title: firebase indexOn for child values
body: <p>I am trying to get all post with mood equal to M1 using firebase Query like below:</p>
  
  <pre><code>db.ref(`Recommendations`).orderByChild(`post/vote/moodTags/${idMood}/key`).equalTo(`${idMood}`).once('value').then(recs =&gt;{
          //Número de recs con el mood
          let recsCount = recs.numChildren();
          console.log(`Número de recs encontradas --&gt; ${recsCount}`);
          ...
      });
  </code></pre>
  
  <p>and firebase console show warning:</p>
  
  <blockquote>
    <p>FIREBASE WARNING: Using an unspecified index. Your data will be downloaded and filtered on the client. Consider adding ".indexOn": "post/vote/moodTags/M1/key" at /Recommendations to your security rules for better performance.</p>
  </blockquote>
  
  <p>but, my indexing rule at firebase site is: </p>
  
  <pre><code>"Recommendations":{
    ".indexOn": ["post/userFirebaseKey","post/user/firebaseKey","userFrom/firebaseKey","post/location/placeId","firebaseKey","placeId"],
    "post":{
      "vote":{
        "moodTags":{
          "$moodId":{
            ".indexOn": "key"
          }
        }
      }
    }
  }
  </code></pre>
  
  <p>so, how can I actually specify the indexOn rule when i use a child level?</p>
  
  <p>my db:</p>
  
  <pre><code>+ Recommendations
    + idRec
      property1:''
      property2:''
      property3:''
      + post
        property21:''
        ...
        + vote
          + moodTags
            + M1
              key:'M1'
              name:''
              url:''
            + M2        
              ...
  </code></pre>