code: func (q *ExistsQuery) Source() (interface{}, error) {
  	// {
  	//   "exists" : {
  	//     "field" : "user"
  	//   }
  	// }
  
  	query := make(map[string]interface{})
  	params := make(map[string]interface{})
  	query["exists"] = params
  
  	params["field"] = q.name
  	if q.queryName != "" {
  		params["_name"] = q.queryName
  	}
  
  	return query, nil
  }
docs: // Source returns the JSON serializable content for this query.