Error when trying to implement a multiple_select

Questions & Help Questions
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defmodule Medica.Medico do
  use Medica.Web, :model

  schema "medico" do
    field :nombre, :string
    field :apellido, :string
    field :especialidad, :string
  end

  @doc """
  Builds a changeset based on the `struct` and `params`.
  """
  def changeset(struct, params \\ %{}) do
    struct
    |> cast(params, [:nombre,:apellido,:especialidad])
    |> validate_required([:nombre,:apellido,:especialidad])
  end
end

This one right?