title: Vue add existing Firebase ID for "relation"
body: <p>I am using Vue and Firebase via VueFire to build a form to relate objects. I have the following form and method:</p>
  
  <pre><code>&lt;form @submit.prevent="addRelation"&gt;
        &lt;select v-model="newRelation.parent"&gt;
          &lt;option v-for="item in content"&gt;{{item.name}}&lt;/option&gt;
        &lt;/select&gt;
        &lt;select v-model="newRelation.child"&gt;
          &lt;option v-for="item in content"&gt;{{item.name}}&lt;/option&gt;
        &lt;/select&gt;
        &lt;input type="submit" value="Add Relation"/&gt;
      &lt;/form&gt;
  </code></pre>
  
  <p>And Vue:</p>
  
  <pre><code>new Vue({
      el: "#app",
      data: {
          related: {},
          newRelation: {
              parent: '',
              child: ''
          }
      },
      firebase: {
          content: contentRef,
          related: relatedRef
      },
      methods: {
          addRelation() {
              relatedRef.push(this.newRelation)
              console.log('relation added')
          }
      },
  </code></pre>
  
  <p>This adds JSON like so in Firebase:</p>
  
  <pre><code>{
      "child": "objectc",
      "parent": "objecta",
      ".key": "-KjFYZn8dqP29uLmJBJP"
    },
  </code></pre>
  
  <p>It pushes a new Firebase ID and nests the data. In order for Firebase to handle 'relationships' correctly they must be structured like so:</p>
  
  <pre><code>{
      ".key":"-Existing ID of object A
      "-Existing ID of object B": true
  }
  </code></pre>
  
  <p>How can I update my method to structure info like so? I have this <a href="http://codepen.io/Auzy/pen/VbzrzE?editors=0011" rel="nofollow noreferrer">CodePen</a> The bottom two entries in the "'Related' Content" section show how I want it to submit (I added manually).</p>