Fixed #12896. Documented the new side-effects of ModelForm validation.

introduced since 1.1, including ``csrf_protect``, ``cache_control`` and anything

created using ``decorator_from_middleware``.

``ModelForm.is_valid()`` and ``ModelForm.errors``

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

Much of the validation work for ModelForms has been moved down to the model

level. As a result, the first time you call ``ModelForm.is_valid()``, access

``ModelForm.errors`` or otherwise trigger form validation, your model will be

cleaned in-place. This conversion used to happen when the model was saved. If

you need an unmodified instance of your model, you should pass a copy to the

``ModelForm`` constructor.

.. _deprecated-features-1.2:

        name = forms.CharField(max_length=100)

        authors = forms.ModelMultipleChoiceField(queryset=Author.objects.all())

The ``is_valid()`` method and ``errors``

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.. versionchanged:: 1.2

The first time you call ``is_valid()`` or access the ``errors`` attribute of a

``ModelForm`` has always triggered form validation, but as of Django 1.2, it

will also trigger :ref:`model validation <validating-objects>`. This has the

side-effect of cleaning the model you pass to the ``ModelForm`` constructor.

For instance, calling ``is_valid()`` on your form will convert any date fields

on your model to actual date objects.

The ``save()`` method

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