title: How to give the information about request.user to clean method
body: <p>I have a form:</p>
  
  <pre><code>class CreateConferenceForm(forms.ModelForm):
      class Meta:
          model = Conference
          fields = ['name', 'participants']
  
      def clean(self):
          cleaned_data = super(CreateConferenceForm, self).clean()
          if not request.user.id in cleaned_data.get('participants'):
              raise forms.ValidationError('Error')
  </code></pre>
  
  <p>But I don't know how to import a request object from view, because method is_valid hasn't additional arguments. How I can do it?</p>