Keep compatibility with existing can :dashboard style ability

lib/rails_admin/extensions/cancancan/authorization_adapter.rb

@@ -5,20 +5,35 @@ module CanCanCan
       class AuthorizationAdapter < RailsAdmin::Extensions::CanCan::AuthorizationAdapter
         def authorize(action, abstract_model = nil, model_object = nil)
           return unless action
-          action, subject = resolve_action_and_subject(action, abstract_model, model_object)
-          @controller.current_ability.authorize!(action, subject)
+          subject = model_object || abstract_model && abstract_model.model
+          if authorized_for_dashboard_in_legacy_way?(action)
+            subject
+          else
+            @controller.current_ability.authorize!(*resolve_with_compatibility(action, subject))
+          end
         end
 
         def authorized?(action, abstract_model = nil, model_object = nil)
           return unless action
-          action, subject = resolve_action_and_subject(action, abstract_model, model_object)
-          @controller.current_ability.can?(action, subject)
+          subject = model_object || abstract_model && abstract_model.model
+          authorized_for_dashboard_in_legacy_way?(action, true) ||
+            @controller.current_ability.can?(*resolve_with_compatibility(action, subject))
         end
 
       private
 
-        def resolve_action_and_subject(action, abstract_model, model_object)
-          subject = model_object || abstract_model && abstract_model.model
+        def authorized_for_dashboard_in_legacy_way?(action, silent = false)
+          return false unless action == :dashboard
+          legacy_ability = @controller.current_ability.permissions[:can][:dashboard]
+          if legacy_ability && (legacy_ability.empty? || legacy_ability.all?(&:empty?))
+            ActiveSupport::Deprecation.warn('RailsAdmin CanCanCan Ability with `can :dashboard` is old and support will be removed in the next major release, use `can :read, :dashboard` instead. See https://github.com/sferik/rails_admin/issues/2901') unless silent
+            true
+          else
+            false
+          end
+        end
+
+        def resolve_with_compatibility(action, subject)
           if subject
             [action, subject]
           else

spec/integration/authorization/cancancan_spec.rb

@@ -340,4 +340,27 @@ def initialize(user)
       end
     end
   end
+
+  describe 'with existing dashboard ability which uses no subject' do
+    class LegacyDashboardAbility
+      include CanCan::Ability
+      def initialize(_)
+        can :access, :rails_admin
+        can :dashboard
+      end
+    end
+
+    before do
+      RailsAdmin.config { |c| c.authorize_with :cancancan, LegacyDashboardAbility }
+      @user = FactoryGirl.create :user
+      login_as @user
+    end
+
+    it 'shows dashboard with instruction on how to migrate to new ability notation' do
+      allow(ActiveSupport::Deprecation).to receive(:warn)
+      expect(ActiveSupport::Deprecation).to receive(:warn).with(/can :read, :dashboard/)
+      visit dashboard_path
+      is_expected.to have_content('Dashboard')
+    end
+  end
 end if defined?(CanCanCan)