Getting argument error when fetching data from API and decoding with Poison

Question

I am new to Elixir/Phoenix and trying to learn by building a small app.

I am fetching data from a 3rd party API and keep getting the following error.

(ArgumentError) argument error :erlang.iolist_to_binary([%{"24h_volume" => "1000", "name" => "some_name"},{...}])

What I have in my controller is:

HTTPoison.start

%HTTPoison.Response{body: body} = HTTPoison.get!(url)
body = body
       |> Poison.decode!(keys: :atoms!)

This does not work. I have used (keys: :atoms) which is discouraged in the Poison documentation.

Here is my schema:

schema "things" do
  field :name, :string
  field :volume_24h, :float

  timestamps()
end

@doc false
def changeset(%Thing{} = thing, attrs) do
  thing
  |> cast(attrs, [:volume_24h, :name])
  |> validate_not_nil([:volume_24h, :name])
end

def validate_not_nil(changeset, fields) do
  Enum.reduce(fields, changeset, fn field, changeset ->
    if get_field(changeset, field) == nil do
      add_error(changeset, field, "nil")
    else
      changeset
    end
  end)
end

I am trying to use a different field name for "24h_volume" and I get this error:

(ArgumentError) argument error :erlang.binary_to_existing_atom("24h_volume", :utf8)

I am clearly missing something here.

Is there a way to pass the desired field name to Poison because "24h_volume" will not be a valid atom? How can I fix these errors?

score 2 · Answer 1 · answered Dec 30 '17 at 23:08

2

You have yo define your atom as follows because your atom normally is illegal to start with a number, but you can go around that by wrapping it with "

So change your atom as follows:

:"24_volume"

answered Dec 30 '17 at 23:08

Andrei Sfat

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This statement is itself correct, but it has nothing to do with the issue. Check my answer for the correct explanation. – Aleksei Matiushkin Dec 31 '17 at 06:42
Yes, I had seen that an atom can not start with a number and I totally missed this :"string" This is clearly a correct answer but being new to this elixir stuff https://stackoverflow.com/users/2035262/mudasobwa 's answer is more informative. Thank you! – alexts Dec 31 '17 at 07:33
@mudasobwa, I see. I thought that the json that he is getting was because the json that he was receiving. – Andrei Sfat Dec 31 '17 at 08:46
@alexts glad you found your answer that you were looking for! – Andrei Sfat Dec 31 '17 at 08:46

Aleksei Matiushkin · Accepted Answer · 2017-12-31T06:43:29.493

You have a mess with volume_24h parameter.

As it is stated in Poison documentation:

Note that keys: :atoms! reuses existing atoms, i.e. if :name was not allocated before the call, you will encounter an argument error message.

This is exactly what happens. The application expects the :volume_24h key to come from the request, but it (for some reason, possibly due to a form misconfiguration of like) receives 24h_volume. By using the permissive atoms call instead of atoms! you did not solve anything, you have the issue hidden. What actually happens, 24h_volume key comes and is being effectively discarded by the call to cast.

What you need is either to fix the fronend/request sender to send volume_24h key, or to fix the controller to accept :"24h_volume" key.

There are two reasons behind discouraging atoms usage. The one is described in the Poison documentation: so called “atoms DOS attack” is possible with the random requests having random keys being issued subsequently, overflowing the atom storage. The second is that by using banged version atoms! one protects themselves from typos/misconfiguration like the one above.

FWIW, the atom for the proper key is being allocated in schema definition.

Thank you for taking the time to answer. I did not know I could use :"24h_volume" that is why I set :volume_24h in my migration file and wanted to change the key i was getting from Poison. I was wondering if there was something like e.g. Poison.decode!(things, keys: :atoms!, fields: {volume_24h: "24h_volume"}) — alexts, Dec 31 '17 at 07:26
I am pretty sure there is not. This transformation has nothing to do with json encoding and I doubt there are meaningful use cases for it. — Aleksei Matiushkin, Dec 31 '17 at 07:37

Getting argument error when fetching data from API and decoding with Poison

2 Answers2