code: func (q *ConstantScoreQuery) Source() (interface{}, error) {
  	// "constant_score" : {
  	//     "filter" : {
  	//         ....
  	//     },
  	//     "boost" : 1.5
  	// }
  
  	query := make(map[string]interface{})
  
  	params := make(map[string]interface{})
  	query["constant_score"] = params
  
  	// filter
  	src, err := q.filter.Source()
  	if err != nil {
  		return nil, err
  	}
  	params["filter"] = src
  
  	// boost
  	if q.boost != nil {
  		params["boost"] = *q.boost
  	}
  
  	return query, nil
  }
docs: // Source returns the query source.