title: Save with firebase with a dynamic key name
body: <p>I am trying to insert in my database the month chosen by the user.</p>
  
  <pre><code>saveInBdd (){
    this.errors = [];
  
    const user = firebase.auth().currentUser;
    user.updateProfile({
      displayName: this.firstname,
    }).then(()=&gt;{
      this.saveUserToUsersRef(user)
    }, error =&gt; {
      this.errors.push(error.message)
    })
  },
  saveUserToUsersRef(user){
    return this.usersRef.child(user.uid).update({
      tab1: { this.months[0].selectedOption : "test"}
    })
  },
  </code></pre>
  
  <p>This code returns this error to me:</p>
  
  <p><a href="https://i.stack.imgur.com/SskwI.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/SskwI.png" alt="enter image description here"></a></p>