title: Concatenate values from different columns with Data Import Handler
body: <p>I have a Django application which connects to a Solr instance for searching. How can I put the firstname and the surname in the same tune_author Solr field (see below)?</p>
  <pre><code>    &lt;entity name=&quot;tune&quot; query=&quot;select id,type,name,start_page,alternate_spellings,item_id from bassculture_tune&quot;&gt;
        &lt;field column=&quot;id&quot; name=&quot;id&quot;/&gt;
        &lt;field column=&quot;type&quot; name=&quot;type&quot;/&gt;
        &lt;field column=&quot;name&quot; name=&quot;name&quot;/&gt;
        &lt;entity name=&quot;tune_author&quot; query=&quot;select id,firstname,surname from bassculture_author where id=${tune.item_id.source_id.author_id}&quot;&gt;
  
          &lt;/entity&gt;
  </code></pre>
  <p>Right now I get the firstname and surname as separate fields in my results, and tune_author is empty. How can I get a tune_author Solr field filled with firstname + surname?</p>
  <p>EDIT</p>
  <p>OK, I think there's also a more basic issue with my understanding of SQL querying.</p>
  <p>My situation is:</p>
  <pre><code>author_table:
  id, firstname, surname ...
      
  source_table:
  id, source_title, author_id ... 
  
  copy_table:
  id, copy_title, source_id ...
  
  tune_table:
  id, tune_title, copy_id ...
  </code></pre>
  <p>So, I want the author's full name of a tune. A tune belongs to a copy. A copy belongs to a source. A source, finally, has the author's ID (and the author_table has the author's details from which I want to reconstruct the full name). I hope this makes things clearer.</p>