
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if not errors:
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manipulator.do_html2python(new_data)
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manipulator.save(new_data)
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# Do a post-after-redirect so that reload works, etc.
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return HttpResponseRedirect("/places/edit/%i/" % place.id)
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else:
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errors = {}
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# This makes sure the form accurate represents the fields of the place.
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new_data = place.__dict__
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form = formfields.FormWrapper(manipulator, new_data, errors)
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t = template_loader.get_template("places/edit_form")
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c = Context(request, {
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'form' : form,
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'place' : place,
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})
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return HttpResponse(t.render(c))
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The only real differences here are:
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* A ``ChangeManipulator`` instead of an ``AddManipulator`` is created;
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The argument to any ``ChangeManipulator`` is the id of the object
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to be changed. As you can see, the initializer will raise an
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``ObjectDoesNotExist`` exception if the id is invalid.
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* ``ChangeManipulator.original_object`` stores the instance of the
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object being edited.
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* We set ``new_data`` to the original object's ``__dict__``; this makes
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sure that the form fields contain the current values of the object.
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``FormWrapper`` does not modify ``new_data`` in any way, and templates
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cannot, so this is perfectly safe.
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* The above example uses a different template so that create and edit can
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be "skinned" differently if needed, but the form chunk itself is
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completely identical to the one in the create form above.
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The astute programmer will notice that the add and create functions are nearly
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identical and could in fact be collapsed into a single view; this is left
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as an exercise for said programmer.
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(However, the even-more-astute programmer will take heed of the note at the top
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of this document and check out the `generic views`_ documentation if all she
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wishes to do is this type of simple create/update).
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Custom forms and manipulators
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=============================
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All the above is fine and dandy if you want to just use the automatically created
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manipulators, but the coolness doesn't end there: you can easily create your
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own custom manipulators for handling custom forms.
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Custom manipulators are pretty simple; here's a manipulator that you might use
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for a "contact" form on a website::
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from django.core import formfields
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urgency_choices = (
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(1, "Extremely urgent"),
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(2, "Urgent"),
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(3, "Normal"),
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(4, "Unimportant"),
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)
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class ContactManipulator(formfields.Manipulator):
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def __init__(self):
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self.fields = (
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formfields.EmailField(field_name="from", is_required=True),
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formfields.TextField(field_name="subject", length=30, maxlength=200, is_required=True),
|
||||
formfields.IntegerField(field_name="urgency", choices=urgency_choices),
|
||||
formfields.LargeTextField(field_name="contents", is_required=True),
|
||||
)
|
||||
|
||||
A certain similarity to Django's models should be apparent. The only required
|
||||
method of a custom manipulator is ``__init__`` which must define the fields
|
||||
present in the manipulator. See the ``django.core.formfields`` module for
|
||||
all the form fields provided by Django.
|
||||
|
||||
You use this custom manipulator exactly as you would use an auto-generated one;
|
||||
here's a simple function that might drive the above form::
|
||||
|
||||
def contact_form(request):
|
||||
manipulator = ContactFormManipulator()
|
||||
if request.POST:
|
||||
new_data = request.POST.copy()
|
||||
errors = manipulator.get_validation_errors(new_data)
|
||||
if not errors:
|
||||
manipulator.do_html2python(new_data)
|
||||
|
||||
# send email using new_data here...
|
||||
|
||||
return HttpResponseRedirect("/contact/thankyou/")
|
||||
else:
|
||||
errors = new_data = {}
|
||||
form = formfields.FormWrapper(manipulator, new_data, errors)
|
||||
t = template_loader.get_template("contact_form")
|
||||
c = Context(request, {
|
||||
'form' : form,
|
||||
})
|
||||
return HttpResponse(t.render(c))
|
||||
|
||||
Validators
|
||||
==========
|
||||
|
||||
One extremely useful feature of manipulators is the automatic validation it
|
||||
performs. Validation is done using a simple validation API: a validator is
|
||||
simple a callable that raises a ``ValidationError`` if there's something wrong
|
||||
with the data. ``django.core.validators`` defines a whole host of validator
|
||||
functions, but defining your own couldn't be easier::
|
||||
|
||||
from django.core import validators, formfields
|
||||
|
||||
class ContactManipulator(formfields.Manipulator):
|
||||
def __init__(self):
|
||||
self.fields = (
|
||||
# ... snip fields as above ...
|
||||
formfields.EmailField(field_name="to", validator_list=[self.isValidToAddress])
|
||||
)
|
||||
|
||||
def isValidToAddress(self, field_data, all_data):
|
||||
if not field_data.endswith("@example.com"):
|
||||
raise ValidationError("You can only send messages to example.com email addresses")
|
||||
|
||||
Above, we've added a "to" field to the contact form, but required that the
|
||||
"to" address end with "@example.com" by adding the ``isValidToAddress``
|
||||
validator to the field's ``validator_list``.
|
||||
|
||||
The arguments to a validator function take a little explanation. ``field_data``
|
||||
is the value of the field in question, and ``all_data`` is a dict of all the
|
||||
data being validated. Note that at the point validators are called all data
|
||||
will still be strings (as ``do_html2python`` hasn't been called yet).
|
||||
|
||||
.. _`generic views`: http://www.djangoproject.com/documentation/generic_views/
|
||||
Loading…
Reference in New Issue