title: Calling a Firebase function in Angular
body: <p>I have made this function: </p>
  
  <pre><code>  getAllUserLikedProperties(): AngularFirestoreCollection&lt;any&gt; {
      return this.afs.collection('users', ref =&gt;
        ref.where('uid', '==', this._auth.currentUserId)
          .where('likedProp', '==', true)
      );
    }
  </code></pre>
  
  <p>and I have tried to call it liked this:</p>
  
  <pre><code>  ngOnInit() {
        this._property.getAllUserLikedProperties().subscribe(likedProp =&gt; {
          console.log(likedProp);
          this.likedProp = likedProp;
        });
  
      }
  </code></pre>
  
  <p>however it returns this error:</p>
  
  <pre><code>ERROR TypeError: this._property.getAllUserLikedProperties is not a function
  </code></pre>
  
  <p>Anyone know why? And know how I should call this?</p>