title: Property 'map' does not exist on type 'Observable<DocumentChangeAction<unknown>[]>'. Question
body: <p>I've come across some error and I am having trouble figuring out what might be the problem. It seems to be related to Typescript and types, but I am not able to fix it by installing the relevant package.</p>
  <pre><code>import { Injectable } from '@angular/core';
  import { AngularFirestore } from 'angularfire2/firestore';
  import { Subject } from 'rxjs/Subject';
  
  import { Exercise } from './exercise.model';
  
  u/Injectable()
  export class TrainingService {
    exerciseChanged = new Subject&lt;Exercise&gt;();
    exercisesChanged = new Subject&lt;Exercise[]&gt;();
    finishedExercisesChanged = new Subject&lt;Exercise[]&gt;();
    private availableExercises: Exercise[] = [];
    private runningExercise: Exercise;
  
    constructor(private db: AngularFirestore) {}
  
    fetchAvailableExercises() {
      this.db
        .collection('availableExercises')
        .snapshotChanges()
        .map(docArray =&gt; {
          return docArray.map(doc =&gt; {
            return {
              id: doc.payload.doc.id,
              name: doc.payload.doc.data().name,
              duration: doc.payload.doc.data().duration,
              calories: doc.payload.doc.data().calories
            };
          });
        })
        .subscribe((exercises: Exercise[]) =&gt; {
          this.availableExercises = exercises;
          this.exercisesChanged.next([...this.availableExercises]);
        });
    }
  }
  </code></pre>
  <p>I tried adding:</p>
  <pre><code>import 'rxjs/add/operator/map'
  </code></pre>
  <p>I also tried installing:</p>
  <pre><code>npm install u/types/rx --save
  </code></pre>
  <p>I am thinking I might have to add something in the module section or something, does someone have a clue?</p>