code: func (b *SearchRequestBuilder) AddDocValueField(field string) *SearchRequestBuilder {
  	// fields field not supported on version >= 5
  	if b.version < 5 {
  		b.customProps["fields"] = []string{"*", "_source"}
  	}
  
  	b.customProps["script_fields"] = make(map[string]interface{})
  
  	if b.version < 5 {
  		b.customProps["fielddata_fields"] = []string{field}
  	} else {
  		b.customProps["docvalue_fields"] = []string{field}
  	}
  
  	return b
  }
docs: // AddDocValueField adds a doc value field to the search request