From bf0a1a219e48007ca51e23ea45d08d295e398fcd Mon Sep 17 00:00:00 2001
From: Dan Goodliffe <dan@randomdan.homeip.net>
Date: Tue, 21 Feb 2023 00:03:08 +0000
Subject: Add helper Selection to insert into a map based on a member value as
 key

---
 lib/persistence.h | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/lib/persistence.h b/lib/persistence.h
index 01b2a7e..d55062a 100644
--- a/lib/persistence.h
+++ b/lib/persistence.h
@@ -256,6 +256,23 @@ namespace Persistence {
 		}
 	};
 
+	template<typename Map, typename Type = typename Map::mapped_type, auto Key = &Type::id>
+	struct MapByMember : public Persistence::SelectionT<std::shared_ptr<Type>> {
+		MapByMember(Map & m) : Persistence::SelectionT<std::shared_ptr<Type>> {s}, map {m} { }
+
+		using Persistence::SelectionT<std::shared_ptr<Type>>::SelectionT;
+		void
+		endObject(Persistence::Stack & stk) override
+		{
+			map.emplace(std::invoke(Key, s), s);
+			stk.pop();
+		}
+
+	private:
+		std::shared_ptr<Type> s;
+		Map & map;
+	};
+
 	struct Persistable {
 		Persistable() = default;
 		virtual ~Persistable() = default;
-- 
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