title: Thinking Sphinx(v4.4) - Error while indexing after including 'where' condition in index definition
body: <p>Thinking sphinx gem version - 4.4.1
  Sphinx version - 3.3.1</p>
  <p>I get</p>
  <blockquote>
  <p>ERROR: index 'article_core': no full text fields in schema, nothing to index!</p>
  </blockquote>
  <p>while indexing after including where condition in the index definition.</p>
  <p>Index definition below</p>
  <pre><code>ThinkingSphinx::Index.define :article, :with =&gt; :active_record do
    indexes title
    # where &quot;text = 'Past Simple'&quot; # type of text column is text: rebuild successful
    # where &quot;id &gt; 1&quot; # type of id column is int: rebuild successful
    where &quot;photo = 'photo'&quot; # type of photo column is String : rebuild fails
  end
  </code></pre>
  <ol>
  <li>The issue occurs when we use where condition inside the index<br />
  definition on a string column(char varying)</li>
  <li>This issue does not occur when we apply where condition with other data types.</li>
  <li>the error statement is 'ERROR: index 'article_core': no full text fields in schema, nothing
  to index!.'</li>
  </ol>