(PECL solr >= 0.9.2)
SolrInputDocument::merge — Merges one input document into another
Merges one input document into another.
sourceDocThe source document.
overwrite
If this is TRUE it will replace matching fields in the destination document.
成功时返回 TRUE， 或者在失败时返回 FALSE。 In the future, this will be modified to return the number of fields in the new document.