survey_form_example.ex

defmodule Example.Survey do
  import Ecto.Query, warn: false
  alias Example.Repo

  alias Example.Surveys.Survey

  def create_survey(attrs \\ %{}) do
    %Survey{}
    |> Survey.changeset(attrs)
    |> Repo.insert()
  end

  def change_survey(%Survey{} = survey) do
    Survey.changeset(survey, %{})
  end
end

defmodule Example.Surveys.Survey do
  use Ecto.Schema
  import Ecto.Changeset

  schema "surveys" do
    field :occurred_at, :utc_datetime
    has_many :answers, Example.Surveys.Answer
  end

  @doc false
  def changeset(survey, attrs) do
    survey
    |> cast(attrs, [:occurred_at])
    |> cast_assoc(:answers, required: true)
    |> validate_required([:occurred_at])
  end
end

defmodule Example.Surveys.Question do
  use Ecto.Schema

  schema "questions" do
    field :question, :string
    has_many :answers, Example.Surveys.Answer
  end
end

defmodule Example.Surveys.Answer do
  use Ecto.Schema
  import Ecto.Changeset

  schema "answers" do
    field :answer, :string
    belongs_to :survey, Example.Surveys.Survey
    belongs_to :question, Example.Surveys.Question
  end

  @doc false
  def changeset(answer, attrs) do
    answer
    |> cast(attrs, [:answer, :question_id])
    |> validate_required([:answer])
    |> assoc_constraint(:question)
  end
end

defmodule ExampleWeb.SurveyController do
  use ExampleWeb, :controller
  
  alias Example.Surveys
  alias Example.Surveys.{Question, Answer, Survey}
  
  def new(conn, _params) do
    answers = for q <- Example.Repo.all(Question) do
      %Example.Surveys.Answer{question: q, question_id: q.id}
    end
    
    changeset = Surveys.change_survey(%Survey{answers: answers})
    render(conn, "new.html", changeset: changeset)
  end
  
  def create(conn, %{"survey" => survey_params}) do
    case Survey.create_survey(survey_params) do
      {:ok, survey} ->
        conn
        |> put_flash(:info, "Survey created successfully.")
        |> redirect(to: Routes.survey_path(conn, :show, survey))

      {:error, %Ecto.Changeset{} = changeset} ->
        answers =
          for answer <- changeset.changes.answers do
            question =
              Example.Repo.get!(Example.Question, answer.changes.question_id)

            %{answer | data: %Answer{question: question, question_id: question.id}}
          end

        changeset = %{changeset | changes: %{changeset.changes | answers: answers}}
        render(conn, "new.html", changeset: changeset)
    end
  end
end


<%= form_for @changeset, @action, fn f -> %>
  <%= if @changeset.action do %>
    <div class="alert alert-danger">
      <p>Oops, something went wrong! Please check the errors below.</p>
    </div>
  <% end %>

  <%= label f, :occurred_at %>
  <%= datetime_select f, :occurred_at %>
  <%= error_tag f, :occurred_at %>

  <%= inputs_for f, :answers, fn a -> %>
    <%= hidden_input a, :question_id %>
    <%= label a, :answer, a.data.question.question %>
    <%= number_input a, :answer %>
    <%= error_tag a, :answer %>
  <% end %>
  
  <div>
    <%= submit "Save" %>
  </div>
<% end %>