cannot save MultipleChoiceField in django

I am having problems saving a MultipleChoiceField on my test server (with Apache). However, it works on my local machine (django server).

In a form, I have checkboxes that I can check and uncheck. When I click on a save button, the data related to the checkboxes are saved in the database and the form is reloaded and the checkboxes updated.

However, this is how it works in local but not on the test server. On the test server, when I click on the save button, it just reloads the form, nothing is saved, nothing is changed.

Here is the code:

class Participant(models.Model):
    databases = models.ManyToManyField(Advertiser, null=True, blank=True, through='ShareDataToBrands')

    @property
    def share_to_brands_list(self):
        brands=[]
        for brand in ShareDataToBrands.objects.all():
            brands.append((brand.advertiser.id, brand.advertiser.brand_name, brand.share_data))

        return brands

class ShareDataToBrandsForm(forms.ModelForm):
    class Meta:
        model = models.Participant
        fields = ('databases', )

    databases=forms.MultipleChoiceField(widget=forms.CheckboxSelectMultiple, required=False)

    def save(self, *args, **kwargs):
        instance=super(ShareDataToBrandsForm, self).save(commit=False)

        #list of brands to share data with
        share_list=map(int, self.cleaned_data.get("databases"))

        participant=self.instance
        for share_data_instance in models.ShareDataToBrands.objects.filter(participant=participant):
            if share_data_instance.advertiser.id in share_list:
                share_data=True
            else:
                share_data=False
            #update record
            share_data_instance.share_data=share_data
            share_data_instance.save()

        return instance

What could possibly be wrong?

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EDIT :

When I check the log, I see that the program never enters in the for loop! However, I have records in the database matching the filter.

I have found the solution, looping through theShareDataToBrands instances related to the participant instead of all the ShareDataToBrands instances.

In the share_to_brands_list property, have changed the following line:

for brand in ShareDataToBrands.objects.all():

with

for brand in ShareDataToBrands.objects.filter(participant=self):