title: Transform XGBosst model for Apache Solr Learning to Rank plugin
body: <p>I would like to use a pairwise ranking model trained with <a href="https://github.com/dmlc/xgboost" rel="nofollow noreferrer">XGBoost</a>* in Apache Solr. I guess the XGBoost model should be handled by the <a href="https://lucene.apache.org/solr/7_7_0//solr-ltr/org/apache/solr/ltr/model/MultipleAdditiveTreesModel.html" rel="nofollow noreferrer"><em>MultipleAdditiveTreesModel</em></a> class from the Solr LTR plugin.</p>
  
  <p>However, when mapping the XGBoost output to the JSON expected by the Solr LTR plugin, it is not clear how to handle the <em>missing</em> condition. Since, XGBoost has a non trivial logic for routing missing values they can not just always be send to the <em>left</em> or <em>right</em> branch of a tree.</p>
  
  <p>How should this issue be handled? Is there an option in XGBoost to enforce that missing values always get routed to the same branch or better, is there an option in the Solr LTR Plugin to handle the case of missing values?</p>
  
  <p>*objective: 'rank:pairwise'</p>