How To Use Ash framework in Phoenix with user permission example

can.ex

defmodule MyApp.Accounts.Checks.Can do
  use Ash.Policy.SimpleCheck

  def describe(_opts) do
    "Check if a user/ actor has permission on a specific resource"
  end

  # @impl true
  def match?(nil, _, _), do: false

  def match?(actor, %{query: query, changeset: changeset}, _opts) do
    # for read actions, the query will be populated
    # for create/update/destroy actions, the changeset will be populated
    subject = query || changeset

    resource =
      subject.resource
      |> Phoenix.Naming.resource_name()
      |> String.downcase()

    # replace `.type` with `.name` for action name
    action_type = convert_action_name(subject.action.type)

    {:ok, actor |> can?(action_type, resource)}
  end

  @doc false
  def can?(user, action_type, resource) do
    # Gather all permissions from the user's groups
    # Check if any user permission matches the action and resource
    Enum.flat_map(user.groups || [], & &1.permissions)
    |> Enum.map(&%{action: &1.name, resource: &1.resource})
    |> Enum.any?(&(&1.action == action_type and &1.resource == resource))
  end

  @doc false
  def cannot?(user, action_type, resource) do
    can?(user, action_type, resource) == false
  end

  defp convert_action_name(action_name) when is_binary(action_name) do
    String.downcase(action_name)
  end

  defp convert_action_name(action_type) when is_atom(action_type) do
    Atom.to_string(action_type)
    |> String.downcase()
  end
end

department.ex

defmodule MyApp.Organisations.Department do
  use Ash.Resource,
    domain: MyApp.Organisations,
    authorizers: [Ash.Policy.Authorizer],
    data_layer: AshPostgres.DataLayer

  postgres do
    table "departments"
    repo MyApp.Repo
  end

  actions do
    default_accept [:name, :description, :status, :type]
    defaults [:read, :destroy, :create, :update]

    create :seed do
      description "Seed default departments"

      accept []
      manual Hr.Organisations.Departments.Actions.SeedDepartment
    end

    read :root do
      filter expr(is_nil(parent_id))
    end

    read :by_parent do
      argument :parent_id, :uuid
      filter expr(parent_id == ^arg(:parent_id))
    end

    read :get_divisions do
      argument :department_id, :uuid
      filter expr(parent_id == ^arg(:parent_id))
    end
  end

  policies do
    policy action_type(:create), authorize_if: MyApp.Accounts.Checks.Can
    policy action_type(:read), authorize_if: MyApp.Accounts.Checks.Can
    policy action_type(:update), authorize_if: MyApp.Accounts.Checks.Can
    policy action_type(:destroy), authorize_if: MyApp.Accounts.Checks.Can
  end


  attributes do
    uuid_primary_key :id
    attribute :name, :string, allow_nil?: false
    attribute :description, :string, allow_nil?: true

    attribute :type, :atom do
      default :department
      constraints one_of: [:department, :division, :others]
    end

    attribute :status, :atom do
      default :active
      constraints one_of: [:active, :inactive]
    end

    timestamps()
  end

  identities do
    identity :unique_name, :name
  end
end

group.ex

defmodule MyApp.Accounts.Group do
  require Ash.Resource.Change.Builtins

  use Ash.Resource,
    domain: MyApp.Accounts,
    data_layer: AshPostgres.DataLayer

  postgres do
    table "groups"
    repo Hr.Repo

    references do
      reference :permissions, on_delete: :nilify
      reference :users, on_delete: :nilify
    end
  end

  actions do
    default_accept [:name, :description]
    defaults [:read, :destroy]

    create :seed do
      accept []
      manual MyApp.Accounts.Actions.SeedGroup
    end

    update :update do
      argument :permission_ids, {:array, :uuid}, allow_nil?: true
      manual MyApp.Accounts.Actions.UpdateGroup
      primary? true
    end

    create :create do
      argument :permission_ids, {:array, :uuid}, allow_nil?: true
      primary? true

      change after_action(&MyApp.Accounts.Group.Permission.add_permissions/3)
    end

   

    update :add_permissions do
      argument :permission_ids, {:array, :uuid}
      require_atomic? false

      change manage_relationship(:permission_ids, :permissions,
               type: :append_and_remove,
               on_no_match: :create
             )
    end

    update :add_users do
      require_atomic? false
      argument :user_ids, {:array, :uuid}

      change manage_relationship(:user_ids, :users,
               type: :append_and_remove,
               on_no_match: :create
             )
    end

    read :search do
      argument :keyword, :string, allow_nil?: false

      filter expr(
               ilike(name, "%#{^arg(:keyword)}%") or
                 ilike(description, "%#{^arg(:keyword)}%")
             )
    end
  end

 
  attributes do
    uuid_primary_key :id
    attribute :name, :string, allow_nil?: false
    attribute :description, :string

    timestamps()
  end

  relationships do
    many_to_many :permissions, MyApp.Accounts.Permission do
      through Hr.Accounts.GroupPermission
      source_attribute_on_join_resource :group_id
      destination_attribute_on_join_resource :permission_id
    end

    many_to_many :users, MyApp.Accounts.User do
      through Hr.Accounts.UserGroup
      source_attribute_on_join_resource :group_id
      destination_attribute_on_join_resource :user_id
    end
  end

  identities do
    identity :unique_name, :name
  end
end

group_permission.ex

defmodule MyApp.Accounts.GroupPermission do
  use Ash.Resource,
    domain: MyApp.Accounts,
    data_layer: AshPostgres.DataLayer

  postgres do
    table "group_permissions"
    repo Hr.Repo
  end

  actions do
    defaults [:read, :destroy, :create, :update]
  end

  multitenancy do
    strategy :context
  end

  attributes do
    timestamps()
  end

  relationships do
    belongs_to :group, MyApp.Accounts.Group do
      writable? true
      allow_nil? false
      primary_key? true
    end

    belongs_to :permission, MyApp.Accounts.Permission do
      writable? true
      allow_nil? false
      primary_key? true
    end
  end
end

permission.ex

defmodule MyApp.Accounts.Permission do
  use Ash.Resource,
    domain: MyApp.Accounts,
    data_layer: AshPostgres.DataLayer

  postgres do
    table "permissions"
    repo MyApp.Repo
  end

  actions do
    default_accept [:name, :resource, :description]
    defaults [:create, :read, :update, :destroy]

    create :seed_permissions do
      description "Seed all application permissions as configured"

      accept []
      manual MyApp.Accounts.Actions.SeedPermission
    end
  end

  attributes do
    uuid_primary_key :id
    attribute :name, :string, allow_nil?: false
    attribute :resource, :string, allow_nil?: false
    attribute :description, :string

    timestamps()
  end

  relationships do
    many_to_many :groups, MyApp.Accounts.Group do
      through MyApp.Accounts.GroupPermission
      source_attribute_on_join_resource :permission_id
      destination_attribute_on_join_resource :group_id
    end
  end

  identities do
    identity :unique_action_resource, [:name, :resource]
  end
end

user.ex

defmodule MyApp.Accounts.User do
 

  use Ash.Resource,
    domain: Hr.Accounts,
    data_layer: AshPostgres.DataLayer,
    # If using policies, enable the policy authorizer:
    # authorizers: [Ash.Policy.Authorizer],
    extensions: [AshAuthentication]

  postgres do
    table "users"
    repo MyApp.Repo
  end

  authentication do
    strategies do
      password :password do
        identity_field :email
      end
    end

    tokens do
      enabled? true
      token_resource MyApp.Accounts.Token
      signing_secret MyApp.Accounts.Secrets
    end
  end

  actions do
    default_accept [:email, :hashed_password]
    defaults [:create, :read, :destroy, update: :*]

    update :add_groups do
      description "Adds Groups to a user"
      argument :group_ids, {:array, :uuid}
      require_atomic? false

      change manage_relationship(:group_ids, :groups,
               type: :append_and_remove,
               on_no_match: :create
             )
    end
  end

  attributes do
    uuid_primary_key :id

    attribute :email, :ci_string do
      allow_nil? false
      public? true
    end

    attribute :current_tenant, :string, allow_nil?: true
    attribute :hashed_password, :string, allow_nil?: false, sensitive?: true
    timestamps()
  end

  relationships do
    many_to_many :groups, MyApp.Accounts.Group do
      through MyApp.Accounts.UserGroup
      source_attribute_on_join_resource :user_id
      destination_attribute_on_join_resource :group_id
    end
  end

  identities do
    identity :unique_email, [:email]
  end
end

user_group.ex

defmodule MyApp.Accounts.UserGroup do
  use Ash.Resource,
    domain: MyApp.Accounts,
    data_layer: AshPostgres.DataLayer

  postgres do
    table "user_groups"
    repo MyApp.Repo
  end

  actions do
    defaults [:create, :read, :update, :destroy]
  end


  attributes do
    timestamps()
  end

  relationships do
    belongs_to :group, MyApp.Accounts.Group, allow_nil?: false, primary_key?: true
    belongs_to :user, MyApp.Accounts.User, allow_nil?: false, primary_key?: true
  end
end

Additionally, there is now a subject key in the authorizer passed to policy functions, so instead of:

  def match?(actor, %{query: query, changeset: changeset}, _opts) do
    subject = query || changeset

    ...
  end

You can now do

  def match?(actor, %{subject: subject}, _opts) do
    ...
  end

The simplest example of a policy would likely be something that requires no custom checks.

defmodule MyApp.Blog do
  use Ash.Domain

  resources do
     resource MyApp.Blog.Post
  end
end

defmodule MyApp.Blog.Post do
  use Ash.Resource,
    data_layer: Ash.DataLayer.Ets,
    domain: MyApp.Blog

  attributes do
    uuid_v7_primary_key :id

    attribute :text, :string, allow_nil?: false
    attribute :published, :boolean, default: false, allow_nil?: false
  end

  actions do
    # default read, create, and update actions
    defaults [:read, create: [:text], update: [:text]]

    # custom update for publishing
    update :publish do
       change set_attribute(:published, true)
    end
  end

  policies do
    policy action_type(:read) do
      description "Admins and writers can see all posts, and everyone else can only see published posts"
      authorize_if actor_attribute_equals(:admin, true)
      authorize_if actor_attribute_equals(:writer, true)

      authorize_if expr(published == true)
    end

    policy action_type([:create, :update]) do 
      description "Admins and writers can create and update posts"  
      authorize_if actor_attribute_equals(:admin, true)
      authorize_if actor_attribute_equals(:writer, true)
    end

    policy action(:publish) do
      description "Only admins can publish posts"
      authorize_if actor_attribute_equals(:admin, true)
    end
  end
end

So now if you were calling actions on this resource:

Ash.create!(MyApp.Blog.Post, %{text: "text"}) # forbidden, no user
Ash.create!(MyApp.Blog.Post, %{text: "text"}, %{id: :fake}) # forbidden, not an admin or a writer

post1 = Ash.create!(MyApp.Blog.Post, %{text: "text"}, %{id: :fake, admin: true}) # authorized, an admin
post2 = Ash.create!(MyApp.Blog.Post, %{text: "text"}, %{id: :fake, writer: true}) # authorized, a writer

Ash.update!(post1, %{text: "new text"})  # forbidden, no user
Ash.update!(post1, %{text: "new text"}, actor: %{id: :fake})  # forbidden, no user
Ash.update!(post1, %{text: "new text"}, %{id: :fake, admin: true}) # authorized, an admin
Ash.update!(post1, %{text: "text text again"}, %{id: :fake, writer: true}) # authorized, a writer

Ash.update!(post2, action: :publish, actor: %{id: :fake, writer: true) # forbidden, a writer
Ash.update!(post2, action: :publish, actor: %{id: :fake, admin: true) # authorized, an admin

Then, for read policies, you can see how they are applied as filters:

# this only returns the published posts for non-logged-in-users
Ash.read!(MyApp.Blog.Post)
# => [%MyApp.Blog.Post{}]

# but for an admin or writer, it returns both
Ash.read!(MyApp.Blog.Post, actor: %{id: :fake, admin: true})
# => [%MyApp.Blog.Post{}]

To be clear: there is nothing wrong with your original example, it's a good one! Just chiming in with some feedback to potentially make it even better 🙇

I like this. Thanks @zachdaniel.

1. I wonder if it would work for attribute on a relationship of the actor. If yes, how would one access the relationship from actor_attribute_equals.

2. Also, would this work if one manually authorizes the action by authorize?: true

1. right now you can't access relationships via actor_attribute_equals. Anything that does that will have to be a custom check.
2. authorize?: true doesn't make the action authorized. It determines if authorization occurs. The default value is true. To bypass authorization, you'd need to do authorize?: false.

Thanks for the clarificatin, @zachdaniel.