          # Return HTTP 400 response (BAD REQUEST)
          raise ErrorResponse(400, detail)
    
  
      def get_bound_form(self, data=None, files=None, method=None):
          """
          Given some content return a Django form bound to that content.
          If form validation is turned off (:attr:`form` class attribute is :const:`None`) then returns :const:`None`.
          """
  
 
</s> remove         Given some content return a Django form bound to that content.
        If form validation is turned off (:attr:`form` class attribute is :const:`None`) then returns :const:`None`.
 </s> add         Returns the form class used to validate this resource. </s> remove         form = super(ModelResource, self).get_bound_form(data, files, method=method)
        
        # Use an explict Form if it exists
        if form:
            return form

        elif self.model:
 </s> add         if not form and self.model: </s> remove         return instance
 </s> add         return self.model_instance
    def get_form_class(self, method=None):
          # Return HTTP 400 response (BAD REQUEST)
          raise ErrorResponse(400, detail)
    
  
    def get_form_class(self, method=None):
          """
          Given some content return a Django form bound to that content.
          If form validation is turned off (:attr:`form` class attribute is :const:`None`) then returns :const:`None`.
          """
  
keep keep keep keep replace keep keep keep keep keep
 FIX: ModelViews can have Resources whose models have unique fields.

ReadModelMixin and UpdateModelMixin store model instance as a property. This allows ModelResource to bind the ModelForm using the model instance making the form validate the input data against the model instance and not a brand new instance. When the latter happened and the model used unique fields, the form validation failed whenever a PUT was maintaining the previuos value of the unique field. 
djangorestframework/resources.py