title: form is not valid... WHY?
body: <p>the model:</p>
  
  <pre><code>class Operation(models.Model):
    operation_type = models.ForeignKey(OperationType)
    category = models.ForeignKey(Category, default=0)
    related_account = models.ForeignKey(Account, related_name = 'related_account', null = True)
    comments = models.TextField(null = True)
  </code></pre>
  
  <p>the code:</p>
  
  <pre><code>def detail(request, category_id):
    class OperationCategoryOnlyForm(forms.ModelForm):
      class Meta:
        model = Operation
        fields = ('operation_type', 'category', 'related_account', )
  
    from django.forms.models import modelformset_factory
    OperationFormSet = modelformset_factory(Operation, form=OperationCategoryOnlyForm)
    if request.method == "POST":
      formset = OperationFormSet(request.POST, queryset=Operation.objects.filter(category=category_id))
      if formset.is_valid():
        formset.save()
  # HERE IS THE FORMSET, WHICH OVERLAPS THE POSTED FORMSET - this was intentionaly to get the current result not result before save, but when you want to debug - you should pay attention at such things.    
  #  formset = OperationFormSet(queryset=Operation.objects.filter(category=category_id))
  
    c = {"formset" : formset,}
    c.update(csrf(request))
    return render_to_response("reports/operation_list.html", c)
  </code></pre>
  
  <p>the template: <strong>UPDATED:</strong></p>
  
  <pre><code>&lt;form method="post" action=""&gt;
    {% csrf_token %}
    {{ formset.management_form }}
    {{ formset.errors }}
    {{ formset.non_field_errors }}
    {{ formset.non_form_errors }}
  &lt;table&gt;
   {% for form in formset.forms %}
  tr&gt;&lt;td&gt; {{ form.errors }} &lt;/td&gt;&lt;td&gt; {{ form.non_field_errors }}&lt;/td&gt;&lt;/tr&gt;
  &lt;tr&gt;&lt;td&gt;{{ form.id }}&lt;/td&gt;&lt;td&gt;{{ form.instance.comments }}&lt;/td&gt;&lt;td&gt;{{ form.operation_type }}&lt;br&gt;{{ form.related_account }}&lt;/td&gt;&lt;td&gt;{{ form.category }}&lt;/td&gt;&lt;/tr&gt;
    {% endfor %}
  &lt;/table&gt;
  &lt;input type="submit" value="submit"&gt;
  &lt;/form&gt;
  </code></pre>
  
  <p>I found that form.is_valid() = false - but i have no idea how to get the reason WHY...</p>
  
  <p><strong>UPD</strong> - I updated the template according to comment - nothing is result....</p>
  
  <p>(this is very stupid rule, that i have to write less code than question - code is self explaining and is the essence of question - Almost always the question reduces into one sentence but there's no way to reduce code)</p>