Showing custom model validation exceptions in the Django admin site

Question

I have a booking model that needs to check if the item being booked out is available. I would like to have the logic behind figuring out if the item is available centralised so

Answer 1

The best way is put the validation one field is use the ModelForm... [ forms.py]

class FormProduct(forms.ModelForm):

class Meta:
    model = Product

def clean_photo(self):
    if self.cleaned_data["photo"] is None:
        raise forms.ValidationError(u"You need set some imagem.")

And set the FORM that you create in respective model admin [ admin.py ]

class ProductAdmin(admin.ModelAdmin):
     form = FormProduct