modelforms.txt

.. _topics-forms-modelforms:

==========================
Creating forms from 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.forms 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``                    ``SlugField``

    ``SmallIntegerField``            ``IntegerField``

    ``TextField``                    ``CharField`` with
                                     ``widget=forms.Textarea``

    ``TimeField``                    ``TimeField``

    ``URLField``                     ``URLField`` with ``verify_exists`` set
                                     to the model field's ``verify_exists``

    ``XMLField``                     ``CharField`` with
                                     ``widget=forms.Textarea``
    ===============================  ========================================


.. versionadded:: 1.0
    The ``FloatField`` form field and ``DecimalField`` model and form fields
    are new in Django 1.0.

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

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

    * ``ManyToManyField`` is represented by
      ``django.forms.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
    from django.forms import ModelForm

    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)
    >>> f.save()

    # Create a form to edit an existing Article, but use
    # POST data to populate the form.
    >>> a = Article.objects.get(pk=1)
    >>> f = ArticleForm(request.POST, instance=a)
    >>> f.save()

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, or if you want to use one of the specialized
:ref:`model saving options <ref-models-force-insert>`. ``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 ``forms`` 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
:ref:`binding-uploaded-files` for more information.

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.

   .. versionchanged:: 1.1

   The form will render the fields in the same order they are specified in the
   ``fields`` attribute.

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::

        author = Author(title='Mr')
        form = PartialAuthorForm(request.POST, instance=author)
        form.save()

    Alternatively, you can use ``save(commit=False)`` and manually set
    any extra required fields::

        form = PartialAuthorForm(request.POST)
        author = form.save(commit=False)
        author.title = 'Mr'
        author.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

.. note::

    If you explicitly instantiate a form field like this, Django assumes that you
    want to completely define its behavior; therefore, default attributes (such as 
    ``max_length`` or ``required``) are not drawn from the corresponding model. If 
    you want to maintain the behavior specified in the model, you must set the
    relevant arguments explicitly when declaring the form field.

    For example, if the ``Article`` model looks like this::

        class Article(models.Model):
            headline = models.CharField(max_length=200, null=True, blank=True, 
                                        help_text="Use puns liberally")
            content = models.TextField()

    and you want to do some custom validation for ``headline``, while keeping
    the ``blank`` and ``help_text`` values as specified, you might define 
    ``ArticleForm`` like this::

        class ArticleForm(ModelForm):
            headline = MyFormField(max_length=200, required=False, 
                                   help_text="Use puns liberally")

            class Meta:
                model = Article

    See the :ref:`form field documentation <ref-forms-fields>` for more information
    on fields and their arguments.

Changing the order of fields
----------------------------

.. versionadded:: 1.1

By default, a ``ModelForm`` will render fields in the same order that they are
defined on the model, with ``ManyToManyField`` instances appearing last. If
you want to change the order in which fields are rendered, you can use the
``fields`` attribute on the ``Meta`` class.

The ``fields`` attribute defines the subset of model fields that will be
rendered, and the order in which they will be rendered. For example given this
model::

    class Book(models.Model):
        author = models.ForeignKey(Author)
        title = models.CharField(max_length=100)

the ``author`` field would be rendered first. If we wanted the title field
to be rendered first, we could specify the following ``ModelForm``::

    >>> class BookForm(ModelForm):
    ...     class Meta:
    ...         model = Book
    ...         fields = ['title', 'author']

.. _overriding-modelform-clean-method:

Overriding the clean() method
-----------------------------

You can override the ``clean()`` method on a model form to provide additional
validation in the same way you can on a normal form.

In this regard, model forms have two specific characteristics when compared to
forms:

By default the ``clean()`` method validates the uniqueness of fields that are
marked as ``unique``, ``unique_together`` or ``unique_for_date|month|year`` on
the model.  Therefore, if you would like to override the ``clean()`` method and
maintain the default validation, you must call the parent class's ``clean()``
method.

Also, a model form instance bound to a model object will contain a
``self.instance`` attribute that gives model form methods access to that
specific model instance.

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.

.. _model-formsets:

Model formsets
==============

Like :ref:`regular formsets <topics-forms-formsets>`, Django provides a couple
of enhanced formset classes that make it easy to work with Django models. Let's
reuse the ``Author`` model from above::

    >>> from django.forms.models import modelformset_factory
    >>> AuthorFormSet = modelformset_factory(Author)

This will create a formset that is capable of working with the data associated
with the ``Author`` model. It works just like a regular formset::

    >>> formset = AuthorFormSet()
    >>> print formset
    <input type="hidden" name="form-TOTAL_FORMS" value="1" id="id_form-TOTAL_FORMS" /><input type="hidden" name="form-INITIAL_FORMS" value="0" id="id_form-INITIAL_FORMS" />
    <tr><th><label for="id_form-0-name">Name:</label></th><td><input id="id_form-0-name" type="text" name="form-0-name" maxlength="100" /></td></tr>
    <tr><th><label for="id_form-0-title">Title:</label></th><td><select name="form-0-title" id="id_form-0-title">
    <option value="" selected="selected">---------</option>
    <option value="MR">Mr.</option>
    <option value="MRS">Mrs.</option>
    <option value="MS">Ms.</option>
    </select></td></tr>
    <tr><th><label for="id_form-0-birth_date">Birth date:</label></th><td><input type="text" name="form-0-birth_date" id="id_form-0-birth_date" /><input type="hidden" name="form-0-id" id="id_form-0-id" /></td></tr>

.. note::
    ``modelformset_factory`` uses ``formset_factory`` to generate formsets.
    This means that a model formset is just an extension of a basic formset
    that knows how to interact with a particular model.

Changing the queryset
---------------------

By default, when you create a formset from a model, the formset will use a
queryset that includes all objects in the model (e.g.,
``Author.objects.all()``). You can override this behavior by using the
``queryset`` argument::

    >>> formset = AuthorFormSet(queryset=Author.objects.filter(name__startswith='O'))

Alternatively, you can create a subclass that sets ``self.queryset`` in
``__init__``::

    from django.forms.models import BaseModelFormSet

    class BaseAuthorFormSet(BaseModelFormSet):
        def __init__(self, *args, **kwargs):
            self.queryset = Author.objects.filter(name__startswith='O')
            super(BaseAuthorFormSet, self).__init__(*args, **kwargs)

Then, pass your ``BaseAuthorFormSet`` class to the factory function::

    >>> AuthorFormSet = modelformset_factory(Author, formset=BaseAuthorFormSet)

Controlling which fields are used with ``fields`` and ``exclude``
-----------------------------------------------------------------

By default, a model formset uses all fields in the model that are not marked
with ``editable=False``. However, this can be overridden at the formset level::

    >>> AuthorFormSet = modelformset_factory(Author, fields=('name', 'title'))

Using ``fields`` restricts the formset to use only the given fields.
Alternatively, you can take an "opt-out" approach, specifying which fields to
exclude::

    >>> AuthorFormSet = modelformset_factory(Author, exclude=('birth_date',))

.. _saving-objects-in-the-formset:

Saving objects in the formset
-----------------------------

As with a ``ModelForm``, you can save the data as a model object. This is done
with the formset's ``save()`` method::

    # Create a formset instance with POST data.
    >>> formset = AuthorFormSet(request.POST)

    # Assuming all is valid, save the data.
    >>> instances = formset.save()

The ``save()`` method returns the instances that have been saved to the
database. If a given instance's data didn't change in the bound data, the
instance won't be saved to the database and won't be included in the return
value (``instances``, in the above example).

Pass ``commit=False`` to return the unsaved model instances::

    # don't save to the database
    >>> instances = formset.save(commit=False)
    >>> for instance in instances:
    ...     # do something with instance
    ...     instance.save()

This gives you the ability to attach data to the instances before saving them
to the database. If your formset contains a ``ManyToManyField``, you'll also
need to call ``formset.save_m2m()`` to ensure the many-to-many relationships
are saved properly.

.. _model-formsets-max-num:

Limiting the number of editable objects
---------------------------------------

As with regular formsets, you can use the ``max_num`` parameter to
``modelformset_factory`` to limit the number of forms displayed. With
model formsets, this property limits the query to select only the maximum
number of objects needed::

    >>> Author.objects.order_by('name')
    [<Author: Charles Baudelaire>, <Author: Paul Verlaine>, <Author: Walt Whitman>]

    >>> AuthorFormSet = modelformset_factory(Author, max_num=2, extra=1)
    >>> formset = AuthorFormSet(queryset=Author.objects.order_by('name'))
    >>> formset.initial
    [{'id': 1, 'name': u'Charles Baudelaire'}, {'id': 3, 'name': u'Paul Verlaine'}]

If the value of ``max_num`` is higher than the number of objects returned, up to
``extra`` additional blank forms will be added to the formset, so long as the
total number of forms does not exceed ``max_num``::

    >>> AuthorFormSet = modelformset_factory(Author, max_num=4, extra=2)
    >>> formset = AuthorFormSet(queryset=Author.objects.order_by('name'))
    >>> for form in formset.forms:
    ...     print form.as_table()
    <tr><th><label for="id_form-0-name">Name:</label></th><td><input id="id_form-0-name" type="text" name="form-0-name" value="Charles Baudelaire" maxlength="100" /><input type="hidden" name="form-0-id" value="1" id="id_form-0-id" /></td></tr>
    <tr><th><label for="id_form-1-name">Name:</label></th><td><input id="id_form-1-name" type="text" name="form-1-name" value="Paul Verlaine" maxlength="100" /><input type="hidden" name="form-1-id" value="3" id="id_form-1-id" /></td></tr>
    <tr><th><label for="id_form-2-name">Name:</label></th><td><input id="id_form-2-name" type="text" name="form-2-name" value="Walt Whitman" maxlength="100" /><input type="hidden" name="form-2-id" value="2" id="id_form-2-id" /></td></tr>
    <tr><th><label for="id_form-3-name">Name:</label></th><td><input id="id_form-3-name" type="text" name="form-3-name" maxlength="100" /><input type="hidden" name="form-3-id" id="id_form-3-id" /></td></tr>

Using a model formset in a view
-------------------------------

Model formsets are very similar to formsets. Let's say we want to present a
formset to edit ``Author`` model instances::

    def manage_authors(request):
        AuthorFormSet = modelformset_factory(Author)
        if request.method == 'POST':
            formset = AuthorFormSet(request.POST, request.FILES)
            if formset.is_valid():
                formset.save()
                # do something.
        else:
            formset = AuthorFormSet()
        return render_to_response("manage_authors.html", {
            "formset": formset,
        })

As you can see, the view logic of a model formset isn't drastically different
than that of a "normal" formset. The only difference is that we call
``formset.save()`` to save the data into the database. (This was described
above, in :ref:`saving-objects-in-the-formset`.)

Overiding ``clean()`` on a ``model_formset``
--------------------------------------------

Just like with ``ModelForms``, by default the ``clean()`` method of a
``model_formset`` will validate that none of the items in the formset violate
the unique constraints on your model (either ``unique``, ``unique_together`` or
``unique_for_date|month|year``).  If you want to overide the ``clean()`` method
on a ``model_formset`` and maintain this validation, you must call the parent
class's ``clean`` method::

    class MyModelFormSet(BaseModelFormSet):
        def clean(self):
            super(MyModelFormSet, self).clean()
            # example custom validation across forms in the formset:
            for form in self.forms:
                # your custom formset validation

Using a custom queryset
-----------------------

As stated earlier, you can override the default queryset used by the model
formset::

    def manage_authors(request):
        AuthorFormSet = modelformset_factory(Author)
        if request.method == "POST":
            formset = AuthorFormSet(request.POST, request.FILES,
                                    queryset=Author.objects.filter(name__startswith='O'))
            if formset.is_valid():
                formset.save()
                # Do something.
        else:
            formset = AuthorFormSet(queryset=Author.objects.filter(name__startswith='O'))
        return render_to_response("manage_authors.html", {
            "formset": formset,
        })

Note that we pass the ``queryset`` argument in both the ``POST`` and ``GET``
cases in this example.

Using the formset in the template
---------------------------------

.. highlight:: html+django

There are three ways to render a formset in a Django template.

First, you can let the formset do most of the work::

    <form method="POST" action="">
        {{ formset }}
    </form>

Second, you can manually render the formset, but let the form deal with
itself::

    <form method="POST" action="">
        {{ formset.management_form }}
        {% for form in formset.forms %}
            {{ form }}
        {% endfor %}
    </form>

When you manually render the forms yourself, be sure to render the management
form as shown above. See the :ref:`management form documentation
<understanding-the-managementform>`.

Third, you can manually render each field::

    <form method="POST" action="">
        {{ formset.management_form }}
        {% for form in formset.forms %}
            {% for field in form %}
                {{ field.label_tag }}: {{ field }}
            {% endfor %}
        {% endfor %}
    </form>

If you opt to use this third method and you don't iterate over the fields with
a ``{% for %}`` loop, you'll need to render the primary key field. For example,
if you were rendering the ``name`` and ``age`` fields of a model::

    <form method="POST" action="">
        {{ formset.management_form }}
        {% for form in formset.forms %}
            {{ form.id }}
            <ul>
                <li>{{ form.name }}</li>
                <li>{{ form.age }}</li>
            </ul>
        {% endfor %}
    </form>

Notice how we need to explicitly render ``{{ form.id }}``. This ensures that
the model formset, in the ``POST`` case, will work correctly. (This example
assumes a primary key named ``id``. If you've explicitly defined your own
primary key that isn't called ``id``, make sure it gets rendered.)

.. highlight:: python

Inline formsets
===============

Inline formsets is a small abstraction layer on top of model formsets. These
simplify the case of working with related objects via a foreign key. Suppose
you have these two models::

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

    class Book(models.Model):
        author = models.ForeignKey(Author)
        title = models.CharField(max_length=100)

If you want to create a formset that allows you to edit books belonging to
a particular author, you could do this::

    >>> from django.forms.models import inlineformset_factory
    >>> BookFormSet = inlineformset_factory(Author, Book)
    >>> author = Author.objects.get(name=u'Mike Royko')
    >>> formset = BookFormSet(instance=author)

.. note::
    ``inlineformset_factory`` uses ``modelformset_factory`` and marks
    ``can_delete=True``.

More than one foreign key to the same model
-------------------------------------------

If your model contains more than one foreign key to the same model, you'll
need to resolve the ambiguity manually using ``fk_name``. For example, consider
the following model::

    class Friendship(models.Model):
        from_friend = models.ForeignKey(Friend)
        to_friend = models.ForeignKey(Friend)
        length_in_months = models.IntegerField()

To resolve this, you can use ``fk_name`` to ``inlineformset_factory``::

    >>> FriendshipFormSet = inlineformset_factory(Friend, Friendship, fk_name="from_friend")

Using an inline formset in a view
---------------------------------

You may want to provide a view that allows a user to edit the related objects
of a model. Here's how you can do that::

    def manage_books(request, author_id):
        author = Author.objects.get(pk=author_id)
        BookInlineFormSet = inlineformset_factory(Author, Book)
        if request.method == "POST":
            formset = BookInlineFormSet(request.POST, request.FILES, instance=author)
            if formset.is_valid():
                formset.save()
                # Do something.
        else:
            formset = BookInlineFormSet(instance=author)
        return render_to_response("manage_books.html", {
            "formset": formset,
        })

Notice how we pass ``instance`` in both the ``POST`` and ``GET`` cases.