Form for a model and related child models in Phoenix 1.2 and Ecto 2.0.

Child.ex

defmodule MyApp.Child do
  use MyApp.Web, :model

  schema "child" do
    field :name, :string
    field :description, :string
    belongs_to :event, MyApp.Parent

    timestamps
  end

  @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 \\ %{}) do
    model
    |> cast(params, [:name, :description])
    |> cast_assoc(:parent, required: True)
    |> validate_required([:name, ])
  end

  def nested_changeset(model, params \\ %{}) do
    # Does not try to cast the parent assoc
    model
    |> cast(params, [:name, :description])
    |> validate_required([:name, ])
  end
end

Parent.ex

defmodule MyApp.Parent do
  use MyApp.Web, :model

  schema "parent" do
    field :name, :string
    field :description, :string
    has_many :children, MyApp.Child

    timestamps
  end
  
    @error_messages %{
    :name => %{
      :required => "Please include a name."
    },
    :description => %{
      :required => "Please include a short description."
    }
  }

  def error_messages,  do: @error_messages
  
  def changeset(model, params \\ %{}) do
    model
    |> cast(params, [:name, :description])
    |> validate_required(:name, message: @error_messages.name.required)
    |> validate_required(:description, message: @error_messages.description.required)
    |> cast_assoc(:location_choices, with: &MyApp.Child.nested_changeset/2)
  end
end

parent_form.html.eex

<%= 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 %>

  <div class="form-group">
    <%= label f, :name, class: "control-label" %>
    <%= text_input f, :name, class: "form-control" %>
    <%= error_tag f.source, :name %>
  </div>

  <div class="form-group">
    <%= label f, :description, class: "control-label" %>
    <%= textarea f, :description, class: "form-control" %>
    <%= error_tag f.source, :description %>
  </div>

  <hr>
  <h3>Children</h3>
    <%= inputs_for f, :children, [append: [Child.nested_changeset(%Child{})]], fn fp -> %>
      <div class="form-group">
        <%= label fp, :name, class: "control-label" %>
        <%= text_input fp, :name, class: "form-control" %>
        <%= error_tag fp, :name %>
      </div>
      <div class="form-group">
        <%= label fp, :description, class: "control-label" %>
        <%= text_input fp, :description, class: "form-control" %>
        <%= error_tag fp, :description %>
      </div>
    <% end %>
    
    <div class="form-group">
      <%= submit "Submit", class: "btn btn-primary" %>
    </div>
<% end %>

ParentController.ex

defmodule MyApp.ParentController do
  use MyApp.Web, :controller
  alias MyApp.Parent
  import Ecto.Changeset

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

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

  def create(conn, %{"parent" => params}, user) do
    changeset = Parent.changeset(%Parent{}, params)
    case Repo.insert(changeset) do
      {:ok, parent} ->
        conn
        |> put_flash(:info, "Event created successfully.")
        |> redirect(to: parent_path(conn, :show, parent))
      {:error, changeset} ->
        render(conn, "new.html", changeset: changeset)
    end
  end

  def edit(conn, %{"id" => id}) do
    event = Repo.get!(Parent, id) |> Repo.preload(:children)
    changeset = Parent.changeset(parent)
    render(conn, "edit.html", parent: parent, changeset: changeset)
  end

  def update(conn, %{"id" => id, "parent" => params}, user) do
    parent = Repo.get!(Parent, id) |> Repo.preload(:children)
    changeset = Parent.changeset(parent, params)
    case Repo.update(changeset) do
      {:ok, parent} ->
        conn
        |> put_flash(:info, "Parent updated successfully.")
        |> redirect(to: event_path(conn, :edit, parent))  # just going back to edit because it is simple example
      {:error, changeset} ->
        render(conn, "edit.html", parent: parent, changeset: changeset)
    end
  end
  
end