modelforms.txt

==========================
Using newforms with models
==========================

``ModelForm``
=============

If you're building a database-driven app, chances are you'll have forms that
map closely to Django models. For instance, you might have a ``BlogComment``
model, and you want to create a form that lets people submit comments. In this
case, it would be redundant to define the field types in your form, because
you've already defined the fields in your model.

For this reason, Django provides a helper class that let you create a ``Form``
class from a Django model.

For example::

    >>> from django.newforms import ModelForm

    # Create the form class.
    >>> class ArticleForm(ModelForm):
    ...     class Meta:
    ...         model = Article

    # Creating a form to add an article.
    >>> form = ArticleForm()

    # Creating a form to change an existing article.
    >>> article = Article.objects.get(pk=1)
    >>> form = ArticleForm(instance=article)

Field types
-----------

The generated ``Form`` class will have a form field for every model field. Each
model field has a corresponding default form field. For example, a
``CharField`` on a model is represented as a ``CharField`` on a form. A
model ``ManyToManyField`` is represented as a ``MultipleChoiceField``. Here is
the full list of conversions:

    ===============================  ========================================
    Model field                      Form field
    ===============================  ========================================
    ``AutoField``                    Not represented in the form
    ``BooleanField``                 ``BooleanField``
    ``CharField``                    ``CharField`` with ``max_length`` set to
                                     the model field's ``max_length``
    ``CommaSeparatedIntegerField``   ``CharField``
    ``DateField``                    ``DateField``
    ``DateTimeField``                ``DateTimeField``
    ``DecimalField``                 ``DecimalField``
    ``EmailField``                   ``EmailField``
    ``FileField``                    ``FileField``
    ``FilePathField``                ``CharField``
    ``FloatField``                   ``FloatField``
    ``ForeignKey``                   ``ModelChoiceField`` (see below)
    ``ImageField``                   ``ImageField``
    ``IntegerField``                 ``IntegerField``
    ``IPAddressField``               ``IPAddressField``
    ``ManyToManyField``              ``ModelMultipleChoiceField`` (see
                                     below)
    ``NullBooleanField``             ``CharField``
    ``PhoneNumberField``             ``USPhoneNumberField``
                                     (from ``django.contrib.localflavor.us``)
    ``PositiveIntegerField``         ``IntegerField``
    ``PositiveSmallIntegerField``    ``IntegerField``
    ``SlugField``                    ``CharField``
    ``SmallIntegerField``            ``IntegerField``
    ``TextField``                    ``CharField`` with ``widget=Textarea``
    ``TimeField``                    ``TimeField``
    ``URLField``                     ``URLField`` with ``verify_exists`` set
                                     to the model field's ``verify_exists``
    ``USStateField``                 ``CharField`` with
                                     ``widget=USStateSelect``
                                     (``USStateSelect`` is from
                                     ``django.contrib.localflavor.us``)
    ``XMLField``                     ``CharField`` with ``widget=Textarea``
    ===============================  ========================================


.. note::
    The ``FloatField`` form field and ``DecimalField`` model and form fields
    are new in the development version.

As you might expect, the ``ForeignKey`` and ``ManyToManyField`` model field
types are special cases:

    * ``ForeignKey`` is represented by ``django.newforms.ModelChoiceField``,
      which is a ``ChoiceField`` whose choices are a model ``QuerySet``.

    * ``ManyToManyField`` is represented by
      ``django.newforms.ModelMultipleChoiceField``, which is a
      ``MultipleChoiceField`` whose choices are a model ``QuerySet``.

In addition, each generated form field has attributes set as follows:

    * If the model field has ``blank=True``, then ``required`` is set to
      ``False`` on the form field. Otherwise, ``required=True``.

    * The form field's ``label`` is set to the ``verbose_name`` of the model
      field, with the first character capitalized.

    * The form field's ``help_text`` is set to the ``help_text`` of the model
      field.

    * If the model field has ``choices`` set, then the form field's ``widget``
      will be set to ``Select``, with choices coming from the model field's
      ``choices``. The choices will normally include the blank choice which is
      selected by default. If the field is required, this forces the user to
      make a selection. The blank choice will not be included if the model
      field has ``blank=False`` and an explicit ``default`` value (the
      ``default`` value will be initially selected instead).

Finally, note that you can override the form field used for a given model
field. See `Overriding the default field types`_ below.

A full example
--------------

Consider this set of models::

    from django.db import models

    TITLE_CHOICES = (
        ('MR', 'Mr.'),
        ('MRS', 'Mrs.'),
        ('MS', 'Ms.'),
    )

    class Author(models.Model):
        name = models.CharField(max_length=100)
        title = models.CharField(max_length=3, choices=TITLE_CHOICES)
        birth_date = models.DateField(blank=True, null=True)

        def __unicode__(self):
            return self.name

    class Book(models.Model):
        name = models.CharField(max_length=100)
        authors = models.ManyToManyField(Author)

    class AuthorForm(ModelForm):
        class Meta:
            model = Author

    class BookForm(ModelForm):
        class Meta:
            model = Book

With these models, the ``ModelForm`` subclasses above would be roughly
equivalent to this (the only difference being the ``save()`` method, which
we'll discuss in a moment.)::

    class AuthorForm(forms.Form):
        name = forms.CharField(max_length=100)
        title = forms.CharField(max_length=3,
                    widget=forms.Select(choices=TITLE_CHOICES))
        birth_date = forms.DateField(required=False)

    class BookForm(forms.Form):
        name = forms.CharField(max_length=100)
        authors = forms.ModelMultipleChoiceField(queryset=Author.objects.all())

The ``save()`` method
---------------------

Every form produced by ``ModelForm`` also has a ``save()``
method. This method creates and saves a database object from the data
bound to the form. A subclass of ``ModelForm`` can accept an existing
model instance as the keyword argument ``instance``; if this is
supplied, ``save()`` will update that instance. If it's not supplied,
``save()`` will create a new instance of the specified model::

    # Create a form instance from POST data.
    >>> f = ArticleForm(request.POST)

    # Save a new Article object from the form's data.
    >>> new_article = f.save()

    # Create a form to edit an existing Article.
    >>> a = Article.objects.get(pk=1)
    >>> f = ArticleForm(instance=a)

Note that ``save()`` will raise a ``ValueError`` if the data in the form
doesn't validate -- i.e., ``if form.errors``.

This ``save()`` method accepts an optional ``commit`` keyword argument, which
accepts either ``True`` or ``False``. If you call ``save()`` with
``commit=False``, then it will return an object that hasn't yet been saved to
the database. In this case, it's up to you to call ``save()`` on the resulting
model instance. This is useful if you want to do custom processing on the
object before saving it. ``commit`` is ``True`` by default.

Another side effect of using ``commit=False`` is seen when your model has
a many-to-many relation with another model. If your model has a many-to-many
relation and you specify ``commit=False`` when you save a form, Django cannot
immediately save the form data for the many-to-many relation. This is because
it isn't possible to save many-to-many data for an instance until the instance
exists in the database.

To work around this problem, every time you save a form using ``commit=False``,
Django adds a ``save_m2m()`` method to your ``ModelForm`` subclass. After
you've manually saved the instance produced by the form, you can invoke
``save_m2m()`` to save the many-to-many form data. For example::

    # Create a form instance with POST data.
    >>> f = AuthorForm(request.POST)

    # Create, but don't save the new author instance.
    >>> new_author = f.save(commit=False)

    # Modify the author in some way.
    >>> new_author.some_field = 'some_value'

    # Save the new instance.
    >>> new_author.save()

    # Now, save the many-to-many data for the form.
    >>> f.save_m2m()

Calling ``save_m2m()`` is only required if you use ``save(commit=False)``.
When you use a simple ``save()`` on a form, all data -- including
many-to-many data -- is saved without the need for any additional method calls.
For example::

    # Create a form instance with POST data.
    >>> a = Author()
    >>> f = AuthorForm(request.POST, instance=a)

    # Create and save the new author instance. There's no need to do anything else.
    >>> new_author = f.save()

Other than the ``save()`` and ``save_m2m()`` methods, a ``ModelForm``
works exactly the same way as any other ``newforms`` form. For
example, the ``is_valid()`` method is used to check for validity, the
``is_multipart()`` method is used to determine whether a form requires
multipart file upload (and hence whether ``request.FILES`` must be
passed to the form), etc.; see `the standard newforms documentation`_
for more information.

.. _the standard newforms documentation: ../newforms/

Using a subset of fields on the form
------------------------------------

In some cases, you may not want all the model fields to appear on the generated
form. There are three ways of telling ``ModelForm`` to use only a subset of the
model fields:

1. Set ``editable=False`` on the model field. As a result, *any* form
   created from the model via ``ModelForm`` will not include that
   field.

2. Use the ``fields`` attribute of the ``ModelForm``'s inner ``Meta``
   class.  This attribute, if given, should be a list of field names
   to include in the form.

3. Use the ``exclude`` attribute of the ``ModelForm``'s inner ``Meta``
   class.  This attribute, if given, should be a list of field names
   to exclude from the form.

For example, if you want a form for the ``Author`` model (defined
above) that includes only the ``name`` and ``title`` fields, you would
specify ``fields`` or ``exclude`` like this::

    class PartialAuthorForm(ModelForm):
        class Meta:
            model = Author
            fields = ('name', 'title')
    
    class PartialAuthorForm(ModelForm):
        class Meta:
            model = Author
            exclude = ('birth_date',)

Since the Author model has only 3 fields, 'name', 'title', and
'birth_date', the forms above will contain exactly the same fields.

.. note::

    If you specify ``fields`` or ``exclude`` when creating a form with
    ``ModelForm``, then the fields that are not in the resulting form will not
    be set by the form's ``save()`` method. Django will prevent any attempt to
    save an incomplete model, so if the model does not allow the missing fields
    to be empty, and does not provide a default value for the missing fields,
    any attempt to ``save()`` a ``ModelForm`` with missing fields will fail.
    To avoid this failure, you must instantiate your model with initial values
    for the missing, but required fields, or use ``save(commit=False)`` and
    manually set any extra required fields::

        instance = Instance(required_field='value')
        form = InstanceForm(request.POST, instance=instance)
        new_instance = form.save()

        instance = form.save(commit=False)
        instance.required_field = 'new value'
        new_instance = instance.save()

    See the `section on saving forms`_ for more details on using
    ``save(commit=False)``.

.. _section on saving forms: `The save() method`_

Overriding the default field types
----------------------------------

The default field types, as described in the `Field types`_ table above, are
sensible defaults. If you have a ``DateField`` in your model, chances are you'd
want that to be represented as a ``DateField`` in your form. But
``ModelForm`` gives you the flexibility of changing the form field type
for a given model field. You do this by declaratively specifying fields like
you would in a regular ``Form``. Declared fields will override the default
ones generated by using the ``model`` attribute.

For example, if you wanted to use ``MyDateFormField`` for the ``pub_date``
field, you could do the following::

    >>> class ArticleForm(ModelForm):
    ...     pub_date = MyDateFormField()
    ...
    ...     class Meta:
    ...         model = Article

If you want to override a field's default widget, then specify the ``widget``
parameter when declaring the form field::

   >>> class ArticleForm(ModelForm):
   ...     pub_date = DateField(widget=MyDateWidget())
   ...
   ...     class Meta:
   ...         model = Article

Form inheritance
----------------

As with basic forms, you can extend and reuse ``ModelForms`` by inheriting
them. This is useful if you need to declare extra fields or extra methods on a
parent class for use in a number of forms derived from models. For example,
using the previous ``ArticleForm`` class::

    >>> class EnhancedArticleForm(ArticleForm):
    ...     def clean_pub_date(self):
    ...         ...

This creates a form that behaves identically to ``ArticleForm``, except there's
some extra validation and cleaning for the ``pub_date`` field.

You can also subclass the parent's ``Meta`` inner class if you want to change
the ``Meta.fields`` or ``Meta.excludes`` lists::

    >>> class RestrictedArticleForm(EnhancedArticleForm):
    ...     class Meta(ArticleForm.Meta):
    ...         exclude = ['body']

This adds the extra method from the ``EnhancedArticleForm`` and modifies
the original ``ArticleForm.Meta`` to remove one field.

There are a couple of things to note, however.

 * Normal Python name resolution rules apply. If you have multiple base
   classes that declare a ``Meta`` inner class, only the first one will be
   used. This means the child's ``Meta``, if it exists, otherwise the
   ``Meta`` of the first parent, etc.

 * For technical reasons, a subclass cannot inherit from both a ``ModelForm``
   and a ``Form`` simultaneously.

Chances are these notes won't affect you unless you're trying to do something
tricky with subclassing.