Inserting JSONB into Postgres via Ecto

I’m trying to pass jsonb data into postgres via ecto. I’d like to be able to take a valid JSON string, add it as a graphql argument, and see that json in my table.

migration

defmodule MyApp.CreateJsonTable do
    use Ecto.Migration

    def change do
      create table(:geodata) do
         add(:json, :map)
         timestamps(type: :utc_datetime)
      end
   end
end

My understanding is that you need to define a struct for Poison for JSONB, and then decode into that when you insert.

defmodule Geodatajson do
  use MyApp, :model

  embedded_schema do
    field(:latitude, :float)
    field(:longitude, :float)
  end
end

now the model:

defmodule MyApp.Geodata do
  use MyApp, :model

  alias MyApp.Repo
  alias MyApp.Geodata

  schema "geodata" do
    embeds_one(:json, Geodatajson)

    timestamps()
  end

  def changeset(struct, params \\ %{}) do
    struct
    |> cast(params, [:json])
  end

  def add_geodata(str) do
    json = str |> Poison.decode!(as: Geodatajson)
    data = %Geodata{json: json}
    Repo.insert(data)
  end
end

I try to pass in the data like this:

iex> MyApp.Geodata.add_geodata("{\"latitude\": 1.23, \"longitude\": 4.56}")

but the JSONB does not get decoded:

{:ok,
 %MyApp.Geodata{
   __meta__: #Ecto.Schema.Metadata<:loaded, "geodata">,
   id: 26,
   inserted_at: ~N[2018-04-28 13:28:42.346382],
   json: %Geodatajson{
     id: "3b22ef94-92eb-4c64-8174-9ce1cb88e8c5",
     latitude: nil,
     longitude: nil
   },
   updated_at: ~N[2018-04-28 13:28:42.346392]
 }}

What can I do to get this data into postgres?