361 lines
14 KiB
Plaintext
361 lines
14 KiB
Plaintext
==========================
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Using newforms with models
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==========================
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``ModelForm``
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=============
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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 helper class that let you create a ``Form``
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class from a Django model.
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For example::
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>>> from django.newforms import ModelForm
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# Create the form class.
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>>> class ArticleForm(ModelForm):
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... class Meta:
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... model = Article
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# Creating a form to add an article.
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>>> form = ArticleForm()
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# Creating a form to change an existing article.
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>>> article = Article.objects.get(pk=1)
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>>> form = ArticleForm(instance=article)
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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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class AuthorForm(ModelForm):
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class Meta:
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model = Author
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class BookForm(ModelForm):
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class Meta:
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model = Book
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With these models, the ``ModelForm`` subclasses above would be roughly
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equivalent to this (the only difference being the ``save()`` method, which
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we'll discuss in a moment.)::
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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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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 ``ModelForm`` also has a ``save()``
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method. This method creates and saves a database object from the data
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bound to the form. A subclass of ``ModelForm`` can accept an existing
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model instance as the keyword argument ``instance``; if this is
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supplied, ``save()`` will update that instance. If it's not supplied,
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``save()`` will create a new instance of the specified model::
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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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# Create a form to edit an existing Article.
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>>> a = Article.objects.get(pk=1)
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>>> f = ArticleForm(instance=a)
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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 your ``ModelForm`` subclass. After
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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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>>> a = Author()
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>>> f = AuthorForm(request.POST, instance=a)
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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 a subset of fields on the form
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------------------------------------
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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 three ways of telling ``ModelForm`` to use only a subset of the
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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 ``ModelForm`` will not include that
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field.
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2. Use the ``fields`` attribute of the ``ModelForm``'s inner ``Meta``
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class. This attribute, if given, should be a list of field names
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to include in the form.
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3. Use the ``exclude`` attribute of the ``ModelForm``'s inner ``Meta``
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class. This attribute, if given, should be a list of field names
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to exclude from the form.
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For example, if you want a form for the ``Author`` model (defined
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above) that includes only the ``name`` and ``title`` fields, you would
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specify ``fields`` or ``exclude`` like this::
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class PartialAuthorForm(ModelForm):
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class Meta:
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model = Author
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fields = ('name', 'title')
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class PartialAuthorForm(ModelForm):
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class Meta:
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model = Author
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exclude = ('birth_date',)
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Since the Author model has only 3 fields, 'name', 'title', and
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'birth_date', the forms above will contain exactly the same fields.
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.. note::
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If you specify ``fields`` or ``exclude`` when creating a form with
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``ModelForm``, then the fields that are not in the resulting form will not
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be set by the form's ``save()`` method. Django will prevent any attempt to
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save an incomplete model, so if the model does not allow the missing fields
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to be empty, and does not provide a default value for the missing fields,
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any attempt to ``save()`` a ``ModelForm`` with missing fields will fail.
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To avoid this failure, you must instantiate your model with initial values
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for the missing, but required fields, or use ``save(commit=False)`` and
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manually set any extra required fields::
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instance = Instance(required_field='value')
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form = InstanceForm(request.POST, instance=instance)
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new_instance = form.save()
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instance = form.save(commit=False)
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instance.required_field = 'new value'
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new_instance = 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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``ModelForm`` gives you the flexibility of changing the form field type
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for a given model field. You do this by declaratively specifying fields like
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you would in a regular ``Form``. Declared fields will override the default
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ones generated by using the ``model`` attribute.
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For example, if you wanted to use ``MyDateFormField`` for the ``pub_date``
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field, you could do the following::
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>>> class ArticleForm(ModelForm):
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... pub_date = MyDateFormField()
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...
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... class Meta:
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... model = Article
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If you want to override a field's default widget, then specify the ``widget``
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parameter when declaring the form field::
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>>> class ArticleForm(ModelForm):
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... pub_date = DateField(widget=MyDateWidget())
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...
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... class Meta:
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... model = Article
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Form inheritance
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----------------
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As with basic forms, you can extend and reuse ``ModelForms`` by inheriting
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them. This is useful if you need to declare extra fields or extra methods on a
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parent class for use in a number of forms derived from models. For example,
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using the previous ``ArticleForm`` class::
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>>> class EnhancedArticleForm(ArticleForm):
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... def clean_pub_date(self):
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... ...
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This creates a form that behaves identically to ``ArticleForm``, except there's
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some extra validation and cleaning for the ``pub_date`` field.
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You can also subclass the parent's ``Meta`` inner class if you want to change
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the ``Meta.fields`` or ``Meta.excludes`` lists::
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>>> class RestrictedArticleForm(EnhancedArticleForm):
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... class Meta(ArticleForm.Meta):
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... exclude = ['body']
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This adds the extra method from the ``EnhancedArticleForm`` and modifies
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the original ``ArticleForm.Meta`` to remove one field.
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There are a couple of things to note, however.
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* Normal Python name resolution rules apply. If you have multiple base
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classes that declare a ``Meta`` inner class, only the first one will be
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used. This means the child's ``Meta``, if it exists, otherwise the
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``Meta`` of the first parent, etc.
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* For technical reasons, a subclass cannot inherit from both a ``ModelForm``
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and a ``Form`` simultaneously.
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Chances are these notes won't affect you unless you're trying to do something
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tricky with subclassing.
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