code: func (r *SearchRequest) Preference(preference string) *SearchRequest {
  	r.preference = &preference
  	return r
  }
docs: // Preference to execute the search. Defaults to randomize across shards.
  // Can be set to "_local" to prefer local shards, "_primary" to execute
  // only on primary shards, or a custom value, which guarantees that the
  // same order will be used across different requests.