Phoenix Scopes

Canonical FP bar: [docs/fcis-engineering-rules.md](../../docs/fcis-engineering-rules.md) — Functional Core, Imperative Shell: pure domain modules; side effects at edges. Authorization decisions should be pure checks on scope/user data; persist only at the edge.

RULES — Follow these with no exceptions

1. Use bracket access in templates — assigns[:current_scope] prevents crashes when unauthenticated 2. Test both authenticated and unauthenticated states — scope-based auth has two distinct code paths 3. Define anonymous/0 for the unauthenticated case — return a Scope with user: nil 4. Pass scope to context functions, not a bare user — centralizes authorization and enables tenant/permission checks 5. Guard mutating events with Scope.can?/2 — enforce authorization server-side; never rely on hidden UI controls

Scope Struct Definition

defmodule MyApp.Scope do
  defstruct [:user, :role, :permissions, :tenant]

  def for_user(%MyApp.Accounts.User{} = user) do
    %__MODULE__{
      user: user,
      role: user.role,
      permissions: permissions_for(user.role)
    }
  end

  def anonymous, do: %__MODULE__{user: nil}

  def authenticated?(%__MODULE__{user: nil}), do: false
  def authenticated?(%__MODULE__{}), do: true

  def can?(%__MODULE__{permissions: perms}, action) when is_list(perms), do: action in perms
  def can?(%__MODULE__{}, _action), do: false

  defp permissions_for(:admin), do: [:read, :write, :delete, :manage_users]
  defp permissions_for(:editor), do: [:read, :write]
  defp permissions_for(_), do: []
end

Using Scopes in LiveViews

defmodule MyAppWeb.DashboardLive do
  use MyAppWeb, :live_view

  @impl true
  def mount(_params, _session, socket) do
    scope = socket.assigns.current_scope

    if Scope.authenticated?(scope) do
      {:ok, assign(socket, :posts, Blog.list_user_posts(scope))}
    else
      {:ok, push_navigate(socket, to: ~p"/login")}
    end
  end

  @impl true
  def handle_event("delete", %{"id" => id}, socket) do
    scope = socket.assigns.current_scope

    if Scope.can?(scope, :delete) do
      # perform delete
      {:noreply, socket}
    else
      {:noreply, put_flash(socket, :error, "Not authorized")}
    end
  end
end

Safe Template Access

<%= if assigns[:current_scope] && Scope.authenticated?(@current_scope) do %>
  <p>Welcome, <%= @current_scope.user.email %></p>
  <.link href={~p"/settings"}>Settings</.link>
<% else %>
  <.link href={~p"/login"}>Log in</.link>
<% end %>

Migration Workflow (current_user → Scopes)

1. Define the Scope struct — create MyApp.Scope with for_user/1, anonymous/0, authenticated?/1, and can?/2 as shown above.
2. Update onmount hooks — replace user assignment with scope assignment (see before/after below). Run mix test test/myappweb/live/ --trace and verify all onmount tests pass before proceeding.
3. Search and replace @currentuser — update all template references to @currentscope.user; use assigns[:current_scope] for optional access. Run mix test and fix any KeyError or FunctionClauseError failures before continuing.
4. Update context functions — pass scope instead of user to functions that need auth context. Re-run mix test to confirm no regressions.
5. Run the full test suite — verify both authenticated and unauthenticated flows still work. If failures occur, revert the most recent step, fix the issue, and re-verify before moving forward.

Before (Phoenix 1.7)

def on_mount(:require_authenticated_user, _params, session, socket) do
  case get_user_from_session(session) do
    nil -> {:halt, redirect(socket, to: ~p"/login")}
    user -> {:cont, assign(socket, :current_user, user)}
  end
end

After (Phoenix 1.8)

def on_mount(:require_authenticated_user, _params, session, socket) do
  scope = get_scope_from_session(session)

  if Scope.authenticated?(scope) do
    {:cont, assign(socket, :current_scope, scope)}
  else
    {:halt, redirect(socket, to: ~p"/login")}
  end
end

Testing Scopes

Cover both code paths. Context tests assert that scope-based filtering returns only the caller's data; LiveView tests assert scope-based access control.

Context test — scope-based filtering (MyApp.DataCase):

defmodule MyApp.BlogTest do
  use MyApp.DataCase, async: true

  alias MyApp.{Blog, Scope}

  test "list_user_posts/1 returns only posts visible to the scope" do
    owner = insert(:user)
    other = insert(:user)
    own_post = insert(:post, user: owner)
    _hidden = insert(:post, user: other)

    scope = Scope.for_user(owner)

    assert Blog.list_user_posts(scope) == [own_post]
  end
end

LiveView test — scope-based access (Phoenix.LiveViewTest):

defmodule MyAppWeb.DashboardLiveTest do
  use MyAppWeb.ConnCase, async: true

  import Phoenix.LiveViewTest

  test "shows dashboard content for an authenticated scope", %{conn: conn} do
    conn = log_in_user(conn, insert(:user))
    assert {:ok, _view, html} = live(conn, ~p"/dashboard")
    assert html =~ "Welcome"
  end

  test "redirects to login for an unauthenticated scope", %{conn: conn} do
    assert {:error, {:redirect, %{to: "/login"}}} = live(conn, ~p"/dashboard")
  end
end

Common Pitfalls

Integration

Companion skills: phoenix-liveview-auth, phoenix-authorization-patterns, phoenix-auth-customization, testing-essentials.