ecto_changeset.ex

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

  schema "users" do
    field :first_name, :string
    field :last_name, :string
    field :email, :string
    field :password_digest, :string
    field :password, :string, virtual: true

    timestamps
  end

  @required_fields ~w(first_name last_name email password_digest)
  @optional_fields ~w()

  def changeset(model, params \\ :empty) do
    model
    |> cast(params, @required_fields, @optional_fields)
    |> generate_password_digest
    |> validate_format(:email, ~r/@/)
    |> unique_constraint(:email)
  end

  defp generate_password_digest(changeset) do
    case changeset.params["password"] do
      nil -> changeset
      password ->
        changeset
        |> put_change(:password_digest, Comeonin.Bcrypt.hashpwsalt(password))
    end
  end
end

Example

alias Meinac.User

params    = %{ email: "mehmetemininac@gmail.com", first_name: "Mehmet Emin", last_name: "INAC", password: "123456" }
changeset = User.changeset(%User{}, params)
changeset.errors # => [password_digest: "can't be blank"]

as you see, it returns blank error for password_digest but it shouldn't.