wip: better foreign key management

src/nut/core/core.pri

@@ -8,6 +8,7 @@ HEADERS += \
     $$PWD/changelogtable.h \
     $$PWD/database.h \
     $$PWD/database_p.h \
+    $$PWD/foreigncontainer.h \
     $$PWD/propertysignalmapper.h \
     $$PWD/query.h \
     $$PWD/table.h \
@@ -19,6 +20,7 @@ SOURCES += \
     $$PWD/bulkinserter.cpp \
     $$PWD/changelogtable.cpp \
     $$PWD/database.cpp \
+    $$PWD/foreigncontainer.cpp \
     $$PWD/propertysignalmapper.cpp \
     $$PWD/query.cpp \
     $$PWD/table.cpp \

src/nut/core/foreigncontainer.cpp

@@ -0,0 +1,2 @@
+#include "foreigncontainer.h"
+

src/nut/core/foreigncontainer.h

@@ -0,0 +1,58 @@
+#ifndef FOREIGNCONTAINER_H
+#define FOREIGNCONTAINER_H
+
+template<class _OBJECT, typename _KEY>
+class ForeignContainer
+{
+    _OBJECT *_object{nullptr};
+    _KEY _key;
+    enum StorageType {
+        Key,
+        ClassValue
+    };
+    StorageType storageType;
+
+public:
+    ForeignContainer()
+    {}
+
+    ForeignContainer<_OBJECT, _KEY> operator =(const _KEY &key)
+    {
+        this->_key = key;
+        this->_object = nullptr;
+        this->storageType = Key;
+        return *this;
+    }
+
+    ForeignContainer<_OBJECT, _KEY> operator =(const _OBJECT *value)
+    {
+        this->_object = value;
+        this->storageType = ClassValue;
+        return *this;
+    }
+
+    _KEY key()
+    {
+        if (this->storageType == Key)
+            return _key;
+        else
+            return _object->primaryValue().template value<_KEY>();
+    }
+
+    _OBJECT *object()
+    {
+        return _object;
+    }
+
+    operator _KEY()
+    {
+        return key();
+    }
+
+    operator _OBJECT()
+    {
+        return object();
+    }
+};
+
+#endif // FOREIGNCONTAINER_H