diff --git a/src/main/java/net/hostsharing/hsadminng/hs/booking/item/HsBookingItemEntity.java b/src/main/java/net/hostsharing/hsadminng/hs/booking/item/HsBookingItemEntity.java
index 8bdb5c8b..a0574bce 100644
--- a/src/main/java/net/hostsharing/hsadminng/hs/booking/item/HsBookingItemEntity.java
+++ b/src/main/java/net/hostsharing/hsadminng/hs/booking/item/HsBookingItemEntity.java
@@ -98,7 +98,15 @@ public class HsBookingItemEntity implements Stringifyable, RbacObject {
     private Map<String, Object> resources = new HashMap<>();
 
     @Transient
-    private PatchableMapWrapper resourcesWrapper;
+    private PatchableMapWrapper<Object> resourcesWrapper;
+
+    public PatchableMapWrapper<Object> getResources() {
+        return PatchableMapWrapper.of(resourcesWrapper, (newWrapper) -> {resourcesWrapper = newWrapper; }, resources );
+    }
+
+    public void putResources(Map<String, Object> newResources) {
+        getResources().assign(newResources);
+    }
 
     public void setValidFrom(final LocalDate validFrom) {
         setValidity(toPostgresDateRange(validFrom, getValidTo()));
@@ -116,20 +124,6 @@ public class HsBookingItemEntity implements Stringifyable, RbacObject {
         return upperInclusiveFromPostgresDateRange(getValidity());
     }
 
-    public PatchableMapWrapper getResources() {
-        if ( resourcesWrapper == null ) {
-            resourcesWrapper = new PatchableMapWrapper(resources);
-        }
-        return resourcesWrapper;
-    }
-
-    public void putResources(Map<String, Object> entries) {
-        if ( resourcesWrapper == null ) {
-            resourcesWrapper = new PatchableMapWrapper(resources);
-        }
-        resourcesWrapper.assign(entries);
-    }
-
     @Override
     public String toString() {
         return stringify.apply(this);
diff --git a/src/main/java/net/hostsharing/hsadminng/hs/hosting/asset/HsHostingAssetEntity.java b/src/main/java/net/hostsharing/hsadminng/hs/hosting/asset/HsHostingAssetEntity.java
index 199e0e7b..462ccd2c 100644
--- a/src/main/java/net/hostsharing/hsadminng/hs/hosting/asset/HsHostingAssetEntity.java
+++ b/src/main/java/net/hostsharing/hsadminng/hs/hosting/asset/HsHostingAssetEntity.java
@@ -105,20 +105,14 @@ public class HsHostingAssetEntity implements Stringifyable, RbacObject {
     private Map<String, Object> config = new HashMap<>();
 
     @Transient
-    private PatchableMapWrapper configWrapper;
+    private PatchableMapWrapper<Object> configWrapper;
 
-    public PatchableMapWrapper getConfig() {
-        if ( configWrapper == null ) {
-            configWrapper = new PatchableMapWrapper(config);
-        }
-        return configWrapper;
+    public PatchableMapWrapper<Object> getConfig() {
+        return PatchableMapWrapper.of(configWrapper, (newWrapper) -> {configWrapper = newWrapper; }, config );
     }
 
-    public void putConfig(Map<String, Object> entries) {
-        if ( configWrapper == null ) {
-            configWrapper = new PatchableMapWrapper(config);
-        }
-        configWrapper.assign(entries);
+    public void putConfig(Map<String, Object> newConfg) {
+        PatchableMapWrapper.of(configWrapper, (newWrapper) -> {configWrapper = newWrapper; }, config).assign(newConfg);
     }
 
     @Override
diff --git a/src/main/java/net/hostsharing/hsadminng/mapper/PatchableMapWrapper.java b/src/main/java/net/hostsharing/hsadminng/mapper/PatchableMapWrapper.java
index 678a68cd..a3d2687e 100644
--- a/src/main/java/net/hostsharing/hsadminng/mapper/PatchableMapWrapper.java
+++ b/src/main/java/net/hostsharing/hsadminng/mapper/PatchableMapWrapper.java
@@ -6,31 +6,41 @@ import jakarta.validation.constraints.NotNull;
 import java.util.Collection;
 import java.util.Map;
 import java.util.Set;
+import java.util.function.Consumer;
 
+import static java.util.Optional.ofNullable;
 import static java.util.stream.Collectors.joining;
 
 /** This class wraps another (usually persistent) map and
  * supports applying `PatchMap` as well as a toString method with stable entry order.
  */
-public class PatchableMapWrapper implements Map<String, Object> {
+public class PatchableMapWrapper<T> implements Map<String, T> {
 
-    private final Map<String, Object> delegate;
+    private final Map<String, T> delegate;
 
-    public PatchableMapWrapper(final Map<String, Object> map) {
+    private PatchableMapWrapper(final Map<String, T> map) {
         delegate = map;
     }
 
+    public static <T> PatchableMapWrapper<T> of(final PatchableMapWrapper<T> currentWrapper, final Consumer<PatchableMapWrapper<T>> setWrapper, final  Map<String, T> target) {
+        return ofNullable(currentWrapper).orElseGet(() -> {
+            final var newWrapper = new PatchableMapWrapper<T>(target);
+            setWrapper.accept(newWrapper);
+            return newWrapper;
+        });
+    }
+
     @NotNull
-    public static ImmutablePair<String, Object> entry(final String key, final Object value) {
+    public static <E> ImmutablePair<String, E> entry(final String key, final E value) {
         return new ImmutablePair<>(key, value);
     }
 
-    public void assign(final Map<String, Object> entries) {
+    public void assign(final Map<String, T> entries) {
         delegate.clear();
         delegate.putAll(entries);
     }
 
-    public void patch(final Map<String, Object> patch) {
+    public void patch(final Map<String, T> patch) {
         patch.forEach((key, value) -> {
             if (value == null) {
                 remove(key);
@@ -73,22 +83,22 @@ public class PatchableMapWrapper implements Map<String, Object> {
     }
 
     @Override
-    public Object get(final Object key) {
+    public T get(final Object key) {
         return delegate.get(key);
     }
 
     @Override
-    public Object put(final String key, final Object value) {
+    public T put(final String key, final T value) {
         return delegate.put(key, value);
     }
 
     @Override
-    public Object remove(final Object key) {
+    public T remove(final Object key) {
         return delegate.remove(key);
     }
 
     @Override
-    public void putAll(final Map<? extends String, ?> m) {
+    public void putAll(final @NotNull Map<? extends String, ? extends T> m) {
         delegate.putAll(m);
     }
 
@@ -103,12 +113,12 @@ public class PatchableMapWrapper implements Map<String, Object> {
     }
 
     @Override
-    public Collection<Object> values() {
+    public Collection<T> values() {
         return delegate.values();
     }
 
     @Override
-    public Set<Entry<String, Object>> entrySet() {
+    public Set<Entry<String, T>> entrySet() {
         return delegate.entrySet();
     }
 }