Question

Code:

Model:

class Machines(models.Model):
    name = models.CharField(max_length=100, verbose_name="hostname", help_text="Host name of the machine")
    ip_addr = models.CharField(max_length=50, primary_key=True, help_text="IP address of the machine")
.... have many other fields.

Form:
Using forms.ModelForm to create a form

View:

def save(request):
if request.method == "POST":
        ip = request.POST.get("ip")
        machine = get_object_or_404(Machines, ip_addr=ip)
        form = MachineForm(instance=machine, data=request.POST)
        if form.is_valid():
            if form.has_changed():
                form.save()
                context = {"message": "Updated successfully"}
            else:   
                context = {"message": "No data to update"}
    return render_to_response("edit.html", context, context_instance=RequestContext(request))

If I change “name” field, form.save() updates the current object properly. But, if I change “ip_addr” field which is primary key, form.save() creates two entries one with old primary key other with new primary key.

If we do same in MySQL(BTW, I am using MySQL as DB)

update machines_table set ip_addr="10.1.1.1" where ip_addr="10.1.1.2";

It works fine, there will not be any duplicate entry.

Can you please help me out.

Answer 1

What you’re doing is updating private key. Django creates new instance in this case. It is is actually Django’s way to copy a record. The INSERT not UPDATE query is performed by ORM.

If you change your code to:

class Machines(models.Model):
    …
    ip_addr = models.CharField(max_length=50, unique=True, …
    …

Django will interprete your edit action correctly as the primary key won’t change.

Next time use django-debug-toolbar to see what query is performed by Django ORM.