diff --git a/LeStorage/Store.swift b/LeStorage/Store.swift
index e7a9ba9..e8130d0 100644
--- a/LeStorage/Store.swift
+++ b/LeStorage/Store.swift
@@ -64,10 +64,10 @@ public class Store {
     
     /// Registers a collection
     /// [synchronize] denotes a collection which modification will be sent to the django server
-    public func registerCollection<T : Storable>(synchronized: Bool) -> StoredCollection<T> {
+    public func registerCollection<T : Storable>(synchronized: Bool, indexed: Bool = false) -> StoredCollection<T> {
         
         // register collection
-        let collection = StoredCollection<T>(synchronized: synchronized, store: Store.main, loadCompletion: nil)
+        let collection = StoredCollection<T>(synchronized: synchronized, store: Store.main, indexed: indexed, loadCompletion: nil)
         self._collections[T.resourceName()] = collection
         
 //        if synchronized { // register additional collection for api calls
diff --git a/LeStorage/StoredCollection.swift b/LeStorage/StoredCollection.swift
index 0904ece..9577538 100644
--- a/LeStorage/StoredCollection.swift
+++ b/LeStorage/StoredCollection.swift
@@ -179,14 +179,22 @@ public class StoredCollection<T: Storable>: RandomAccessCollection, SomeCollecti
     }
     
     /// Inserts the whole sequence into the items array, no updates
-    public func append(contentOfs sequence: any Sequence<T>) throws {
+    public func addOrUpdate(contentOfs sequence: any Sequence<T>) throws {
         defer {
             self._hasChanged = true
         }
-        self.items.append(contentsOf: sequence)
+        
         for instance in sequence {
-            try self._sendInsertionIfNecessary(instance)
+            if let index = self.items.firstIndex(where: { $0.id == instance.id }) {
+                self.items[index] = instance
+                try self._sendUpdateIfNecessary(instance)
+            } else { // insert
+                self.items.append(instance)
+                self._index?[instance.stringId] = instance
+                try self._sendInsertionIfNecessary(instance)
+            }
         }
+        
     }
     
     /// Returns the instance corresponding to the provided [id]
@@ -226,7 +234,7 @@ public class StoredCollection<T: Storable>: RandomAccessCollection, SomeCollecti
     /// Schedules a write operation
     fileprivate func _scheduleWrite() {
         if self.asynchronousIO {
-            DispatchQueue(label: "lestorage.queue.write", qos: .utility).sync { // sync to make sure we don't have writes performed at the same time
+            DispatchQueue(label: "lestorage.queue.write", qos: .utility).asyncAndWait { // sync to make sure we don't have writes performed at the same time
                 self._write()
             }
         } else {