user.ex

defmodule Sandbox.User do
  use Sandbox.Web, :model

  schema "users" do
    field :first_name, :string
    field :last_name, :string

    timestamps
  end

  @required_fields ~w(first_name last_name)
  @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)
    |> validate_length(:first_name, min: 2)
    |> validate_length(:last_name, max: 256)
  end
end

user_controller.ex

defmodule Sandbox.UserController do
  alias Sandbox.User

  plug :scrub_params, "user" when action in [:create, :update]

  def index(conn, _params) do
    users = Repo.all(User)
    render conn, "index.html", users: users
  end

  def new(conn, _params) do
    changeset = User.changeset(%User{})
    render conn, "new.html", changeset: changeset
  end

  def create(conn, %{"user" => user_params}) do
    changeset = User.changeset(%User{}, user_params)

    case Repo.insert(changeset) do
      {:ok, _user} ->
        conn
        |> put_flash(:info, "User created successfully")
        |> redirect(to: user_path(conn, :index))
      {:error, changeset} ->
        render conn, "new.html", changeset: changeset
    end
  end
end