Elixir convert struct to 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

Maybe results is a list of these structs? Not only one struct? — NoDisplayName
– NoDisplayName, Commented Apr 9, 2016 at 4:58

Juliano · Accepted Answer · 2017-01-19 22:10:22Z

129

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"}

answered Jan 19, 2017 at 22:10

Juliano

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1 Comment

Darragh Enright Over a year ago

Haha! Judging by the upvote I've been here before... perhaps I should get a tattoo :)

radzserg · Accepted Answer · 2016-10-13 15:43:57Z

17

There's another trick

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

answered Oct 13, 2016 at 15:43

radzserg

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3 Comments

nelsonic Over a year ago

mymap = Map.delete(mystruct, :__meta__) |> Map.delete(:__struct__)

radzserg Over a year ago

@nelsonic - I actually like the approach that Juliano suggested Map.from_struct(%User{name: "valim"}) is better way to do that

nelsonic Over a year ago

Yeah, Map.from_struct/1 is definitely good for regular structs. But when the struct is an Ecto changeset Map.from_struct/1 won't fully work so we need: mymap = Map.from_struct(changeset) |> Map.delete(:__meta__)

Sasha Fonseca · Accepted Answer · 2016-04-09 11:11:02Z

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.

Fred the Magic Wonder Dog · Accepted Answer · 2016-04-11 22:26:00Z

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.

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