diff --git a/src/blackmisc/valuecache.cpp b/src/blackmisc/valuecache.cpp
index d22b5b8bd..53d2ec9d8 100644
--- a/src/blackmisc/valuecache.cpp
+++ b/src/blackmisc/valuecache.cpp
@@ -312,10 +312,8 @@ namespace BlackMisc
             {
                 return CStatusMessage(this).error("Invalid JSON format in %1") << file.fileName();
             }
-            CVariantMap storedValues;
-            storedValues.convertFromJson(json.object());
-            storedValues.insert(*it);
-            json.setObject(storedValues.toJson());
+            auto object = json.object();
+            json.setObject(it->mergeToJson(object));
 
             if (!(file.seek(0) && file.resize(0) && file.write(json.toJson()) > 0 && file.checkedClose()))
             {
@@ -343,10 +341,25 @@ namespace BlackMisc
             return CStatusMessage(this).error("Failed to create directory %1") << dir;
         }
 
-        const QStringList entries(QDir(dir).entryList({ "*.json" }, QDir::Files));
-        for (const auto &filename : entries)
+        QMap<QString, QStringList> keysInFiles;
+        for (const auto &key : keys)
         {
-            QFile file(dir + "/" + filename);
+            keysInFiles[key.section('/', 0, 0)].push_back(key);
+        }
+        if (keys.isEmpty())
+        {
+            for (const auto &filename : QDir(dir).entryInfoList({ "*.json" }, QDir::Files))
+            {
+                keysInFiles.insert(filename.completeBaseName(), {});
+            }
+        }
+        for (auto it = keysInFiles.cbegin(); it != keysInFiles.cend(); ++it)
+        {
+            QFile file(dir + "/" + it.key() + ".json");
+            if (! file.exists())
+            {
+                continue;
+            }
             if (! file.open(QFile::ReadOnly | QFile::Text))
             {
                 return CStatusMessage(this).error("Failed to open %1: %2") << file.fileName() << file.errorString();
@@ -357,11 +370,8 @@ namespace BlackMisc
                 return CStatusMessage(this).error("Invalid JSON format in %1") << file.fileName();
             }
             CVariantMap temp;
-            temp.convertFromJson(json.object());
-            if (! keys.isEmpty())
-            {
-                temp.removeByKeyIf([&keys](const QString & key) { return ! keys.contains(key); }); // TODO optimize by skipping files
-            }
+            temp.convertFromJson(json.object(), it.value());
+            if (it.value().isEmpty()) { temp.convertFromJson(json.object()); }
             temp.removeDuplicates(currentValues);
             o_values.insert(temp, QFileInfo(file).lastModified().toMSecsSinceEpoch());
         }
diff --git a/src/blackmisc/variantmap.cpp b/src/blackmisc/variantmap.cpp
index 199d931b8..8915e9dd3 100644
--- a/src/blackmisc/variantmap.cpp
+++ b/src/blackmisc/variantmap.cpp
@@ -14,9 +14,8 @@
 namespace BlackMisc
 {
 
-    QJsonObject CVariantMap::toJson() const
+    QJsonObject &CVariantMap::mergeToJson(QJsonObject &json) const
     {
-        QJsonObject json;
         for (auto it = cbegin(); it != cend(); ++it)
         {
             json.insert(it.key(), it.value().toJson());
@@ -24,6 +23,13 @@ namespace BlackMisc
         return json;
     }
 
+    QJsonObject CVariantMap::toJson() const
+    {
+        QJsonObject json;
+        mergeToJson(json);
+        return json;
+    }
+
     void CVariantMap::convertFromJson(const QJsonObject &json)
     {
         clear();
@@ -35,4 +41,17 @@ namespace BlackMisc
         }
     }
 
+    void CVariantMap::convertFromJson(const QJsonObject &json, const QStringList &keys)
+    {
+        clear();
+        for (const auto &key : keys)
+        {
+            auto value = json.value(key);
+            if (value.isUndefined()) { continue; }
+            CVariant var;
+            var.convertFromJson(value.toObject());
+            insert(key, var);
+        }
+    }
+
 }
diff --git a/src/blackmisc/variantmap.h b/src/blackmisc/variantmap.h
index 3481dcf31..09a2aa118 100644
--- a/src/blackmisc/variantmap.h
+++ b/src/blackmisc/variantmap.h
@@ -56,11 +56,18 @@ namespace BlackMisc
         //! Move assignment operator.
         CVariantMap &operator =(CVariantMap &&other) noexcept { CDictionary::operator =(std::move(other)); return *this; }
 
+        //! Insert values from this map into an existing JSON object.
+        QJsonObject &mergeToJson(QJsonObject &json) const;
+
         //! \copydoc BlackMisc::CValueObject::toJson
         QJsonObject toJson() const;
 
         //! \copydoc BlackMisc::CValueObject::convertFromJson
         void convertFromJson(const QJsonObject &json);
+
+        //! \copydoc BlackMisc::CValueObject::convertFromJson
+        //! Convert only keys present in list argument.
+        void convertFromJson(const QJsonObject &json, const QStringList &keys);
     };
 
 }