Elixir将struct转换为map

Question

I'm trying to convert a struct to a map to be able to clean all the nil values

I'm currently using this code

  case Nadia.get_updates  do
    {:ok, results} ->
      Map.from_struct(results)
      |> Enum.filter(fn {_, v} -> v != nil end)
      |> Enum.into(%{})

Note: Nadia.get_updates returns the following structure: https://hexdocs.pm/nadia/Nadia.Model.Update.html#t:t/0

Yet I'm always receiving the following error: no function clause matching in Map.from_struct/1

Answer 1

Since v0.15 we have Map.from_struct/1 which does exactly this.

defmodule User do
  defstruct [:name]
end

Map.from_struct(%User{name: "valim"})
#=> %{name: "valim"}

Answer 2

There's another trick

my_struct = %MyStruct{}
my_map = Map.delete my_struct, :__struct__

Answer 3

get_updates([{atom, any}]) ::
  {:ok, [Nadia.Model.Update.t]} |
  {:error, Nadia.Model.Error.t}

If successful it returns a list of Nadia.Model.Update.t. That's why you get that error.

Answer 4

FWIW, I wrote a library that will do this kind of thing for you automagically w/o having to pull the data structure apart.

https://github.com/philosophers-stone/transform

This is from the test code:

test "implement scrub of empty values from map" do
  data = %{ :a => nil, :b => "", :c => "a"}
  replace_empty = fn(string, _d) -> if( string == "", do: nil , else: string) end
  replace_nil = fn(map, _depth) ->  for {k, v} <- map, v != nil , into: %{}, do: {k, v} end
  potion = %{ BitString => replace_empty, Map => replace_nil}

  assert PhStTransform.transform(data, potion) == %{:c => "a"}

end

Note, if data was a list of maps this would still work just fine. The PhStTransform library will just keep pulling the data apart until it finds a map. In your case you would use a replace_nil function that worked on the kinds of structs you are interested in.