field.ex

defmodule Ph_14Version.Field do
  use Ph_14Version.Web, :model
  use Ecto.Model.Callbacks

  before_insert :set_map_coords

  schema "fields" do
    field :name, :string
    field :map_coords, Geo.MultiPolygon

    timestamps
  end

  @required_fields ~w(name map_coords)
  @optional_fields ~w()

  @doc """
  Creates a changeset based on the `model` and `params`.

  If no params are provided, an invalid changeset is returned
  with no validation performed.
  """
  def changeset(model, params \\ :empty) do
    model
    |> cast(params, @required_fields, @optional_fields)
  end

  def set_map_coords(changeset) do
    map_coords = Ecto.Changeset.get_field(changeset, :map_coords)
    change(changeset, %{map_coords: Geo.JSON.decode(map_coords)})
  end
  
end

I need to parse this to insert a record

{"type":"MultiPolygon","crs":{"type":"name","properties":{"name":"EPSG4326"}},"coordinates":[[[[-85.62224328517914,42.09954388363239],[-85.62217891216278,42.092203790811375],[-85.62700688838957,42.09218786777775],[-85.6271356344223,42.098970718243926],[-85.62554776668549,42.09892295422769],[-85.62529027462006,42.099496120047895],[-85.62224328517914,42.09954388363239]]]]}

This will decode it to be a Geo.MultiPolygon
Geo.JSON.decode(map_coords)