Add AssignDefaultRole change for automatic role assignment

- Assigns 'Mitglied' role to new users if no role is set

lib/accounts/user/changes/assign_default_role.ex

@@ -0,0 +1,103 @@
+defmodule Mv.Accounts.User.Changes.AssignDefaultRole do
+  @moduledoc """
+  Assigns the default "Mitglied" role to new users if no role is explicitly provided.
+
+  This change runs during user creation actions (`:create_user`, `:register_with_password`, 
+  `:register_with_rauthy`) and ensures that all users have a role assigned.
+
+  ## Behavior
+
+  - Skips assignment if `role_id` is already set in the changeset, data, or arguments
+  - Loads the "Mitglied" role without authorization (safe for system operations)
+  - Returns unchanged changeset if "Mitglied" role doesn't exist (test environments)
+  - Adds error to changeset on unexpected failures
+
+  ## Important Notes
+
+  - Works with upserts: When combined with `upsert_fields`, only new users get the role
+  - Uses `authorize?: false` to avoid circular dependencies during user creation
+  - The "Mitglied" role must exist (created by seeds or migration)
+
+  ## Examples
+
+      # Automatically assigns "Mitglied" role during user creation:
+      {:ok, user} =
+        User
+        |> Ash.Changeset.for_create(:create_user, %{email: "new@example.com"})
+        |> Ash.create()
+      
+      # User now has "Mitglied" role assigned
+      {:ok, user_with_role} = Ash.load(user, :role)
+      assert user_with_role.role.name == "Mitglied"
+
+      # Skips assignment if role is already set:
+      {:ok, user} =
+        User
+        |> Ash.Changeset.for_create(:create_user, %{email: "admin@example.com", role_id: admin_role.id})
+        |> Ash.create()
+      
+      # User has the explicitly set role, not "Mitglied"
+      {:ok, user_with_role} = Ash.load(user, :role)
+      assert user_with_role.role.name == "Admin"
+  """
+  use Ash.Resource.Change
+
+  @impl true
+  @spec change(Ash.Changeset.t(), keyword(), map()) :: Ash.Changeset.t()
+  def change(changeset, _opts, _context) do
+    # Check role_id in changeset attributes (for new assignments)
+    role_id_in_changeset = Ash.Changeset.get_attribute(changeset, :role_id)
+
+    # Check role_id in existing data (for upserts)
+    role_id_in_data = Map.get(changeset.data, :role_id)
+
+    # Check if role is being set via argument
+    role_arg = Ash.Changeset.get_argument(changeset, :role)
+
+    # Check if role relationship is already being managed
+    # Relationships are stored as a list of tuples: [{record_or_changes, opts}]
+    role_relationship = Map.get(changeset.relationships || %{}, :role)
+
+    # Skip if role is already set anywhere (changeset, data, argument, or relationship)
+    has_role =
+      not is_nil(role_id_in_changeset) or
+        not is_nil(role_id_in_data) or
+        not is_nil(role_arg) or
+        (not is_nil(role_relationship) and role_relationship != [])
+
+    if has_role do
+      changeset
+    else
+      assign_default_role(changeset)
+    end
+  end
+
+  @spec assign_default_role(Ash.Changeset.t()) :: Ash.Changeset.t()
+  defp assign_default_role(changeset) do
+    # Load the "Mitglied" role without authorization
+    # This is safe because:
+    # 1. We're only reading a public system role (no sensitive data)
+    # 2. This runs during user creation (bootstrap phase)
+    # 3. Using SystemActor here would create circular dependency (SystemActor needs a user)
+    case Mv.Authorization.Role.get_mitglied_role() do
+      {:ok, %Mv.Authorization.Role{} = mitglied_role} ->
+        # Assign the role using manage_relationship
+        # Note: :append_and_remove is the correct type for Ash 2.0+ (replaces :replace)
+        Ash.Changeset.manage_relationship(changeset, :role, mitglied_role,
+          type: :append_and_remove
+        )
+
+      {:ok, nil} ->
+        # Role doesn't exist - skip assignment (common in test environments)
+        # In production, the migration will have created the role
+        changeset
+
+      {:error, error} ->
+        # Unexpected error during role lookup
+        Ash.Changeset.add_error(changeset,
+          field: :role_id,
+          message: "Failed to load default role: #{inspect(error)}"
+        )
+    end
+  end
+end