adding joins clause to accessible_by when conditions are across associations

lib/cancan/ability.rb

@@ -200,6 +200,16 @@ module CanCan
       end
     end
     
+    # Returns the associations used in conditions. This is usually used in the :joins option for a search.
+    # See ActiveRecordAdditions#accessible_by for use in Active Record.
+    def association_joins(action, subject)
+      can_definition = matching_can_definition(action, subject)
+      if can_definition
+        raise Error, "Cannot determine association joins from block for #{action.inspect} #{subject.inspect}" if can_definition.block
+        can_definition.association_joins
+      end
+    end
+
     private
     
     def can_definitions

lib/cancan/active_record_additions.rb

@@ -21,10 +21,11 @@ module CanCan
       # internally uses Ability#conditions method, see that for more information.
       def accessible_by(ability, action = :read)
         conditions = ability.conditions(action, self) || {:id => nil}
+        joins = ability.association_joins(action, self)
         if respond_to? :where
-          where(conditions)
+          where(conditions).joins(joins)
         else
-          scoped(:conditions => conditions)
+          scoped(:conditions => conditions, :joins => joins)
         end
       end
     end

lib/cancan/can_definition.rb

@@ -25,6 +25,21 @@ module CanCan
       result = can_without_base_behavior?(action, subject, extra_args)
       @base_behavior ? result : !result
     end
+
+    def association_joins(conditions = @conditions)
+      joins = []
+      conditions.each do |name, value|
+        if value.kind_of? Hash
+          nested = association_joins(value)
+          if nested
+            joins << {name => nested}
+          else
+            joins << name
+          end
+        end
+      end
+      joins unless joins.empty?
+    end
     
     private