Default value for a field in changeset

Question

I have a model and changeset method. I want to have an optional field to which I want to set a default value if it doesn't get passed to chargeset. How can I do that?

Answer 1

When you create your schema, you can give it a default value by passing it the :default atom.

schema "foo" do
  field :name, :string, default: "bar"
end

Answer 2

Justin's answer is definitely most elegant if your needs are simple, but in my case I needed a little more flexibility. I needed to be able to generate a unique, cryptographically secure value.

I did it in my changeset pipeline:

def changeset(build_proxy, attrs) do
  build_proxy
  |> cast(attrs, [:avatar, :channel, :username, :service_base_url])
  |> gen_api_token_if_empty()
  |> validate_required([:avatar, :channel, :username, :service_base_url])
end                                                                                   

defp gen_api_token_if_empty(changeset) do
  case get_change(changeset, :api_token) do
    nil -> put_change(changeset, :api_token, gen_api_token())
     "" -> put_change(changeset, :api_token, gen_api_token())
      _ -> changeset
  end
end

defp gen_api_token() do
  :crypto.strong_rand_bytes(80)
  |> Base.url_encode64
  |> binary_part(0, length)
end