how to delete one to many associations in flutter firebase - firebase

Question

I am developing a flutter application and I am using firebase database as a database service.
Now my since my app is a task management app my database have three main collections:
+roles
+tasks
+users
every role is a document with a unique id and name
every task is a document with a unique id a name and a role id
now in the app the user is able to add roles in the database I just add a new role document to the roles collection
but I am stuck when it comes to deleting a role.
Obviously roles and tasks are a one-to-many association since one role can be associated with none or more than one task but one task can at most have only one role.
So when I want to delete a role I have to delete the associated tasks with it as well and only deleting the role will then generate a null error in the task since it doesn't find the specified role in the database anymore.
Now I really don't find any hint in the flutter firebase documentation on how to delete more than one document based on a selected criteria.
for example deleting one document based on its document id is straightforward
Future<void> deleteRole(Role role) {
final reference = databaseInstance.document('/roles/${role.id}');
return reference.delete();
}
if i want to read a task based on certain criteria like someId i will just write something like this
Stream<List<Task>> readTasksStream({#required String someId}) {
Stream<QuerySnapshot> snapshots = databaseInstance
.collection('tasks')
.where('someId', isEqualTo: someId)
.snapshots();
//now deserialize the data to objects and return it
return snapshots.map((snapshot) => snapshot.documents
.map((snapshot) => Task.fromMap(snapshot.data))
.toList());
}
but how can i actually do the same for deleting a task based on someId in this case the roleId.
Something like this:
deleteTasksAssociatedWithARole({#required String roleId}) {
Stream<QuerySnapshot> snapshots = databaseInstance
.collection('tasks')
.where('someId', isEqualTo: someId)
.snapshots();
//now I want to delete the selected document from the querysnapshot
// but how ????????
}
I hope you get what I want to do here.
Any suggestion or help is very welcomed.
Thank you so much

Answer 1

To delete all documents matches by the query, you loop over the stream for forEach and then delete them each in turn.
Something like:
snapshots.forEach((snapshot) => snapshot.documents
.forEach((snapshot) => Task.fromMap(snapshot.data))
.toList());
}
snapshots.forEach((document) => document.reference.delete());
I find it easiest to figure out how to do this by following the types in the reference documentation, in this case from the snapshots property.

Answer 2

Frank van Puffelen
thank you so much it worked now.
so the final code is like this:
Future<void> deleteTasksAssociatedWithARole({#required Role role}) {
Stream<QuerySnapshot> snapshots = databaseInstance
.collection('tasks')
.where('roleId', isEqualTo: role.id)
.snapshots();
return snapshots.forEach((snapshot) =>
snapshot.documents.forEach((document) => document.reference.delete()));
}
so instead of deserializing the document snapshots to objects you loop through them with a forEach loop extract its reference to delete the document.