Fixed a few class references in the model field docs.

:attr:`~Field.blank`).

Avoid using :attr:`~Field.null` on string-based fields such as

:class:`CharField` and :class:`TextField` unless you have an excellent reason.

If a string-based field has ``null=True``, that means it has two possible values

for "no data": ``NULL``, and the empty string. In most cases, it's redundant to

have two possible values for "no data;" Django convention is to use the empty

string, not ``NULL``.

:class:`~django.db.models.CharField` and :class:`~django.db.models.TextField`

unless you have an excellent reason. If a string-based field has ``null=True``,

that means it has two possible values for "no data": ``NULL``, and the empty

string. In most cases, it's redundant to have two possible values for "no

data;" Django convention is to use the empty string, not ``NULL``.

.. note::

Finally, note that choices can be any iterable object -- not necessarily a list

or tuple. This lets you construct choices dynamically. But if you find yourself

hacking :attr:`~Field.choices` to be dynamic, you're probably better off using a

proper database table with a :class:`ForeignKey`. :attr:`~Field.choices` is

meant for static data that doesn't change much, if ever.

proper database table with a :class:`~django.db.models.ForeignKey`.

:attr:`~Field.choices` is meant for static data that doesn't change much, if

ever.

``db_column``

-------------

If ``True``, this field is the primary key for the model.

If you don't specify ``primary_key=True`` for any fields in your model, Django

will automatically add an :class:`IntegerField` to hold the primary key, so you

don't need to set ``primary_key=True`` on any of your fields unless you want to

override the default primary-key behavior. For more, see

:ref:`automatic-primary-key-fields`.

will automatically add an :class:`~django.db.models.IntegerField` to hold the

primary key, so you don't need to set ``primary_key=True`` on any of your

fields unless you want to override the default primary-key behavior. For more,

see :ref:`automatic-primary-key-fields`.

``primary_key=True`` implies :attr:`null=False <Field.null>` and :attr:`unique=True <Field.unique>`.

Only one primary key is allowed on an object.

field, a :exc:`django.db.IntegrityError` will be raised by the model's

:meth:`~django.db.models.Model.save` method.

This option is valid on all field types except :class:`ManyToManyField` and

:class:`FileField`.

This option is valid on all field types except

:class:`~django.db.models.ManyToManyField` and

:class:`~django.db.models.FileField`.

``unique_for_date``

-------------------

.. attribute:: Field.unique_for_date

Set this to the name of a :class:`DateField` or :class:`DateTimeField` to

require that this field be unique for the value of the date field.

Set this to the name of a :class:`~django.db.models.DateField` or

:class:`~django.db.models.DateTimeField` to require that this field be unique

for the value of the date field.

For example, if you have a field ``title`` that has

``unique_for_date="pub_date"``, then Django wouldn't allow the entry of two