Add persistence using mod storage

api.lua

@@ -2,6 +2,25 @@ local _groups = {}
 local _action_by_name = {}
 local _actions = {}
 
+-- Load data from MetaDataRef
+function classroom.load_from(meta)
+	local groups_str = meta:get("groups")
+	if not groups_str then
+		return
+	end
+
+	_groups = minetest.deserialize(groups_str)
+end
+
+-- Save data to MetaDataRef
+function classroom.save_to(meta)
+	meta:set_string("groups", minetest.serialize(_groups))
+end
+
+function classroom.save()
+	-- Overridden in init.lua
+end
+
 function classroom.get_students()
 	local students = {}
 	for _, player in pairs(classroom.get_connected_players()) do
@@ -66,6 +85,8 @@ function classroom.create_group(name)
 
 	_groups[name] = group
 
+	classroom.save()
+
 	return group
 end
 
@@ -79,6 +100,8 @@ function classroom.add_student_to_group(name, student)
 		end
 
 		group.students[#group.students + 1] = student
+
+		classroom.save()
 	end
 end
 
@@ -88,6 +111,8 @@ function classroom.remove_student_from_group(name, student)
 		for i=1, #group.students do
 			if group.students[i] == student then
 				table.remove(group.students, i)
+
+				classroom.save()
 			end
 		end
 	end

init.lua

@@ -20,3 +20,14 @@ classroom.check_player_privs = minetest.check_player_privs
 
 -- Required MT version
 assert(minetest.features.formspec_version_element, "Minetest 5.1 or later is required.")
+
+-- Mod storage for persistence
+local storage = minetest.get_mod_storage()
+
+classroom.load_from(storage)
+
+function classroom.save()
+	classroom.save_to(storage)
+end
+
+minetest.register_on_shutdown(classroom.save)