Map.new(%Foo{}, fn {key, _} -> {key, json[Atom.to_string(key)]} end)

Quick example

defmodule Foo do
  @derive [Poison.Encoder]
  defstruct [:bar]
end

defmodule Example do
  def test do
    s = "{\"bar\":\"baz\"}"
    Poison.decode!(s, as: %Foo{})
  end
end

iex(1)> Example.test
%Foo{bar: “baz”}

There are probably more modern ways to do it, but this is was I have been using. You can also nest structs this way.

The number of json encode/decoders seems to grow everyday. https://package-rank.com/wp/hex/poison/-vs-/hex/jason

You can also always decode manually and completely explicitly. It is more typing but it also allows you to change the struct or the parameters independently. It is not always the best way but sometimes is.

defmodule Foo do
  defstruct [:bar, :baz]
end

defmodule Example do
  def test do
    s = '{"bar":"abc", "baz": 42}'
    json = Poison.decode!(s)
    %Foo{
      bar: json["bar"],
      baz: json["baz"],
    }
  end
end