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class ProductForm(ModelForm):
    class Meta:
        model = Product
        exclude = ('owned_by_group',)
        widgets = {
            'category': SelectMultiple(attrs={'class':'selectfilter', }),
        }
    
    def clean_sku(self):        
        if Product.objects.filter(sku__iexact=self.cleaned_data['sku']):
            raise forms.ValidationError("This sku is already in use. Please supply a different sku.")
        return self.cleaned_data['sku']