title: Solr delta-import across multiple databases
body: <p>I have created a Solr Index whose documents are formed from Two databases db1 and db2.
  I can successfully do a full-import as follows:  </p>
  
  <pre><code>    &lt;entity name="ent1" dataSource="db1" query="select ID_DB1 from table1"&gt;  
         &lt;field column="ID_DB1" name="ID_DB1"/&gt;  
  
         &lt;entity name="ent2" dataSource="db2" query="select ID_DB2 from table2 where ID_DB2='${ent1.ID_DB1}'"&gt;  
         &lt;field column="ID_DB2" name="ID_DB2"/&gt;  
         &lt;/entity&gt;    
  
      &lt;/entity&gt;
  </code></pre>
  
  <p>But delta import throws NullPointerException.<br>
  How to write delta-query, parent-delta-query and delta-import-query for this scenario?<br>
  Any help is appreciated!</p>