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426 lines
17 KiB
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426 lines
17 KiB
Plaintext
Generating forms for models
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===========================
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.. admonition:: Note
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The APIs described in this document have been deprecated. If you're
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developing new code, use `ModelForms`_ instead.
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.. _ModelForms: ../modelforms/
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If you're building a database-driven app, chances are you'll have forms that
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map closely to Django models. For instance, you might have a ``BlogComment``
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model, and you want to create a form that lets people submit comments. In this
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case, it would be redundant to define the field types in your form, because
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you've already defined the fields in your model.
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For this reason, Django provides a few helper functions that let you create a
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``Form`` class from a Django model.
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``form_for_model()``
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--------------------
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The method ``django.newforms.form_for_model()`` creates a form based on the
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definition of a specific model. Pass it the model class, and it will return a
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``Form`` class that contains a form field for each model field.
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For example::
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>>> from django.newforms import form_for_model
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# Create the form class.
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>>> ArticleForm = form_for_model(Article)
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# Create an empty form instance.
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>>> f = ArticleForm()
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It bears repeating that ``form_for_model()`` takes the model *class*, not a
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model instance, and it returns a ``Form`` *class*, not a ``Form`` instance.
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Field types
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~~~~~~~~~~~
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The generated ``Form`` class will have a form field for every model field. Each
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model field has a corresponding default form field. For example, a
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``CharField`` on a model is represented as a ``CharField`` on a form. A
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model ``ManyToManyField`` is represented as a ``MultipleChoiceField``. Here is
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the full list of conversions:
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=============================== ========================================
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Model field Form field
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=============================== ========================================
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``AutoField`` Not represented in the form
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``BooleanField`` ``BooleanField``
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``CharField`` ``CharField`` with ``max_length`` set to
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the model field's ``max_length``
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``CommaSeparatedIntegerField`` ``CharField``
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``DateField`` ``DateField``
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``DateTimeField`` ``DateTimeField``
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``DecimalField`` ``DecimalField``
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``EmailField`` ``EmailField``
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``FileField`` ``FileField``
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``FilePathField`` ``CharField``
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``FloatField`` ``FloatField``
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``ForeignKey`` ``ModelChoiceField`` (see below)
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``ImageField`` ``ImageField``
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``IntegerField`` ``IntegerField``
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``IPAddressField`` ``IPAddressField``
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``ManyToManyField`` ``ModelMultipleChoiceField`` (see
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below)
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``NullBooleanField`` ``CharField``
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``PhoneNumberField`` ``USPhoneNumberField``
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(from ``django.contrib.localflavor.us``)
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``PositiveIntegerField`` ``IntegerField``
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``PositiveSmallIntegerField`` ``IntegerField``
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``SlugField`` ``CharField``
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``SmallIntegerField`` ``IntegerField``
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``TextField`` ``CharField`` with ``widget=Textarea``
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``TimeField`` ``TimeField``
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``URLField`` ``URLField`` with ``verify_exists`` set
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to the model field's ``verify_exists``
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``USStateField`` ``CharField`` with
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``widget=USStateSelect``
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(``USStateSelect`` is from
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``django.contrib.localflavor.us``)
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``XMLField`` ``CharField`` with ``widget=Textarea``
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=============================== ========================================
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.. note::
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The ``FloatField`` form field and ``DecimalField`` model and form fields
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are new in the development version.
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As you might expect, the ``ForeignKey`` and ``ManyToManyField`` model field
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types are special cases:
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* ``ForeignKey`` is represented by ``django.newforms.ModelChoiceField``,
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which is a ``ChoiceField`` whose choices are a model ``QuerySet``.
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* ``ManyToManyField`` is represented by
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``django.newforms.ModelMultipleChoiceField``, which is a
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``MultipleChoiceField`` whose choices are a model ``QuerySet``.
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In addition, each generated form field has attributes set as follows:
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* If the model field has ``blank=True``, then ``required`` is set to
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``False`` on the form field. Otherwise, ``required=True``.
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* The form field's ``label`` is set to the ``verbose_name`` of the model
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field, with the first character capitalized.
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* The form field's ``help_text`` is set to the ``help_text`` of the model
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field.
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* If the model field has ``choices`` set, then the form field's ``widget``
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will be set to ``Select``, with choices coming from the model field's
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``choices``. The choices will normally include the blank choice which is
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selected by default. If the field is required, this forces the user to
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make a selection. The blank choice will not be included if the model
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field has ``blank=False`` and an explicit ``default`` value (the
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``default`` value will be initially selected instead).
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Finally, note that you can override the form field used for a given model
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field. See "Overriding the default field types" below.
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A full example
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~~~~~~~~~~~~~~
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Consider this set of models::
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from django.db import models
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TITLE_CHOICES = (
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('MR', 'Mr.'),
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('MRS', 'Mrs.'),
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('MS', 'Ms.'),
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)
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class Author(models.Model):
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name = models.CharField(max_length=100)
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title = models.CharField(max_length=3, choices=TITLE_CHOICES)
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birth_date = models.DateField(blank=True, null=True)
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def __unicode__(self):
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return self.name
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class Book(models.Model):
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name = models.CharField(max_length=100)
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authors = models.ManyToManyField(Author)
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With these models, a call to ``form_for_model(Author)`` would return a ``Form``
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class equivalent to this::
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class AuthorForm(forms.Form):
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name = forms.CharField(max_length=100)
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title = forms.CharField(max_length=3,
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widget=forms.Select(choices=TITLE_CHOICES))
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birth_date = forms.DateField(required=False)
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A call to ``form_for_model(Book)`` would return a ``Form`` class equivalent to
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this::
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class BookForm(forms.Form):
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name = forms.CharField(max_length=100)
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authors = forms.ModelMultipleChoiceField(queryset=Author.objects.all())
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The ``save()`` method
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~~~~~~~~~~~~~~~~~~~~~
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Every form produced by ``form_for_model()`` also has a ``save()`` method. This
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method creates and saves a database object from the data bound to the form. For
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example::
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# Create a form instance from POST data.
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>>> f = ArticleForm(request.POST)
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# Save a new Article object from the form's data.
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>>> new_article = f.save()
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Note that ``save()`` will raise a ``ValueError`` if the data in the form
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doesn't validate -- i.e., ``if form.errors``.
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This ``save()`` method accepts an optional ``commit`` keyword argument, which
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accepts either ``True`` or ``False``. If you call ``save()`` with
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``commit=False``, then it will return an object that hasn't yet been saved to
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the database. In this case, it's up to you to call ``save()`` on the resulting
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model instance. This is useful if you want to do custom processing on the
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object before saving it. ``commit`` is ``True`` by default.
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Another side effect of using ``commit=False`` is seen when your model has
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a many-to-many relation with another model. If your model has a many-to-many
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relation and you specify ``commit=False`` when you save a form, Django cannot
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immediately save the form data for the many-to-many relation. This is because
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it isn't possible to save many-to-many data for an instance until the instance
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exists in the database.
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To work around this problem, every time you save a form using ``commit=False``,
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Django adds a ``save_m2m()`` method to the form created by ``form_for_model``.
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After you've manually saved the instance produced by the form, you can invoke
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``save_m2m()`` to save the many-to-many form data. For example::
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# Create a form instance with POST data.
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>>> f = AuthorForm(request.POST)
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# Create, but don't save the new author instance.
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>>> new_author = f.save(commit=False)
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# Modify the author in some way.
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>>> new_author.some_field = 'some_value'
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# Save the new instance.
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>>> new_author.save()
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# Now, save the many-to-many data for the form.
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>>> f.save_m2m()
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Calling ``save_m2m()`` is only required if you use ``save(commit=False)``.
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When you use a simple ``save()`` on a form, all data -- including
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many-to-many data -- is saved without the need for any additional method calls.
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For example::
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# Create a form instance with POST data.
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>>> f = AuthorForm(request.POST)
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# Create and save the new author instance. There's no need to do anything else.
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>>> new_author = f.save()
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Using an alternate base class
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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If you want to add custom methods to the form generated by
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``form_for_model()``, write a class that extends ``django.newforms.BaseForm``
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and contains your custom methods. Then, use the ``form`` argument to
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``form_for_model()`` to tell it to use your custom form as its base class.
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For example::
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# Create the new base class.
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>>> class MyBase(BaseForm):
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... def my_method(self):
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... # Do whatever the method does
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# Create the form class with a different base class.
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>>> ArticleForm = form_for_model(Article, form=MyBase)
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# Instantiate the form.
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>>> f = ArticleForm()
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# Use the base class method.
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>>> f.my_method()
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Using a subset of fields on the form
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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**New in Django development version**
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In some cases, you may not want all the model fields to appear on the generated
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form. There are two ways of telling ``form_for_model()`` to use only a subset
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of the model fields:
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1. Set ``editable=False`` on the model field. As a result, *any* form
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created from the model via ``form_for_model()`` will not include that
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field.
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2. Use the ``fields`` argument to ``form_for_model()``. This argument, if
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given, should be a list of field names to include in the form.
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For example, if you want a form for the ``Author`` model (defined above)
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that includes only the ``name`` and ``title`` fields, you would specify
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``fields`` like this::
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PartialArticleForm = form_for_model(Author, fields=('name', 'title'))
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.. note::
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If you specify ``fields`` when creating a form with ``form_for_model()``,
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then the fields that are *not* specified will not be set by the form's
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``save()`` method. Django will prevent any attempt to save an incomplete
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model, so if the model does not allow the missing fields to be empty, and
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does not provide a default value for the missing fields, any attempt to
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``save()`` a ``form_for_model`` with missing fields will fail. To avoid
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this failure, you must use ``save(commit=False)`` and manually set any
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extra required fields::
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instance = form.save(commit=False)
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instance.required_field = 'new value'
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instance.save()
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See the `section on saving forms`_ for more details on using
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``save(commit=False)``.
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.. _section on saving forms: `The save() method`_
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Overriding the default field types
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The default field types, as described in the "Field types" table above, are
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sensible defaults; if you have a ``DateField`` in your model, chances are you'd
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want that to be represented as a ``DateField`` in your form. But
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``form_for_model()`` gives you the flexibility of changing the form field type
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for a given model field. You do this by specifying a **formfield callback**.
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A formfield callback is a function that, when provided with a model field,
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returns a form field instance. When constructing a form, ``form_for_model()``
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asks the formfield callback to provide form field types.
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By default, ``form_for_model()`` calls the ``formfield()`` method on the model
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field::
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def default_callback(field, **kwargs):
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return field.formfield(**kwargs)
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The ``kwargs`` are any keyword arguments that might be passed to the form
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field, such as ``required=True`` or ``label='Foo'``.
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For example, if you wanted to use ``MyDateFormField`` for any ``DateField``
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field on the model, you could define the callback::
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>>> def my_callback(field, **kwargs):
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... if isinstance(field, models.DateField):
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... return MyDateFormField(**kwargs)
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... else:
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... return field.formfield(**kwargs)
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>>> ArticleForm = form_for_model(Article, formfield_callback=my_callback)
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Note that your callback needs to handle *all* possible model field types, not
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just the ones that you want to behave differently to the default. That's why
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this example has an ``else`` clause that implements the default behavior.
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.. warning::
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The field that is passed into the ``formfield_callback`` function in
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``form_for_model()`` and ``form_for_instance`` is the field instance from
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your model's class. You **must not** alter that object at all; treat it
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as read-only!
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If you make any alterations to that object, it will affect any future
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users of the model class, because you will have changed the field object
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used to construct the class. This is almost certainly what you don't want
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to have happen.
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Finding the model associated with a form
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The model class that was used to construct the form is available
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using the ``_model`` property of the generated form::
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>>> ArticleForm = form_for_model(Article)
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>>> ArticleForm._model
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<class 'myapp.models.Article'>
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``form_for_instance()``
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-----------------------
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``form_for_instance()`` is like ``form_for_model()``, but it takes a model
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instance instead of a model class::
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# Create an Author.
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>>> a = Author(name='Joe Smith', title='MR', birth_date=None)
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>>> a.save()
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# Create a form for this particular Author.
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>>> AuthorForm = form_for_instance(a)
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# Instantiate the form.
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>>> f = AuthorForm()
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When a form created by ``form_for_instance()`` is created, the initial data
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values for the form fields are drawn from the instance. However, this data is
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not bound to the form. You will need to bind data to the form before the form
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can be saved.
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Unlike ``form_for_model()``, a choice field in form created by
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``form_for_instance()`` will not include the blank choice if the respective
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model field has ``blank=False``. The initial choice is drawn from the instance.
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When you call ``save()`` on a form created by ``form_for_instance()``,
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the database instance will be updated. As in ``form_for_model()``, ``save()``
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will raise ``ValueError`` if the data doesn't validate.
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``form_for_instance()`` has ``form``, ``fields`` and ``formfield_callback``
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arguments that behave the same way as they do for ``form_for_model()``.
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Let's modify the earlier `contact form`_ view example a little bit. Suppose we
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have a ``Message`` model that holds each contact submission. Something like::
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class Message(models.Model):
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subject = models.CharField(max_length=100)
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message = models.TextField()
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sender = models.EmailField()
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cc_myself = models.BooleanField(required=False)
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You could use this model to create a form (using ``form_for_model()``). You
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could also use existing ``Message`` instances to create a form for editing
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messages. The `simple example view`_ can be changed slightly to accept the ``id`` value
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of an existing ``Message`` and present it for editing::
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def contact_edit(request, msg_id):
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# Create the form from the message id.
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message = get_object_or_404(Message, id=msg_id)
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ContactForm = form_for_instance(message)
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if request.method == 'POST':
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form = ContactForm(request.POST)
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if form.is_valid():
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form.save()
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return HttpResponseRedirect('/url/on_success/')
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else:
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form = ContactForm()
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return render_to_response('contact.html', {'form': form})
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Aside from how we create the ``ContactForm`` class here, the main point to
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note is that the form display in the ``GET`` branch of the function
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will use the values from the ``message`` instance as initial values for the
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form field.
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.. _contact form: ../newforms/#simple-view-example
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.. _`simple example view`: ../newforms/#simple-view-example
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When should you use ``form_for_model()`` and ``form_for_instance()``?
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The ``form_for_model()`` and ``form_for_instance()`` functions are meant to be
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shortcuts for the common case. If you want to create a form whose fields map to
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more than one model, or a form that contains fields that *aren't* on a model,
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you shouldn't use these shortcuts. Creating a ``Form`` class the "long" way
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isn't that difficult, after all.
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