The Web Dev

To filter ForeignKey choices in a Python Django ModelForm, we set the quertset property to the queryset with the filtered data.

For instance, we write

class ClientAdminForm(forms.ModelForm):
    def __init__(self, *args, **kwargs):
        super(ClientAdminForm, self).__init__(*args, **kwargs)
        # access object through self.instance...
        self.fields['base_rate'].queryset = Rate.objects.filter(company=self.instance.company)

class ClientAdmin(admin.ModelAdmin):
    form = ClientAdminForm

to add the ClientAdminForm that sets the base_rate field’s queryset to the filtered Rate query set with

Rate.objects.filter(company=self.instance.company)

Then we add the ClientAdminForm form as the form to use in the ClientAdmin section of Django admin.

To capture URL parameters in request.GET with Python Django, we can add a view to match a URL pattern that takes a URL parameter.

Then we can get the parameter value from the view function’s parameter.

For instance, we write

(r'^user/(?P<username>\w{0,50})/$', views.profile_page,),

to add a view that matches the r'^user/(?P<username>\w{0,50})/$' route.

Then in our view file, we add a view with

def profile_page(request, username):
    # ...

to get the username URL parameter value from username.

To make Python Django serve downloadable files, we can return a response with some special values.

For instance, we write

from django.utils.encoding import smart_str

response = HttpResponse(content_type='application/force-download')
response['Content-Disposition'] = 'attachment; filename=%s' % smart_str(file_name)
response['X-Sendfile'] = smart_str(path_to_file)
return response

to create a HttpResponse instance and assign it to response.

Then we set the Content-Disposition response header with

response['Content-Disposition'] = 'attachment; filename=%s' % smart_str(file_name)

And then we set the X-Sendfile header to the path of the file with

response['X-Sendfile'] = smart_str(path_to_file)

And then we return the response in our view.

To create a JSON response using Django and Python, we can use the JsonResponse class.

For instance, we write

from django.http import JsonResponse
return JsonResponse({'foo':'bar'})

to create a JsonResponse instance with a dictionary that we want to return as the response JSON body.

To check if a field has changed when saving with Python Django, we can override the `init method of the model class to keep a copt of the original value.

For instance, we write

class Person(models.Model):

    name = models.CharField()

    __original_name = None

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.__original_name = self.name

    def save(
        self,
        force_insert=False,
        force_update=False,
        *args,
        **kwargs
        ):
        if self.name != self.__original_name:
            super().save(force_insert, force_update, *args, **kwargs)
            self.__original_name = self.name

to add the __init__ method that sets the __original_name variable to self.name to keep the original name value.

And then in the if block in save, we check self.name to see if it’s different from self.__original_name to see if it’s saved with a new value.