title: Extending the admin import form for django import_export
body: <p>I'm using <a href="https://github.com/django-import-export/django-import-export" rel="noreferrer">Django import_export</a> to implement CSV upload in my admin pages. Now I have one model, that contains a foreign key column, but the foreign key column will have only one value for each import. Therefore I would like to allow the user to choose the related model instance from a drop-down instead of forcing the user to append the columns themselves. In order to do this I need to customize the import form, which requires overriding the default methods <code>import_action</code> and <code>process_import</code>, but my efforts so far have shown no effect. Here is what I have so far: </p>
  
  <pre><code>from django import forms
  from import_export.forms import ImportForm  
  from .models import MyModel, RelatedModel
  
  class CustomImportForm(ImportForm):
      """Add a model choice field for a given model to the standard form."""
      appended_instance = forms.ModelChoiceField(queryset=None)
  
      def __init__(self, choice_model, import_formats, *args, **kwargs):
          super(CustomImportForm, self).__init__(import_formats, *args, **kwargs)
          self.fields['appended_instance'].queryset = choice_model.objects.all()
  
  @admin.register(MyModel)
  class MyModelAdmin(ImportExportModelAdmin):
      resource_class = SomeResource
  
  
      def import_action(self, request, *args, **kwargs):
          super().import_action(self, request, *args, **kwargs)
          form = CustomImportForm(RelatedModel, 
                                  import_formats,
                                  request.POST or None,
                                  request.FILES or None)
  </code></pre>
  
  <p>Now when I go the import page I get the <code>AttributeError MyModelAdmin has no attribute 'POST'</code> and in the local vars I can see that the <code>request object</code> is actually the <code>MyModelAdmin</code> class, which is I believe is not what it's supposed to be.</p>