title: Flutter & Firebase: Remove Item(Map) from Array in firebase
body: <p>I'm currently struggling to remove a map item from a list in firebase.
  E.g. I want to remove item "0" from the list (completely).</p>
  
  <p><a href="https://i.stack.imgur.com/FF9ML.png" rel="noreferrer"><img src="https://i.stack.imgur.com/FF9ML.png" alt="enter image description here"></a></p>
  
  <p>I add the single array items in another place via FieldValue.arrayUnion with the updateData Method. This works perfectly fine. In another place in my I then want to have the possibility to remove the item from a list. I load the data via StreamBuilder and display ListTiles via ListView.builder. Now there I tried to replace arrayUnion by arrayRemove, but it doesn't work and I can't figure out why.
  Here's my code:</p>
  
  <pre><code>onPressed: () async {
    try {
      await Firestore.instance
          .collection('users')
          .document(futSnap.data.uid)
          .collection('trainings')
          .document(widget.trainingID)
          .updateData(
        {
          'trainingTitle': widget.trainingTitle,
          'trainingID': widget.trainingID,
          'trainingDate': widget.trainingDate,
          'category': widget.trainingCategory,
          'trainingExercises':
              FieldValue.arrayRemove(
            [
              {
                'title': trainingExercises[i]
                    ['title'],
                'description':
                    trainingExercises[i]
                        ['description'],
                'imageURL': trainingExercises[i]
                    ['imageUrl'],
                'repetitions':
                    trainingExercises[i]
                        ['repetitions'],
              },
            ],
          )
        },
      );
    } catch (e) {
      print(e);
    }
  },
  </code></pre>
  
  <p>I don't receive any error or something.. Still a beginner to flutter and programming, so help would be highly appreciated :)</p>