Django ArrayField rendered using SelectMultiple not showing the selected optons as 'selected'

Question

I am trying to render postgres Arrayfield values using SelectMultiple widget like this

In models.py:

SLOT_CHOICES = (('A','Slot A'),('B','Slot B'),('C','Slot C'),('D','Slot D'),('E','Slot E'),('F','Slot F'),('G','Slot G'),('H','Slot H'),('P','Slot P'),('Q','Slot Q'),('R','Slot R'),('S','Slot S'),('T','Slot T'),)
core_slots = ArrayField(models.CharField(max_length=1,choices=SLOT_CHOICES),blank=True,null=True)

In forms.py:

    self.fields['core_slots'].widget = forms.SelectMultiple(attrs={
            'placeholder': 'Choose slots',
            'class': 'multi-select-input',
        },choices=self.Meta.model.SLOT_CHOICES)

If I select a single option and submit and then try to get the filled form using 'instance' then it works fine and shows the previously selected option as 'selected'. But if I select more than one option, then on submitting all selected values get correctly inserted into database, but if I try to get the filled form using 'instance' then it doesn't show any option as selected.

This problem doesn't happen for ManyToManyField.

In models.py:

past_courses = models.ManyToManyField(Course,blank=True)

In forms.py:

    self.fields['past_courses'].queryset = Course.objects.filter(~Q(dept=self.instance.dept)).order_by('name')
    self.fields['past_courses'].widget.attrs.update({
            'placeholder': 'Choose past courses',
            'class': 'multi-select-input',
        })

This one works fine. Only ArrayField rendered using SelectMultiple widget has problem. I want to display submitted options in template, is there any way to fix this or is there any alternate way to display selected options in template.

Answer 1

What I found was that this seems to be an issue with the fact that the SelectMultiple widget uses ChoiceWidget which has a format_value method that handles multiple selections incorrectly. At least this is in Django 1.11. The format_value does the following:

def format_value(self, value):
    """Return selected values as a list."""
    if value is None and self.allow_multiple_selected:
        return []
    if not isinstance(value, (tuple, list)):
        value = [value]
    return [force_text(v) if v is not None else '' for v in value]

Which as you can see says that it is supposed to return the selected values as a list. But the code that runs if it is not a tuple or list is wrong. It just essentially makes the items in the value be a single entry in the list. So when you have multiple items selected like 'A', 'B', and 'C' - they come through to this format_value function as 'A, B, C' - and this really just makes it be ['A, B, C'] - which is wrong. It needs to be ['A', 'B', 'C']. So I just put in code to override this function with the following:

def format_value(self, value):
    """Return selected values as a list."""
    if value is None and self.allow_multiple_selected:
        return []
    if not isinstance(value, (tuple, list)):
        # This means it should be a comma delimited list of items so parse it
        value = value.split(',')

    return [force_text(v) if v is not None else '' for v in value]

This seems to work fine for me now. I tested on Django 1.11 for picking a single item, multiple items, and NO items and everything saved to the DB properly and showed up on the Django Admin fine as well. Hope this helps.

Answer 2

Had the same problem, and this is basically the same answer as above, but with a drop in replacement widget.

from django.forms import CheckboxSelectMultiple


class ArrayFieldCheckboxSelectMultiple(CheckboxSelectMultiple):

    def format_value(self, value):
        """Return selected values as a list."""
        if value is None and self.allow_multiple_selected:
            return []
        elif self.allow_multiple_selected:
            value = [v for v in value.split(",")]

        if not isinstance(value, (tuple, list)):
            value = [value]
            
        results = [str(v) if v is not None else '' for v in value]
        return results