code: def _add_entry(self, formdata=None, data=unset_value, index=None):
          '''
          Fill the form with previous data if necessary to handle partial update
          '''
          if formdata:
              prefix = '-'.join((self.name, str(index)))
              basekey = '-'.join((prefix, '{0}'))
              idkey = basekey.format('id')
              if prefix in formdata:
                  formdata[idkey] = formdata.pop(prefix)
              if hasattr(self.nested_model, 'id') and idkey in formdata:
                  id = self.nested_model.id.to_python(formdata[idkey])
                  data = get_by(self.initial_data, 'id', id)
  
                  initial = flatten_json(self.nested_form,
                                         data.to_mongo(),
                                         prefix)
  
                  for key, value in initial.items():
                      if key not in formdata:
                          formdata[key] = value
              else:
                  data = None
          return super(NestedModelList, self)._add_entry(formdata, data, indexf _add_entry(self, formdata=None, data=unset_value, index=None):
          '''
          Fill the form with previous data if necessary to handle partial update
          '''
          if formdata:
              prefix = '-'.join((self.name, str(index)))
              basekey = '-'.join((prefix, '{0}'))
              idkey = basekey.format('id')
              if prefix in formdata:
                  formdata[idkey] = formdata.pop(prefix)
              if hasattr(self.nested_model, 'id') and idkey in formdata:
                  id = self.nested_model.id.to_python(formdata[idkey])
                  data = get_by(self.initial_data, 'id', id)
  
                  initial = flatten_json(self.nested_form,
                                         data.to_mongo(),
                                         prefix)
  
                  for key, value in initial.items():
                      if key not in formdata:
                          formdata[key] = value
              else:
                  data = None
          return super(NestedModelList, self)._add_entry(formdata, data, index)
docs: Fill the form with previous data if necessary to handle partial update