How do I create a rate calculator in django?

The most complete example is now from @TokenMacGuy above. and I understand what you mean in the business logic in the form. Though I think you weren't considering progressive rates that would have made it even longer lasting.

I am answering my own question after you completed this issue, and in the hope that someone else can inject and educate django users about business logic cases.

so this is what i did:

first created a betting model to be stored in the database. I have marked each speed with a speed level to be able to handle progressive grading.

class Rate(models.Model):
    area_of_supply = models.ForeignKey(SupplyArea)
    description = models.TextField()
    rate_level = models.CharField(max_length=2,choices=RATE_LEVEL_CHOICES,)
    rate_per_1k_gallons = models.DecimalField(max_digits=10,decimal_places=2)

      

        
        
        
      

    

Then I created in my views.py a view that will handle the request from the form (in my case both in ajax and normal form). Like "business logic", in my case "traffic_calc ()"

def tariff(request):
    if request.method == 'POST':
        form = TariffForm(request.POST)
        if form.is_valid():
            gallons = form.cleaned_data['gallons']
            gallons = int(gallons)
            area_id = form.cleaned_data['tariff']
            area = area_id.name
            usage = tarrif_calc(request, area_id, gallons)    
            response_dict = {'gallons': gallons,'area': area,'usage': usage}
            if request.is_ajax():
                return HttpResponse(simplejson.dumps(response_dict), mimetype='application/json')
            return render_to_response('rates/tariff_form.html', response_dict)
    else:
        form = TariffForm()
    return render_to_response('rates/tariff_form.html', {'form': form})

def tarrif_calc(request, area, gallons):
    r_levels = gallons//1000
    remainder = gallons%1000
    usage = 0
    i = 1

    if r_levels < 1:
        rate = Rate.objects.get(area_of_supply=area,rate_level=1)
        usage = rate.rate_per_1k_gallons    
    else:
        if r_levels >= 8:
            while i <= 9:
                rate = Rate.objects.get(area_of_supply=area,rate_level=i)
                usage += rate.rate_per_1k_gallons
                i += 1

            if gallons > 9000:
                remainder = gallons - 9000
            remainder_rate = Rate.objects.get(area_of_supply=area,rate_level=9)
            usage += remainder * remainder_rate.rate_per_1k_gallons/1000
            usage = round(usage,2)
        else:    
            while i <= r_levels:
                rate = Rate.objects.get(area_of_supply=area,rate_level=i)
                usage += rate.rate_per_1k_gallons
                i += 1
            remainder_level = r_levels+1
            if remainder_level >= 9:
                remainder_rate = Rate.objects.get(area_of_supply=area,rate_level=9)
            else:
                remainder_rate = Rate.objects.get(area_of_supply=area, rate_level=remainder_level)    
                usage += remainder * remainder_rate.rate_per_1k_gallons/1000
                usage = round(usage,2)
    return usage

      

        
        
        
      

    

I agree with @TokenMacGuy .. I should have probably used GET, but used POST, because if js is not available the regex works just as well. I also agree that at this level @TokenMacGuy can probably only import tariff_scal from views and use it in their .py forms.

Then I created the required forms and templates and javascript that handles the ajax posting (although not required)

// JavaScript Document
$(document).ready(function()    {

$('#tariff_form').submit(function(event){
    event.preventDefault();  // stop the form from submitting and refreshing the page
    var form = this;               // in jQuery, this refers to the current element, which is #article_form

    // grab each form value
    var data = {};
    gallons = $('#id_gallons').val();
    tariff = $('#id_tariff').val();

    // now send the data to the server via AJAX
    if(gallons && tariff)
    {
        $.ajax({
            type: "POST",
            url: "/tariff-calculator/",
            data: "gallons="+gallons+"&tariff="+tariff,
            dataType: "json",
            success: function(json){
                $('#gallons_cont').html(json['gallons']);
                $('#area_cont').html(json['area']);
                $('#usage_cont').html(json['usage'])
                $('#results_json').show('slow');            
            },
            });
    }
    else
    {
        alert('Please Fill All Fields')
    }
}); 

});

      

        
        
        
      

    

I guess both methods are similar and both work (although I haven't tried @ TokenMacGuy's), except for the tariff_calc () function, it seems that MackGuy solved the problem, so I'll give it the correct answer, although I would like more comments on business logic in forms.py as I am not completely convinced or think there is a huge difference.