Mapping map keys from one to the other ...

mapper.ex

defmodule Mapper do
	def map(from, mapping) do
		find_keys = Map.keys(mapping)

		result = Map.new(find_keys, fn(key) -> 
			new_key = Map.get(mapping, key) || key

			{ new_key, Map.get(from, key) }
		end)

		Map.drop(from, find_keys) |> Map.merge(result)
	end
end

start = %{ achievementPoints: 1000, lastModified: 1231231, testing: 123 }
mapping = %{ achievementPoints: :achievement_points, lastModified: :last_modified }
# result = %{achievement_points: 1000, last_modified: 1231231, testing: 123}

result = Mapper.map(start, mapping)

IO.puts("start = #{inspect(start)}")
IO.puts("result = #{inspect(result)}")