Ensure that custom primary keys are always shown on a formset. Refs #7938....

    def add_fields(self, form, index):

        """Add a hidden field for the object's primary key."""

        if self.model._meta.has_auto_field:

            self._pk_field_name = self.model._meta.pk.attname

            form.fields[self._pk_field_name] = IntegerField(required=False, widget=HiddenInput)

        super(BaseModelFormSet, self).add_fields(form, index)

    def __unicode__(self):

        return self.name

class CustomPrimaryKey(models.Model):

    my_pk = models.CharField(max_length=10, primary_key=True)

    some_field = models.CharField(max_length=100)

__test__ = {'API_TESTS': """

<p><label for="id_test-0-title">Title:</label> <input id="id_test-0-title" type="text" name="test-0-title" maxlength="100" /><input type="hidden" name="test-0-id" id="id_test-0-id" /></p>

<p><label for="id_test-1-title">Title:</label> <input id="id_test-1-title" type="text" name="test-1-title" maxlength="100" /><input type="hidden" name="test-1-id" id="id_test-1-id" /></p>

# Test a custom primary key ###################################################

We need to ensure that it is displayed

>>> CustomPrimaryKeyFormSet = modelformset_factory(CustomPrimaryKey)

>>> formset = CustomPrimaryKeyFormSet()

>>> for form in formset.forms:

...     print form.as_p()

<p><label for="id_form-0-my_pk">My pk:</label> <input id="id_form-0-my_pk" type="text" name="form-0-my_pk" maxlength="10" /></p>

<p><label for="id_form-0-some_field">Some field:</label> <input id="id_form-0-some_field" type="text" name="form-0-some_field" maxlength="100" /></p>

"""}