mitgliederverwaltung/lib/mv/authorization/checks/has_permission.ex

defmodule Mv.Authorization.Checks.HasPermission do
  @moduledoc """
  Custom Ash Policy Check that evaluates permissions from the PermissionSets module.

  This check:
  1. Reads the actor's role and permission_set_name
  2. Looks up permissions from PermissionSets.get_permissions/1
  3. Finds matching permission for current resource + action
  4. Applies scope filter (:own, :linked, :all)

  ## Usage in Ash Resource

      policies do
        policy action_type(:read) do
          authorize_if Mv.Authorization.Checks.HasPermission
        end
      end

  ## Scope Behavior

  - **:all** - Authorizes without filtering (returns all records)
  - **:own** - Filters to records where record.id == actor.id
  - **:linked** - Filters based on resource type:
    - Member: member.user.id == actor.id (via has_one :user relationship)
    - CustomFieldValue: custom_field_value.member.user.id == actor.id (traverses member → user relationship!)

  ## Error Handling

  Returns `false` for:
  - Missing actor
  - Actor without role
  - Invalid permission_set_name
  - No matching permission found

  All errors result in Forbidden (policy fails).

  ## Examples

      # In a resource policy
      policies do
        policy action_type([:read, :create, :update, :destroy]) do
          authorize_if Mv.Authorization.Checks.HasPermission
        end
      end
  """

  use Ash.Policy.Check
  require Ash.Query
  import Ash.Expr
  alias Mv.Authorization.PermissionSets
  require Logger

  @impl true
  def describe(_opts) do
    "checks if actor has permission via their role's permission set"
  end

  @impl true
  def strict_check(actor, authorizer, _opts) do
    resource = authorizer.resource
    action = get_action_from_authorizer(authorizer)

    cond do
      is_nil(actor) ->
        log_auth_failure(actor, resource, action, "no actor")
        {:ok, false}

      is_nil(action) ->
        log_auth_failure(
          actor,
          resource,
          action,
          "authorizer subject shape unsupported (no action)"
        )

        {:ok, false}

      true ->
        strict_check_with_permissions(actor, resource, action)
    end
  end

  # Helper function to reduce nesting depth
  defp strict_check_with_permissions(actor, resource, action) do
    with %{role: %{permission_set_name: ps_name}} when not is_nil(ps_name) <- actor,
         {:ok, ps_atom} <- PermissionSets.permission_set_name_to_atom(ps_name),
         permissions <- PermissionSets.get_permissions(ps_atom),
         resource_name <- get_resource_name(resource) do
      case check_permission(
             permissions.resources,
             resource_name,
             action,
             actor,
             resource_name
           ) do
        :authorized -> {:ok, true}
        {:filter, _} -> {:ok, :unknown}
        false -> {:ok, false}
      end
    else
      %{role: nil} ->
        log_auth_failure(actor, resource, action, "no role assigned")
        {:ok, false}

      %{role: %{permission_set_name: nil}} ->
        log_auth_failure(actor, resource, action, "role has no permission_set_name")
        {:ok, false}

      {:error, :invalid_permission_set} ->
        log_auth_failure(actor, resource, action, "invalid permission_set_name")
        {:ok, false}

      _ ->
        log_auth_failure(actor, resource, action, "missing data")
        {:ok, false}
    end
  end

  @impl true
  def auto_filter(actor, authorizer, _opts) do
    resource = authorizer.resource
    action = get_action_from_authorizer(authorizer)

    cond do
      is_nil(actor) -> nil
      is_nil(action) -> nil
      true -> auto_filter_with_permissions(actor, resource, action)
    end
  end

  # Helper function to reduce nesting depth
  defp auto_filter_with_permissions(actor, resource, action) do
    with %{role: %{permission_set_name: ps_name}} when not is_nil(ps_name) <- actor,
         {:ok, ps_atom} <- PermissionSets.permission_set_name_to_atom(ps_name),
         permissions <- PermissionSets.get_permissions(ps_atom),
         resource_name <- get_resource_name(resource) do
      case check_permission(
             permissions.resources,
             resource_name,
             action,
             actor,
             resource_name
           ) do
        :authorized -> nil
        {:filter, filter_expr} -> filter_expr
        false -> nil
      end
    else
      _ -> nil
    end
  end

  # Helper to extract action from authorizer
  defp get_action_from_authorizer(authorizer) do
    case authorizer.subject do
      %{action: %{name: action}} -> action
      %{action: action} when is_atom(action) -> action
      _ -> nil
    end
  end

  # Extract resource name from module (e.g., Mv.Membership.Member -> "Member")
  defp get_resource_name(resource) when is_atom(resource) do
    resource |> Module.split() |> List.last()
  end

  # Find matching permission and apply scope
  defp check_permission(resource_perms, resource_name, action, actor, resource_name_for_logging) do
    case Enum.find(resource_perms, fn perm ->
           perm.resource == resource_name and perm.action == action and perm.granted
         end) do
      nil ->
        log_auth_failure(actor, resource_name_for_logging, action, "no matching permission found")
        false

      perm ->
        apply_scope(perm.scope, actor, resource_name)
    end
  end

  # Scope: all - No filtering, access to all records
  defp apply_scope(:all, _actor, _resource) do
    :authorized
  end

  # Scope: own - Filter to records where record.id == actor.id
  # Used for User resource (users can access their own user record)
  defp apply_scope(:own, actor, _resource) do
    {:filter, expr(id == ^actor.id)}
  end

  # Scope: linked - Filter based on user relationship (resource-specific!)
  # Uses Ash relationships: Member has_one :user, CustomFieldValue belongs_to :member
  defp apply_scope(:linked, actor, resource_name) do
    case resource_name do
      "Member" ->
        # Member has_one :user → filter by user.id == actor.id
        {:filter, expr(user.id == ^actor.id)}

      "CustomFieldValue" ->
        # CustomFieldValue belongs_to :member → member has_one :user
        # Traverse: custom_field_value.member.user.id == actor.id
        {:filter, expr(member.user.id == ^actor.id)}

      _ ->
        # Fallback for other resources: try user relationship first, then user_id
        {:filter, expr(user.id == ^actor.id or user_id == ^actor.id)}
    end
  end

  # Log authorization failures for debugging (lazy evaluation)
  defp log_auth_failure(actor, resource, action, reason) do
    Logger.debug(fn ->
      actor_id = if is_map(actor), do: Map.get(actor, :id), else: "nil"
      resource_name = get_resource_name_for_logging(resource)

      """
      Authorization failed:
        Actor: #{actor_id}
        Resource: #{resource_name}
        Action: #{inspect(action)}
        Reason: #{reason}
      """
    end)
  end

  # Helper to extract resource name for logging (handles both atoms and strings)
  defp get_resource_name_for_logging(resource) when is_atom(resource) do
    resource |> Module.split() |> List.last()
  end

  defp get_resource_name_for_logging(resource) when is_binary(resource) do
    resource
  end

  defp get_resource_name_for_logging(_resource) do
    "unknown"
  end
end