avoid multiple queries on error pages (#566)

lib/cancan/rule.rb

@@ -29,6 +29,13 @@ def initialize(base_behavior, action, subject, *extra_args, &block)
       @block = block
     end
 
+    def inspect
+      repr = "#<#{self.class.name}"
+      repr << "#{@base_behavior ? 'can' : 'cannot'} #{@actions.inspect}, #{@subjects.inspect}, #{@attributes.inspect}"
+      repr << @conditions.inspect.to_s if [Hash, String].include?(@conditions.class)
+      repr << '>'
+    end
+
     def can_rule?
       base_behavior
     end

spec/cancan/rule_spec.rb

@@ -58,4 +58,35 @@
     expect(rule2.attributes).to eq []
     expect(rule2.conditions).to eq %i[foo bar]
   end
+
+  describe '#inspect' do
+    def count_queries(&block)
+      count = 0
+      counter_f = lambda { |_name, _started, _finished, _unique_id, payload|
+        count += 1 unless payload[:name].in? %w[CACHE SCHEMA]
+      }
+      ActiveSupport::Notifications.subscribed(counter_f, 'sql.active_record', &block)
+      count
+    end
+
+    before do
+      ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ':memory:')
+      ActiveRecord::Migration.verbose = false
+      ActiveRecord::Schema.define do
+        create_table(:watermelons) do |t|
+          t.boolean :visible
+        end
+      end
+
+      class Watermelon < ActiveRecord::Base
+        scope :visible, -> { where(visible: true) }
+      end
+    end
+
+    it 'does not evaluate the conditions when they are scopes' do
+      rule = CanCan::Rule.new(true, :read, Watermelon, Watermelon.visible, {}, {})
+      count = count_queries { rule.inspect }
+      expect(count).to eq 0
+    end
+  end
 end