Document that model field error_messages don't carry over to forms

[Jimmy Merrild Krag]

I have the following field in a model:

    reference = models.SlugField(
        max_length=20,
        blank=False, null=False,
        error_messages={
            'invalid': _('A reference may only contain letters, numbers, underscores and hyphens.'),
        })

however, when submitting the field in a ModelForm with invalid content (e.g. containing a space character) the error message is the default one for SlugField and not the one I set.

[Tim Graham]

I was surprised about this behavior but according to ticket:13693#comment:10, it's expected. The documentation should clarify this.

[Jimmy Merrild Krag]

So how should I override my error messages?
Right now I do

    def __init__(self, *args, **kwargs):
        super(MyForm, self).__init__(*args, **kwargs)

        # Correct error message for reference field
        reference_field = Plan._meta.get_field('reference')
        self.fields['reference'].error_messages.update(reference_field.error_messages)

which I think is ugly

[Taavi Teska]

​PR 8316

[Rivo Laks]

LGTM

[Tim Graham <timograham@…>]

In 5dbf1c4b:

Fixed #27947 -- Doc'd that model Field.error_messages often don't propagate to forms.

[Tim Graham <timograham@…>]

In c3d3aee:

[1.11.x] Fixed #27947 -- Doc'd that model Field.error_messages often don't propagate to forms.

Backport of 5dbf1c4b23cda915369f4895be293369575238d0 from master