Json / map in Phoenix form

workout_form.ex

<%= form_for @changeset, @action, fn f -> %>
  <%= if @changeset.action do %>
    <div class="alert alert-danger">
      <p>Oops, something went wrong! Please check the errors below.</p>
    </div>
  <% end %>

  <div class="form-group">
    <%= label f, :type, class: "control-label" %>
    <%= select f, :type, @workout_types, prompt: "Choose workout type", class: "form-control" %>
    <%= error_tag f, :type %>
  </div>

  <div class="form-group">
    <%= label f, :details, class: "control-label" %>
    <%= textarea f, :details, rows: 10, class: "form-control" %>
    <%= error_tag f, :details %>
  </div>

  <div class="form-group">
    <%= submit "Submit", class: "btn btn-primary" %>
  </div>
<% end %>

workout_migration.exs

defmodule GetFit.Repo.Migrations.CreateWorkout do
  use Ecto.Migration

  def change do
    create table(:workouts) do
      add :description, :string
      add :type, :string
      add :details, :map

      timestamps()
    end

    create unique_index(:workouts, [:details])
  end
end

workout_model.ex

defmodule GetFit.Workout do
  use GetFit.Web, :model

  alias GetFit.{Movement, Repo}

  schema "workouts" do
    field :description, :string
    field :type, :string
    field :details, :map
    has_many :results, GetFit.Result

    timestamps()
  end

  def changeset(struct, params \\ %{}) do
    struct
    |> cast(filter(params), [:description, :type, :details])
    |> validate_required([:type, :details])
    |> unique_constraint(:details)
  end

  defp filter(%{"details" => details} = params) when is_binary(details) do
    Map.put params, "details", Poison.decode!(details, keys: :atoms)
  end
  defp filter(params), do: params
end