Nested attributes in Phoenix (Live) Views:

To use nested attributes in a form you have to use the inputs_for helper:

<%= f = form_for @changeset, Routes.user_path(@socket, :create), phx_submit: :submit, as: :user %>
  <%= input f, :name %>
  <%= inputs_for f, :address, fn a -> %>
    <%= input a, :city %>
    <%= input a, :zipcode %>
    <% end %><
  <%= submit "Create" %>
</form>

And then you can use cast_assoc and specify the changeset for the nested schema you want to validate:

defmodule MyApp.User do
  def registration_changeset(user, attrs, opts \\ []) do
    user
      |> cast(attrs, [:name])
      |> cast_assoc(:address, with: &MyApp.Address.changeset/2)
      |> validate_required([:name, :address])
  end
end

defmodule MyApp.Address do
  def changeset(address, attrs, opts \\ []) do
    user
      |> cast(attrs, [:city, :zipcode])
      |> validate_required([:city, :zipcode])
  end
end