Django invalid form when using modelchoicefield -

i new django , have been struggling selected value of forms.modelchoicefield in views. form (post) not valid , noticed request.post returns possible values in modelchoicefield. how model selected user? thank in advance help.

model

i have sensor model measurements model...

class sensor(models.model):     # contains information sensor, type, location, , other     type = models.charfield(max_length=16)     ...  class measurement(models.model):     #this contains measurements of sensors @ given date     sensor = models.foreignkey('sensor')     date = models.datefield()     temperature = models.floatfield()     pressure = models.floatfield()     ...     class meta:         unique_together = ('sensor', 'date') 

form

... , form allows user choose sensor , date:

class measurementform(forms.modelform):     #the user allowed choose sensor related measurement table     sensor = forms.modelchoicefield(queryset=sensor.objects.exclude(measurement=none))     date =  forms.datefield(widget=dateinput(attrs={'format':'%y-%m-%d'}), initial=datetime.today) 

so far good, gets rendered method , user able select both sensor , date. however, following view fails @ post, after submitting:

views

class measurementview(view):     form_class = measurementform     my_template = "measurement.html"      def get(self, request, *args, **kwargs):         form = self.form_class()         return render(request, self.my_template, {'form': form, 'data':none})      def post(self, request, *args, **kwargs):         print(request.post)         print(new_form.errors)         new_form = self.form_class(data=request.post)         if new_form.is_valid():             print('form valid')             data = get_data_for_sensor_date(                         new_form.cleaned_data['sensor'],                         new_form.cleaned_data['date']))             return render(request, self.my_template, {'form':new_form, 'data':data})         else:             return render(request, self.my_template, {'form':new_form, 'data':none}) 

console output

<querydict: {'sensor': ['', '1'],               'csrfmiddlewaretoken': ['yupfrdhdbqdaa7eynx2sjehdbxmqxttw'],               'date': ['2016-11-07']}>  <ul class="errorlist">   <li>__all__     <ul class="errorlist nonfield">       <li>measurement sensor , date exists.       </li>     </ul>   </li> </ul> 

i found link django formview returns object exists error modelform

to quote: "ok. found solution error. caused model exists error modelform checkproductform. codename attribute unique, validation returned false. did change orginal modelform form. solved whole issue. , form_invalid in form_valid issue. i've overwritten form's clean_codename function raise validationerror if product doesnt exist."

i still not understand why had error in first place. however, changing form code below made trick.

form

class measurementform(forms.form):     #the user allowed choose sensor related measurement table     sensor = forms.modelchoicefield(queryset=sensor.objects.exclude(measurement=none))     date =  forms.datefield(widget=dateinput(attrs={'format':'%y-%m-%d'}), initial=datetime.today)