code: protected String prependJoin(String queryString, @Nullable SolrDataQuery query, @Nullable Class<?> domainType) {
  		if (query == null || query.getJoin() == null) {
  			return queryString;
  		}
  
  		String fromIndex = query.getJoin().getFromIndex() != null ? " fromIndex=" + query.getJoin().getFromIndex() : "";
  		return "{!join from=" + getMappedFieldName(query.getJoin().getFrom(), domainType) + " to="
  				+ getMappedFieldName(query.getJoin().getTo(), domainType) + fromIndex + "}" + queryString;
  	}
docs: Prepend {@code !join from= to=} to given queryString
  
  @param queryString
  @param query
  @param domainType
  @return