title: Angular Firebase push key on item
body: <p>I'am new on angular and Firebase and I don't understand something.
  i want to push url picture with unique key in my database.
  Now i have an array of url and I send it to my database like that:</p>
  
  <p><img src="https://i.stack.imgur.com/n62Ty.png" alt="My db now"></p>
  
  <p>and instead of 0,1,2 I want to push a unique ID</p>
  
  <p>my code for push the data:</p>
  
  <pre><code>export class SpotsService {
    spotsRef: AngularFireList&lt;any&gt;;
    spotRef: AngularFireObject&lt;any&gt;;
    spotiD: string;
    picsUrl: string[];
    picurl: string;
  
    constructor(private db: AngularFireDatabase) {
      this.picsUrl = [];
     }
  
    addSpot(spot: Spot){
      this.spotsRef.push({
        title: spot.title,
        synopsis: spot.synopsis,
        userId: spot.userId,
        markerLatLng: spot.markerLatLng,
        markerLatLngTwo: spot.markerLatLngTwo,
        photo: spot.photo,
        categories: spot.categories
      });
    }
  }
  </code></pre>
  
  <p>my interface:</p>
  
  <pre><code>export interface Spot {
      $key: string;
      title: string;
      synopsis: string;
      userId: string;
      markerLatLng: number[];
      markerLatLngTwo: number[];
      photo: string[];
      categories: string[];
  }
  </code></pre>
  
  <p>If someone can help me :) Thx a lot</p>