title: Firestore / Flutter - How can I get document Id?
body: <p>this is my code</p>
  
  <pre><code>Future _save() async {
  
  final StorageReference storageRef = FirebaseStorage.instance.ref().child('product/'+nameController.text+'.jpg');
  final StorageUploadTask task = storageRef.putFile(_image);
  StorageTaskSnapshot taskSnapshot = await task.onComplete;
  String downloadUrl = await taskSnapshot.ref.getDownloadURL();
  StorageMetadata created = await taskSnapshot.ref.getMetadata();
  
  Firestore.instance.collection('product').document()
      .setData({
    'name': nameController.text,
    'price': int.tryParse(priceController.text),
    'description': descriptionController.text,
    'creator': widget.user.uid,
    'created': DateTime.fromMillisecondsSinceEpoch(created.creationTimeMillis, isUtc: true).toString(),
    'modified': DateTime.fromMillisecondsSinceEpoch(created.updatedTimeMillis, isUtc: true).toString(),
    'url': downloadUrl,
    'id': //I want to set document Id here //
  });
  }
  </code></pre>
  
  <p>how can I get this random generated document's ID?
  Thank you for your help</p>