Using nested forms in conjunction with Ecto.Multis and changesets

Question

On my app's signup page, I'm trying to use nested forms (using form_for/3 and inputs_for/4) in conjunction with Ecto.Multis and changesets.

Goal: To re-render the form with previously-entered information if one or more server-side validations fail, so that the user can make corrections as opposed to starting from scratch.

---

The app has an Organization schema, which has one User who is the creator/owner:

schema "organizations" do
  field :owner_id, :integer
  has_one :owner, MyApp.Accounts.User, references: :owner_id, foreign_key: :id
end

The Organizations context has this create_organization function:

def create_organization(org_attrs, user_attrs) do
  Ecto.Multi.new()
  |> Ecto.Multi.insert(:organization, organization_changeset(%Organization{}, org_attrs))
  |> Ecto.Multi.run(:user, fn _repo, %{organization: organization} ->
    %User{organization_id: organization.id}
    |> Accounts.register_user_changeset(user_attrs)
    |> Repo.insert()
  end)
  |> Ecto.Multi.run(:update_organization, fn(_repo, %{user: user, organization: organization}) ->
    organization
    |> organization_creator_changeset(%{owner_id: user.id})
    |> Repo.update()
  end)
  |> Repo.transaction()
end

(The reason I'm using an Ecto.Multi for this is because if, for instance, the user insertion fails, I want the entire operation to be rolled back.)

My controller actions:

def new(conn, _params) do
  changeset = Organizations.create_organization_changeset(%Organization{owner: %User{}})
  conn
  |> render("new.html", changeset: changeset)
end


def create(conn, %{"organization" => %{"owner" => user_params}} = org_params) do
  case Organizations.create_organization(org_params, user_params) do
    {:ok, %{user: user}} ->
      conn
      |> put_status(:created)
      |> put_view(MyAppWeb.Accounts.AccountView)
      |> render("confirm.html", email: user.email)
    {:error, _resource, changeset, _changes} ->
      conn
      |> put_status(:unprocessable_entity)
      |> put_flash(:error, MyAppWeb.ErrorHelpers.transform_errors(changeset))
      |> render("new.html", changeset: changeset)
  end
end

And my form:

<%= form_for @changeset, account_path(@conn, :create), fn f -> %>

  <div class="row">
    <%= text_input :organization, :name, class: "form-control", required: true %>
    <%= label f, :organization_name %>
    <%= error_tag f, :name %>
  </div>

  <%= inputs_for f, :owner, fn u -> %>

    <div class="row">
      <%= email_input u, :email, class: "form-control", required: true %>
      <%= label u, :email %>
      <%= error_tag u, :email %>
    </div>

    <div class="row">
      <%= password_input u, :password, class: "form-control", pattern: ".{8,}", title: "Password must have 8 or more characters.", required: true %>
      <%= label u, :password %>
      <%= error_tag u, :password %>
    </div>

    <div class="row">
      <%= password_input u, :password_confirmation, class: "form-control", required: true %>
      <%= label u, :password_confirmation %>
      <%= error_tag u, :password_confirmation %>
    </div>

  <% end %>

<% end %>

The Problem: If everything goes well, the organization and the user are created successfully and confirm.html renders. However, if there are changeset errors, I get this:

could not generate inputs for :owner from MyApp.Accounts.User. Check the field exists and it is one of embeds_one, embeds_many, has_one, has_many, belongs_to or many_to_many

I believe this is because the Ecto.Changeset that is passed into new.html is a stand-alone %User{} changeset:

#Ecto.Changeset<
  action: :insert,
  changes: %{
    email: "test@test.com",
    organization: #Ecto.Changeset<action: :update, changes: %{}, errors: [],
     data: #MyApp.Organizations.Organization<>, valid?: true>,
    password: "test",
    password_confirmation: "test",
    password_hash: "bla"
  },
  errors: [
    email: {"has already been taken",
     [constraint: :unique, constraint_name: "accounts_users_email_index"]}
  ],
  data: #MyApp.Accounts.User<>,
  valid?: false
>

What is the best way to solve this problem?

Answer 1

Instead of introducing your own reimplementation for inserting several nested / related records into different tables, you probably should better go with what Ecto provides out of the box: Ecto.Changeset.put_assoc/4 in User’s changeset.

It will perform all the validations for you and return a not valid changeset is something went wrong.