fixed backend test compilation

include/matador/object/object_ptr.hpp

@@ -1,16 +1,38 @@
 #ifndef OBJECT_PTR_HPP
 #define OBJECT_PTR_HPP
 
+#include "matador/utils/identifier.hpp"
+
+#include <memory>
+
 namespace matador::object {
 template <typename Type>
 class object_ptr {
 public:
-    using value_type = Type;
-    Type* operator->() { return ptr; };
-    Type& operator*() { return *ptr; };
+  object_ptr() = default;
+  object_ptr(Type *obj) : ptr_(obj) {}
+  explicit object_ptr(std::shared_ptr<Type> obj) : ptr_(obj) {}
+  object_ptr(const object_ptr &other) : ptr_(other.ptr_) {}
+  object_ptr(object_ptr &&other) noexcept : ptr_(std::move(other.ptr_)) {}
+  object_ptr &operator=(const object_ptr &other)  = default;
+  object_ptr &operator=(object_ptr &&other) = default;
 
+  using value_type = Type;
+  Type* operator->() { return ptr_.get(); }
+  Type& operator*() const { return *ptr_; }
+
+  [[nodiscard]] bool empty() const { return ptr_ == nullptr; }
+
+  Type* get() { return ptr_.get(); }
+  const Type* get() const { return ptr_.get(); }
+  operator bool() { return valid(); }
+  bool valid() { return ptr_ != nullptr; }
+
+  [[nodiscard]] const utils::identifier& primary_key() const { return pk_; }
+  void primary_key(const utils::identifier &pk) { pk_ = pk; }
 private:
-    Type* ptr{nullptr};
+    std::shared_ptr<Type> ptr_{};
+    utils::identifier pk_;
 };
 
 }