title: .orderByChild() is not working on Firebase (vue.js)
body: <p>I'm trying to sort my data by using .orderByChild, but the data are filter by the name of the node.</p>
  
  <p>I added this on Firebase : </p>
  
  <pre><code>   "concerts": {
    ".indexOn": "expires"
  }
  </code></pre>
  
  <p>And here is my code in my index.js file : </p>
  
  <pre><code>loadConcerts ({commit}) {
    commit('setLoading', true)
    firebase.database().ref("concerts").orderByChild("expires").startAt(Date.now()/1e3).limitToFirst(7).once("value")
      .then((data) =&gt; {
        const concerts = []
        const obj = data.val()
        for (let key in obj) {
          concerts.push({
            expires: obj[key].expires,
            ...
          })
        }
        commit('setLoadedConcerts', concerts)
        commit('setLoading', false)
      })
  </code></pre>
  
  <p>I also add this : </p>
  
  <pre><code>getters: {
  loadedConcerts (state) {
    return state.loadedConcerts.sort((concertA, concertB) =&gt; {
      return concertA.expires &gt; concertB.expires
    })
  },
  loadedConcert (state) {
    return (concertId) =&gt; {
      return state.loadedConcerts.find((concert) =&gt; {
        return concert.id === concertId
      }) 
     }
  </code></pre>
  
  <p>Maybe someone has a clue ? Thank you</p>