title: What does a dollar sign mean in mongodb in terms of groups?
body: <p>According to the docs, a "$" is reserved for operators. If you look at the group operator however, values need to have a dollar prefixed. These values are not operators. What does it mean in this context then? Example below:</p>
  
  <pre><code>db.article.aggregate(
      { $group : {
          _id : "$author",
          docsPerAuthor : { $sum : 1 },
          viewsPerAuthor : { $sum : "$pageViews" }
      }}
  );
  </code></pre>
  
  <p>Why does pageViews need a leading dollar sign? I've tried it locally and it doesn't work without the dollar sign.</p>