.. _ref-forms-fields:

===========
Form fields
===========

.. module:: django.forms.fields
   :synopsis: Django's built-in form fields.

.. currentmodule:: django.forms

.. class:: Field(**kwargs)

When you create a ``Form`` class, the most important part is defining the
fields of the form. Each field has custom validation logic, along with a few
other hooks.

.. method:: Field.clean(value)

Although the primary way you'll use ``Field`` classes is in ``Form`` classes,
you can also instantiate them and use them directly to get a better idea of
how they work. Each ``Field`` instance has a ``clean()`` method, which takes
a single argument and either raises a ``django.forms.ValidationError``
exception or returns the clean value::

    >>> f = forms.EmailField()
    >>> f.clean('foo@example.com')
    u'foo@example.com'
    >>> f.clean(u'foo@example.com')
    u'foo@example.com'
    >>> f.clean('invalid e-mail address')
    Traceback (most recent call last):
    ...
    ValidationError: [u'Enter a valid e-mail address.']

Core field arguments
--------------------

Each ``Field`` class constructor takes at least these arguments. Some
``Field`` classes take additional, field-specific arguments, but the following
should *always* be accepted:

``required``
~~~~~~~~~~~~

.. attribute:: Field.required

By default, each ``Field`` class assumes the value is required, so if you pass
an empty value -- either ``None`` or the empty string (``""``) -- then
``clean()`` will raise a ``ValidationError`` exception::

    >>> f = forms.CharField()
    >>> f.clean('foo')
    u'foo'
    >>> f.clean('')
    Traceback (most recent call last):
    ...
    ValidationError: [u'This field is required.']
    >>> f.clean(None)
    Traceback (most recent call last):
    ...
    ValidationError: [u'This field is required.']
    >>> f.clean(' ')
    u' '
    >>> f.clean(0)
    u'0'
    >>> f.clean(True)
    u'True'
    >>> f.clean(False)
    u'False'

To specify that a field is *not* required, pass ``required=False`` to the
``Field`` constructor::

    >>> f = forms.CharField(required=False)
    >>> f.clean('foo')
    u'foo'
    >>> f.clean('')
    u''
    >>> f.clean(None)
    u''
    >>> f.clean(0)
    u'0'
    >>> f.clean(True)
    u'True'
    >>> f.clean(False)
    u'False'

If a ``Field`` has ``required=False`` and you pass ``clean()`` an empty value,
then ``clean()`` will return a *normalized* empty value rather than raising
``ValidationError``. For ``CharField``, this will be a Unicode empty string.
For other ``Field`` classes, it might be ``None``. (This varies from field to
field.)

``label``
~~~~~~~~~

.. attribute:: Field.label

The ``label`` argument lets you specify the "human-friendly" label for this
field. This is used when the ``Field`` is displayed in a ``Form``.

As explained in "Outputting forms as HTML" above, the default label for a
``Field`` is generated from the field name by converting all underscores to
spaces and upper-casing the first letter. Specify ``label`` if that default
behavior doesn't result in an adequate label.

Here's a full example ``Form`` that implements ``label`` for two of its fields.
We've specified ``auto_id=False`` to simplify the output::

    >>> class CommentForm(forms.Form):
    ...     name = forms.CharField(label='Your name')
    ...     url = forms.URLField(label='Your Web site', required=False)
    ...     comment = forms.CharField()
    >>> f = CommentForm(auto_id=False)
    >>> print f
    <tr><th>Your name:</th><td><input type="text" name="name" /></td></tr>
    <tr><th>Your Web site:</th><td><input type="text" name="url" /></td></tr>
    <tr><th>Comment:</th><td><input type="text" name="comment" /></td></tr>

``initial``
~~~~~~~~~~~

.. attribute:: Field.initial

The ``initial`` argument lets you specify the initial value to use when
rendering this ``Field`` in an unbound ``Form``.

The use-case for this is when you want to display an "empty" form in which a
field is initialized to a particular value. For example::

    >>> class CommentForm(forms.Form):
    ...     name = forms.CharField(initial='Your name')
    ...     url = forms.URLField(initial='http://')
    ...     comment = forms.CharField()
    >>> f = CommentForm(auto_id=False)
    >>> print f
    <tr><th>Name:</th><td><input type="text" name="name" value="Your name" /></td></tr>
    <tr><th>Url:</th><td><input type="text" name="url" value="http://" /></td></tr>
    <tr><th>Comment:</th><td><input type="text" name="comment" /></td></tr>

You may be thinking, why not just pass a dictionary of the initial values as
data when displaying the form? Well, if you do that, you'll trigger validation,
and the HTML output will include any validation errors::

    >>> class CommentForm(forms.Form):
    ...     name = forms.CharField()
    ...     url = forms.URLField()
    ...     comment = forms.CharField()
    >>> default_data = {'name': 'Your name', 'url': 'http://'}
    >>> f = CommentForm(default_data, auto_id=False)
    >>> print f
    <tr><th>Name:</th><td><input type="text" name="name" value="Your name" /></td></tr>
    <tr><th>Url:</th><td><ul class="errorlist"><li>Enter a valid URL.</li></ul><input type="text" name="url" value="http://" /></td></tr>
    <tr><th>Comment:</th><td><ul class="errorlist"><li>This field is required.</li></ul><input type="text" name="comment" /></td></tr>

This is why ``initial`` values are only displayed for unbound forms. For bound
forms, the HTML output will use the bound data.

Also note that ``initial`` values are *not* used as "fallback" data in
validation if a particular field's value is not given. ``initial`` values are
*only* intended for initial form display::

    >>> class CommentForm(forms.Form):
    ...     name = forms.CharField(initial='Your name')
    ...     url = forms.URLField(initial='http://')
    ...     comment = forms.CharField()
    >>> data = {'name': '', 'url': '', 'comment': 'Foo'}
    >>> f = CommentForm(data)
    >>> f.is_valid()
    False
    # The form does *not* fall back to using the initial values.
    >>> f.errors
    {'url': [u'This field is required.'], 'name': [u'This field is required.']}

``widget``
~~~~~~~~~~

.. attribute:: Field.widget

The ``widget`` argument lets you specify a ``Widget`` class to use when
rendering this ``Field``. See :ref:`ref-forms-widgets` for more information.

``help_text``
~~~~~~~~~~~~~

.. attribute:: Field.help_text

The ``help_text`` argument lets you specify descriptive text for this
``Field``. If you provide ``help_text``, it will be displayed next to the
``Field`` when the ``Field`` is rendered by one of the convenience ``Form``
methods (e.g., ``as_ul()``).

Here's a full example ``Form`` that implements ``help_text`` for two of its
fields. We've specified ``auto_id=False`` to simplify the output::

    >>> class HelpTextContactForm(forms.Form):
    ...     subject = forms.CharField(max_length=100, help_text='100 characters max.')
    ...     message = forms.CharField()
    ...     sender = forms.EmailField(help_text='A valid e-mail address, please.')
    ...     cc_myself = forms.BooleanField(required=False)
    >>> f = HelpTextContactForm(auto_id=False)
    >>> print f.as_table()
    <tr><th>Subject:</th><td><input type="text" name="subject" maxlength="100" /><br />100 characters max.</td></tr>
    <tr><th>Message:</th><td><input type="text" name="message" /></td></tr>
    <tr><th>Sender:</th><td><input type="text" name="sender" /><br />A valid e-mail address, please.</td></tr>
    <tr><th>Cc myself:</th><td><input type="checkbox" name="cc_myself" /></td></tr>
    >>> print f.as_ul()
    <li>Subject: <input type="text" name="subject" maxlength="100" /> 100 characters max.</li>
    <li>Message: <input type="text" name="message" /></li>
    <li>Sender: <input type="text" name="sender" /> A valid e-mail address, please.</li>
    <li>Cc myself: <input type="checkbox" name="cc_myself" /></li>
    >>> print f.as_p()
    <p>Subject: <input type="text" name="subject" maxlength="100" /> 100 characters max.</p>
    <p>Message: <input type="text" name="message" /></p>
    <p>Sender: <input type="text" name="sender" /> A valid e-mail address, please.</p>
    <p>Cc myself: <input type="checkbox" name="cc_myself" /></p>

``error_messages``
~~~~~~~~~~~~~~~~~~

.. versionadded:: 1.0

.. attribute:: Field.error_messages


The ``error_messages`` argument lets you override the default messages that the
field will raise. Pass in a dictionary with keys matching the error messages you
want to override. For example, here is the default error message::

    >>> generic = forms.CharField()
    >>> generic.clean('')
    Traceback (most recent call last):
      ...
    ValidationError: [u'This field is required.']

And here is a custom error message::

    >>> name = forms.CharField(error_messages={'required': 'Please enter your name'})
    >>> name.clean('')
    Traceback (most recent call last):
      ...
    ValidationError: [u'Please enter your name']

In the `built-in Field classes`_ section below, each ``Field`` defines the
error message keys it uses.

Dynamic initial values
----------------------

The ``initial`` argument to ``Field`` (explained above) lets you hard-code the
initial value for a ``Field`` -- but what if you want to declare the initial
value at runtime? For example, you might want to fill in a ``username`` field
with the username of the current session.

To accomplish this, use the ``initial`` argument to a ``Form``. This argument,
if given, should be a dictionary mapping field names to initial values. Only
include the fields for which you're specifying an initial value; it's not
necessary to include every field in your form. For example::

    >>> class CommentForm(forms.Form):
    ...     name = forms.CharField()
    ...     url = forms.URLField()
    ...     comment = forms.CharField()
    >>> f = CommentForm(initial={'name': 'your username'}, auto_id=False)
    >>> print f
    <tr><th>Name:</th><td><input type="text" name="name" value="your username" /></td></tr>
    <tr><th>Url:</th><td><input type="text" name="url" /></td></tr>
    <tr><th>Comment:</th><td><input type="text" name="comment" /></td></tr>
    >>> f = CommentForm(initial={'name': 'another username'}, auto_id=False)
    >>> print f
    <tr><th>Name:</th><td><input type="text" name="name" value="another username" /></td></tr>
    <tr><th>Url:</th><td><input type="text" name="url" /></td></tr>
    <tr><th>Comment:</th><td><input type="text" name="comment" /></td></tr>

Just like the ``initial`` parameter to ``Field``, these values are only
displayed for unbound forms, and they're not used as fallback values if a
particular value isn't provided.

Finally, note that if a ``Field`` defines ``initial`` *and* you include
``initial`` when instantiating the ``Form``, then the latter ``initial`` will
have precedence. In this example, ``initial`` is provided both at the field
level and at the form instance level, and the latter gets precedence::

    >>> class CommentForm(forms.Form):
    ...     name = forms.CharField(initial='class')
    ...     url = forms.URLField()
    ...     comment = forms.CharField()
    >>> f = CommentForm(initial={'name': 'instance'}, auto_id=False)
    >>> print f
    <tr><th>Name:</th><td><input type="text" name="name" value="instance" /></td></tr>
    <tr><th>Url:</th><td><input type="text" name="url" /></td></tr>
    <tr><th>Comment:</th><td><input type="text" name="comment" /></td></tr>


Built-in ``Field`` classes
--------------------------

Naturally, the ``forms`` library comes with a set of ``Field`` classes that
represent common validation needs. This section documents each built-in field.

For each field, we describe the default widget used if you don't specify
``widget``. We also specify the value returned when you provide an empty value
(see the section on ``required`` above to understand what that means).

``BooleanField``
~~~~~~~~~~~~~~~~

.. class:: BooleanField(**kwargs)

    * Default widget: ``CheckboxInput``
    * Empty value: ``False``
    * Normalizes to: A Python ``True`` or ``False`` value.
    * Validates that the check box is checked (i.e. the value is ``True``) if
      the field has ``required=True``.
    * Error message keys: ``required``

.. versionchanged:: 1.0
   The empty value for a ``CheckboxInput`` (and hence the standard ``BooleanField``)
   has changed to return ``False`` instead of ``None`` in the development version.

.. note::

    Since all ``Field`` subclasses have ``required=True`` by default, the
    validation condition here is important. If you want to include a checkbox
    in your form that can be either checked or unchecked, you must remember to
    pass in ``required=False`` when creating the ``BooleanField``.

``CharField``
~~~~~~~~~~~~~

.. class:: CharField(**kwargs)

    * Default widget: ``TextInput``
    * Empty value: ``''`` (an empty string)
    * Normalizes to: A Unicode object.
    * Validates ``max_length`` or ``min_length``, if they are provided.
      Otherwise, all inputs are valid.
    * Error message keys: ``required``, ``max_length``, ``min_length``

Has two optional arguments for validation:

.. attribute:: CharField.max_length
.. attribute:: CharField.min_length

    If provided, these arguments ensure that the string is at most or at least
    the given length.

``ChoiceField``
~~~~~~~~~~~~~~~

.. class:: ChoiceField(**kwargs)

    * Default widget: ``Select``
    * Empty value: ``''`` (an empty string)
    * Normalizes to: A Unicode object.
    * Validates that the given value exists in the list of choices.
    * Error message keys: ``required``, ``invalid_choice``

Takes one extra required argument:

.. attribute:: ChoiceField.choices

    An iterable (e.g., a list or tuple) of 2-tuples to use as choices for this
    field.
    
``TypedChoiceField``
~~~~~~~~~~~~~~~~~~~~

.. class:: TypedChoiceField(**kwargs)

Just like a :class:`ChoiceField`, except :class:`TypedChoiceField` takes an
extra ``coerce`` argument.

    * Default widget: ``Select``
    * Empty value: Whatever you've given as ``empty_value``
    * Normalizes to: the value returned by the ``coerce`` argument.
    * Validates that the given value exists in the list of choices.
    * Error message keys: ``required``, ``invalid_choice``

Takes extra arguments:

.. attribute:: TypedChoiceField.coerce

    A function that takes one argument and returns a coerced value. Examples
    include the built-in ``int``, ``float``, ``bool`` and other types. Defaults
    to an identity function.

.. attribute:: TypedChoiceField.empty_value

    The value to use to represent "empty." Defaults to the empty string;
    ``None`` is another common choice here.

``DateField``
~~~~~~~~~~~~~

.. class:: DateField(**kwargs)

    * Default widget: ``TextInput``
    * Empty value: ``None``
    * Normalizes to: A Python ``datetime.date`` object.
    * Validates that the given value is either a ``datetime.date``,
      ``datetime.datetime`` or string formatted in a particular date format.
    * Error message keys: ``required``, ``invalid``

Takes one optional argument:

.. attribute:: DateField.input_formats

    A list of formats used to attempt to convert a string to a valid
    ``datetime.date`` object.

If no ``input_formats`` argument is provided, the default input formats are::

    '%Y-%m-%d', '%m/%d/%Y', '%m/%d/%y', # '2006-10-25', '10/25/2006', '10/25/06'
    '%b %d %Y', '%b %d, %Y',            # 'Oct 25 2006', 'Oct 25, 2006'
    '%d %b %Y', '%d %b, %Y',            # '25 Oct 2006', '25 Oct, 2006'
    '%B %d %Y', '%B %d, %Y',            # 'October 25 2006', 'October 25, 2006'
    '%d %B %Y', '%d %B, %Y',            # '25 October 2006', '25 October, 2006'

``DateTimeField``
~~~~~~~~~~~~~~~~~

.. class:: DateTimeField(**kwargs)

    * Default widget: ``DateTimeInput``
    * Empty value: ``None``
    * Normalizes to: A Python ``datetime.datetime`` object.
    * Validates that the given value is either a ``datetime.datetime``,
      ``datetime.date`` or string formatted in a particular datetime format.
    * Error message keys: ``required``, ``invalid``

Takes one optional argument:

.. attribute:: DateTimeField.input_formats

    A list of formats used to attempt to convert a string to a valid
    ``datetime.datetime`` object.

If no ``input_formats`` argument is provided, the default input formats are::

    '%Y-%m-%d %H:%M:%S',     # '2006-10-25 14:30:59'
    '%Y-%m-%d %H:%M',        # '2006-10-25 14:30'
    '%Y-%m-%d',              # '2006-10-25'
    '%m/%d/%Y %H:%M:%S',     # '10/25/2006 14:30:59'
    '%m/%d/%Y %H:%M',        # '10/25/2006 14:30'
    '%m/%d/%Y',              # '10/25/2006'
    '%m/%d/%y %H:%M:%S',     # '10/25/06 14:30:59'
    '%m/%d/%y %H:%M',        # '10/25/06 14:30'
    '%m/%d/%y',              # '10/25/06'

.. versionchanged:: 1.0
   The ``DateTimeField`` used to use a ``TextInput`` widget by default. This has now changed.

``DecimalField``
~~~~~~~~~~~~~~~~

.. versionadded:: 1.0

.. class:: DecimalField(**kwargs)

    * Default widget: ``TextInput``
    * Empty value: ``None``
    * Normalizes to: A Python ``decimal``.
    * Validates that the given value is a decimal. Leading and trailing
      whitespace is ignored.
    * Error message keys: ``required``, ``invalid``, ``max_value``,
      ``min_value``, ``max_digits``, ``max_decimal_places``,
      ``max_whole_digits``

Takes four optional arguments: 

.. attribute:: DecimalField.max_value
.. attribute:: DecimalField.min_value

    These attributes define the limits for the fields value.

.. attribute:: DecimalField.max_digits

    The maximum number of digits (those before the decimal point plus those
    after the decimal point, with leading zeros stripped) permitted in the
    value.
    
.. attribute:: DecimalField.decimal_places

    The maximum number of decimal places permitted.

``EmailField``
~~~~~~~~~~~~~~

.. class:: EmailField(**kwargs)

    * Default widget: ``TextInput``
    * Empty value: ``''`` (an empty string)
    * Normalizes to: A Unicode object.
    * Validates that the given value is a valid e-mail address, using a
      moderately complex regular expression.
    * Error message keys: ``required``, ``invalid``

Has two optional arguments for validation, ``max_length`` and ``min_length``.
If provided, these arguments ensure that the string is at most or at least the
given length.

``FileField``
~~~~~~~~~~~~~

.. versionadded:: 1.0

.. class:: FileField(**kwargs)

    * Default widget: ``FileInput``
    * Empty value: ``None``
    * Normalizes to: An ``UploadedFile`` object that wraps the file content
      and file name into a single object.
    * Validates that non-empty file data has been bound to the form.
    * Error message keys: ``required``, ``invalid``, ``missing``, ``empty``

To learn more about the ``UploadedFile`` object, see the :ref:`file uploads
documentation <topics-file-uploads>`.

When you use a ``FileField`` in a form, you must also remember to
:ref:`bind the file data to the form <topics-file-uploads>`.

``FilePathField``
~~~~~~~~~~~~~~~~~

.. versionadded:: 1.0

.. class:: FilePathField(**kwargs)

    * Default widget: ``Select``
    * Empty value: ``None``
    * Normalizes to: A unicode object
    * Validates that the selected choice exists in the list of choices.
    * Error message keys: ``required``, ``invalid_choice``

The field allows choosing from files inside a certain directory. It takes three
extra arguments; only ``path`` is required:

.. attribute:: FilePathField.path

    The absolute path to the directory whose contents you want listed. This
    directory must exist.

.. attribute:: FilePathField.recursive

    If ``False`` (the default) only the direct contents of ``path`` will be
    offered as choices. If ``True``, the directory will be descended into
    recursively and all descendants will be listed as choices.

.. attribute:: FilePathField.match

    A regular expression pattern; only files with names matching this expression
    will be allowed as choices.

``FloatField`` 
~~~~~~~~~~~~~~ 

    * Default widget: ``TextInput`` 
    * Empty value: ``None`` 
    * Normalizes to: A Python float. 
    * Validates that the given value is an float. Leading and trailing 
      whitespace is allowed, as in Python's ``float()`` function. 
    * Error message keys: ``required``, ``invalid``, ``max_value``, 
      ``min_value`` 
	 
Takes two optional arguments for validation, ``max_value`` and ``min_value``. 
These control the range of values permitted in the field.

``ImageField``
~~~~~~~~~~~~~~

.. versionadded:: 1.0

.. class:: ImageField(**kwargs)

    * Default widget: ``FileInput``
    * Empty value: ``None``
    * Normalizes to: An ``UploadedFile`` object that wraps the file content
      and file name into a single object.
    * Validates that file data has been bound to the form, and that the
      file is of an image format understood by PIL.
    * Error message keys: ``required``, ``invalid``, ``missing``, ``empty``,
      ``invalid_image``

Using an ImageField requires that the `Python Imaging Library`_ is installed.

When you use an ``ImageField`` on a form, you must also remember to
:ref:`bind the file data to the form <topics-file-uploads>`.

.. _Python Imaging Library: http://www.pythonware.com/products/pil/

``IntegerField``
~~~~~~~~~~~~~~~~

.. class:: IntegerField(**kwargs)

    * Default widget: ``TextInput``
    * Empty value: ``None``
    * Normalizes to: A Python integer or long integer.
    * Validates that the given value is an integer. Leading and trailing
      whitespace is allowed, as in Python's ``int()`` function.
    * Error message keys: ``required``, ``invalid``, ``max_value``,
      ``min_value``

Takes two optional arguments for validation:

.. attribute:: IntegerField.max_value
.. attribute:: IntegerField.min_value

    These control the range of values permitted in the field.

``IPAddressField``
~~~~~~~~~~~~~~~~~~

.. class:: IPAddressField(**kwargs)

    * Default widget: ``TextInput``
    * Empty value: ``''`` (an empty string)
    * Normalizes to: A Unicode object.
    * Validates that the given value is a valid IPv4 address, using a regular
      expression.
    * Error message keys: ``required``, ``invalid``

``MultipleChoiceField``
~~~~~~~~~~~~~~~~~~~~~~~

.. class:: MultipleChoiceField(**kwargs)

    * Default widget: ``SelectMultiple``
    * Empty value: ``[]`` (an empty list)
    * Normalizes to: A list of Unicode objects.
    * Validates that every value in the given list of values exists in the list
      of choices.
    * Error message keys: ``required``, ``invalid_choice``, ``invalid_list``

Takes one extra argument, ``choices``, as for ``ChoiceField``.

``NullBooleanField``
~~~~~~~~~~~~~~~~~~~~

.. class:: NullBooleanField(**kwargs)

    * Default widget: ``NullBooleanSelect``
    * Empty value: ``None``
    * Normalizes to: A Python ``True``, ``False`` or ``None`` value.
    * Validates nothing (i.e., it never raises a ``ValidationError``).

``RegexField``
~~~~~~~~~~~~~~

.. class:: RegexField(**kwargs)

    * Default widget: ``TextInput``
    * Empty value: ``''`` (an empty string)
    * Normalizes to: A Unicode object.
    * Validates that the given value matches against a certain regular
      expression.
    * Error message keys: ``required``, ``invalid``

Takes one required argument:

.. attribute:: RegexField.regex

    A regular expression specified either as a string or a compiled regular
    expression object.

Also takes ``max_length`` and ``min_length``, which work just as they do for
``CharField``.

The optional argument ``error_message`` is also accepted for backwards
compatibility. The preferred way to provide an error message is to use the
``error_messages`` argument, passing a dictionary with ``'invalid'`` as a key
and the error message as the value.

``TimeField``
~~~~~~~~~~~~~

.. class:: TimeField(**kwargs)

    * Default widget: ``TextInput``
    * Empty value: ``None``
    * Normalizes to: A Python ``datetime.time`` object.
    * Validates that the given value is either a ``datetime.time`` or string
      formatted in a particular time format.
    * Error message keys: ``required``, ``invalid``

Takes one optional argument:

.. attribute:: TimeField.input_formats

    A list of formats used to attempt to convert a string to a valid
    ``datetime.time`` object.

If no ``input_formats`` argument is provided, the default input formats are::

    '%H:%M:%S',     # '14:30:59'
    '%H:%M',        # '14:30'

``URLField``
~~~~~~~~~~~~

.. class:: URLField(**kwargs)

    * Default widget: ``TextInput``
    * Empty value: ``''`` (an empty string)
    * Normalizes to: A Unicode object.
    * Validates that the given value is a valid URL.
    * Error message keys: ``required``, ``invalid``, ``invalid_link``

Takes the following optional arguments:


.. attribute:: URLField.max_length
.. attribute:: URLField.min_length

    Same as ``CharField.max_length`` and ``CharField.min_length``.

.. attribute:: URLField.verify_exists

    If ``True``, the validator will attempt to load the given URL, raising
    ``ValidationError`` if the page gives a 404. Defaults to ``False``.

.. attribute:: URLField.validator_user_agent

    String used as the user-agent used when checking for a URL's existence.
    Defaults to the value of the ``URL_VALIDATOR_USER_AGENT`` setting.

Slightly complex built-in ``Field`` classes
-------------------------------------------

The following are not yet documented.

.. class:: ComboField(**kwargs)

.. class:: MultiValueField(**kwargs)

.. class:: SplitDateTimeField(**kwargs)

Fields which handle relationships
---------------------------------

For representing relationships between models, two fields are
provided which can derive their choices from a ``QuerySet``:

.. class:: ModelChoiceField(**kwargs)
.. class:: ModelMultipleChoiceField(**kwargs)

These fields place one or more model objects into the ``cleaned_data``
dictionary of forms in which they're used. Both of these fields have an
additional required argument:

.. attribute:: ModelChoiceField.queryset

    A ``QuerySet`` of model objects from which the choices for the
    field will be derived, and which will be used to validate the
    user's selection.

``ModelChoiceField``
~~~~~~~~~~~~~~~~~~~~

Allows the selection of a single model object, suitable for
representing a foreign key.

The ``__unicode__`` method of the model will be called to generate
string representations of the objects for use in the field's choices;
to provide customized representations, subclass ``ModelChoiceField``
and override ``label_from_instance``. This method will receive a model
object, and should return a string suitable for representing it. For
example::

    class MyModelChoiceField(ModelChoiceField):
        def label_from_instance(self, obj):
            return "My Object #%i" % obj.id

``ModelMultipleChoiceField``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Allows the selection of one or more model objects, suitable for
representing a many-to-many relation. As with ``ModelChoiceField``,
you can use ``label_from_instance`` to customize the object
representations.

Creating custom fields
----------------------

If the built-in ``Field`` classes don't meet your needs, you can easily create
custom ``Field`` classes. To do this, just create a subclass of
``django.forms.Field``. Its only requirements are that it implement a
``clean()`` method and that its ``__init__()`` method accept the core arguments
mentioned above (``required``, ``label``, ``initial``, ``widget``,
``help_text``).
