Django: correctly display multiple error messages for one field

Question

I'm using django 1.5.5. for my project. I have some models, some of which have a many-to-many field to another:

class ScreeningFormat(CorecrmModel):
    name = models.CharField(max_length=100)

class Film(CorecrmModel):
    title = models.CharField(max_length=255)
    screening_formats = models.ManyToManyField(ScreeningFormat)

class Screen(CorecrmModel):
    name = models.CharField(max_length=100)
    screening_formats = models.ManyToManyField(ScreeningFormat)

class FilmShow(CorecrmModel):
    film = models.ForeignKey(Film)
    screen = models.ForeignKey(Screen)
    screening_formats = models.ManyToManyField(ScreeningFormat)

I've created a custom admin form for FilmShow which has a clean_screening_formats method:

class FilmShowAdminForm(admin.ModelForm):
    def clean_screening_formats(self):
        # Make sure the selected screening format exists in those marked for the film
        screening_formats = self.cleaned_data['screening_formats']
        film_formats = self.cleaned_data['film'].screening_formats.all()
        sf_errors = []
        for (counter, sf) in enumerate(screening_formats):
            if sf not in film_formats:
                sf_error = forms.ValidationError('%s is not a valid screening format for %s' % (sf.name, self.cleaned_data['film'].title), code='error%s' % counter)
                sf_errors.append(sf_error)
        if any(sf_errors):
            raise forms.ValidationError(sf_errors)

        return formats

The actual validation check is working fine, but the output of these error messages on the admin form is a bit off. Whereas a single error message is output as (for example):

This field is required.

The output for a list of messages looks like this:

[u'35mm Film is not a valid screening format for Thor: The Dark World']
[u'Blu Ray is not a valid screening format for Thor: The Dark World']

Can anyone suggest how I could make these lists of error messages display correctly?

Edit: I think this problem is arising from the fact that django is storing the messages slightly differently when more than one error is being raised. Example:

>>> from django import forms
>>> error = forms.ValidationError('This is the error message', code='lone_error')
>>> error.messages
[u'This is the error message']

>>> e1 = forms.ValidationError('This is the first error message', code='error1')
>>> e2 = forms.ValidationError('This is the second error message', code='error2')
>>> error_list = [e1, e2]
>>> el = forms.ValidationError(error_list)
>>> el.messages
[u"[u'This is the first error message']", u"[u'This is the second error message']"]

Could this be a bug in Django?

Answer 1

The implementation that you have made is only valid from Django 1.6+. Compare: 1.6 docs to 1.5.

Before 1.6, messages were immediately converted to strings in django.core.exceptions.ValidationError (see code here):

class ValidationError(Exception):
    """An error while validating data."""
    def __init__(self, message, code=None, params=None):
        import operator
        from django.utils.encoding import force_text
        """
        ValidationError can be passed any object that can be printed (usually
        a string), a list of objects or a dictionary.
        """
        if isinstance(message, dict):
            self.message_dict = message
            # Reduce each list of messages into a single list.
            message = reduce(operator.add, message.values())

        if isinstance(message, list):
            self.messages = [force_text(msg) for msg in message]  #! Will output "u'<message>'"

In your case, instead of passing a list of ValidationError instances, just pass a list of strings:

>>> e1 = 'This is the first error message'
>>> e2 = 'This is the second error message'
>>> error_list = [e1, e2]
>>> el = forms.ValidationError(error_list)
>>> el.messages
[u'This is the first error message', u'This is the second error message']

Answer 2

It worked for me so keep trying the following:

def clean(self):
if self.acci_anios_residencia == None:
    lista = ["Especifique un año"]
    raise ValidationError({'acci_anios_residencia': lista})