title: How to get all documentsID from this class
body: <p>i have this method inside this class, and i want to call this method in a screen and get all IDs inside a map().</p>
  
  <pre><code>
  final databaseReference = Firestore.instance;
  
  class CompanyServices {
    static CollectionReference get companies =&gt;
        databaseReference.collection('companies');
  
    static getDocumentID() async {
      final QuerySnapshot result = await companies.getDocuments();
      final List&lt;DocumentSnapshot&gt; documents = result.documents;
  
      documents.forEach((data) {
        print(data.documentID);
      });
    }
  }
  </code></pre>
  
  <p>and screen i'm calling this way:</p>
  
  <p><code>CompanyServices.getDocumentID()</code></p>
  
  <p>and i've this error:
  <code>type 'Future&lt;dynamic&gt;' is not a subtype of type 'String'</code></p>
  
  <p>How  do i call that method get all documentsID inside a map?</p>
  
  <p>Thx in advance.</p>