diff --git a/lib/cancan/ability.rb b/lib/cancan/ability.rb
index 535663e..7e4d6f0 100644
--- a/lib/cancan/ability.rb
+++ b/lib/cancan/ability.rb
@@ -49,8 +49,12 @@ module CanCan
     # 
     def can?(action, subject, *extra_args)
       raise Error, "Nom nom nom. I eated it." if action == :has && subject == :cheezburger
-      can_definition = matching_can_definition(action, subject)
-      can_definition && can_definition.can?(action, subject, extra_args)
+      matching_can_definition(action, subject) do |can_definition|
+        unless (can = can_definition.can?(action, subject, extra_args)) == :_NOT_MATCHED
+          return can
+        end
+      end
+      false
     end
     
     # Convenience method which works the same as "can?" but returns the opposite value.
@@ -179,11 +183,15 @@ module CanCan
       @aliased_actions = {}
     end
     
-    # Returns a hash of conditions which match the given ability. This is useful if you need to generate a database
-    # query based on the current ability.
+    # Returns an array of arrays composing from desired action and hash of conditions which match the given ability.
+    # This is useful if you need to generate a database query based on the current ability.
     # 
     #   can :read, Article, :visible => true
-    #   conditions :read, Article # returns { :visible => true }
+    #   conditions :read, Article # returns [ [ true, { :visible => true } ] ]
+    #
+    #   can :read, Article, :visible => true
+    #   cannot :read, Article, :blocked => true
+    #   conditions :read, Article # returns [ [ false, { :blocked => true } ], [ true, { :visible => true } ] ]
     # 
     # Normally you will not call this method directly, but instead go through ActiveRecordAdditions#accessible_by method.
     # 
@@ -191,25 +199,74 @@ module CanCan
     # If the ability is defined using a block then this will raise an exception since a hash of conditions cannot be
     # determined from that.
     def conditions(action, subject)
-      can_definition = matching_can_definition(action, subject)
-      if can_definition
-        raise Error, "Cannot determine ability conditions from block for #{action.inspect} #{subject.inspect}" if can_definition.block
-        can_definition.conditions || {}
+      matched = matching_can_definition(action, subject)
+      unless matched.empty?
+        if matched.any?{|can_definition| can_definition.conditions.nil? && can_definition.block }
+          raise Error, "Cannot determine ability conditions from block for #{action.inspect} #{subject.inspect}"
+        end
+        matched.map{|can_definition|
+            [can_definition.base_behavior, can_definition.conditions]
+        }
       else
         false
       end
     end
     
+    # Returns sql conditions for object, which responds to :sanitize_sql .
+    # This is useful if you need to generate a database query based on the current ability.
+    # 
+    #   can :manage, User, :id => 1
+    #   can :manage, User, :manager_id => 1
+    #   cannot :manage, User, :self_managed => true
+    #   sql_conditions :manage, User # returns not (self_managed = 't') AND ((manager_id = 1) OR (id = 1))
+    #
+    # Normally you will not call this method directly, but instead go through ActiveRecordAdditions#accessible_by method.
+    # 
+    # If the ability is not defined then false is returned so be sure to take that into consideration.
+    # If there is just one :can ability, it conditions returned untouched.
+    # If the ability is defined using a block then this will raise an exception since a hash of conditions cannot be
+    # determined from that.   
+    def sql_conditions(action, subject)
+      conds = conditions(action, subject)
+      return false if conds == false
+      return (conds[0][1] || {}) if conds.size==1 && conds[0][0] == true # to match previous spec
+      
+      true_cond = subject.send(:sanitize_sql, ['?=?', true, true])
+      false_cond = subject.send(:sanitize_sql, ['?=?', true, false])
+      conds.reverse.inject(nil) do |sql, action|
+        behavior, condition = action
+        if condition
+          condition = "#{subject.send(:sanitize_sql, condition)}"
+          condition = "not (#{condition})" if behavior == false
+        else
+          condition = behavior ? true_cond : false_cond
+        end
+        case sql
+          when nil then condition
+          when true_cond
+            behavior ? true_cond : condition
+          when false_cond
+            behavior ? condition : false_cond
+          else
+            behavior ? "(#{condition}) OR (#{sql})" : "#{condition} AND (#{sql})"
+        end
+      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
+      can_definitions = matching_can_definition(action, subject)
+      unless can_definitions.empty?
+        if can_definitions.any?{|can_definition| can_definition.conditions.nil? && can_definition.block }
+          raise Error, "Cannot determine association joins from block for #{action.inspect} #{subject.inspect}"
+        end
+        collect_association_joins(can_definitions)
+      else
+        nil
       end
     end
-
+    
     private
     
     def can_definitions
@@ -217,9 +274,18 @@ module CanCan
     end
     
     def matching_can_definition(action, subject)
-      can_definitions.reverse.detect do |can_definition|
-        can_definition.expand_actions(aliased_actions)
-        can_definition.matches? action, subject
+      if block_given?
+        can_definitions.reverse.each do |can_definition|
+          can_definition.expand_actions(aliased_actions)
+          if can_definition.matches? action, subject
+            yield can_definition
+            break if can_definition.conditions.nil? && can_definition.block.nil?
+          end
+        end
+      else
+        matched = []
+        matching_can_definition(action, subject){|can_definition| matched << can_definition}
+        matched
       end
     end
     
@@ -230,5 +296,32 @@ module CanCan
         :update => [:edit],
       }
     end
+    
+    def collect_association_joins(can_definitions)
+      joins = []
+      can_definitions.each do |can_definition|
+        merge_association_joins(joins, can_definition.association_joins || [])
+      end
+      clear_association_joins(joins)
+    end
+    
+    def merge_association_joins(what, with)
+      with.each do |join|
+        name, nested = join.each_pair.first
+        if at = what.detect{|h| h.has_key?(name) }
+          at[name] = merge_association_joins(at[name], nested)
+        else
+          what << join
+        end
+      end
+    end
+    
+    def clear_association_joins(joins)
+      joins.map do |join| 
+        name, nested = join.each_pair.first
+        nested.empty? ? name : {name => clear_association_joins(nested)}
+      end
+    end
+
   end
 end
diff --git a/lib/cancan/active_record_additions.rb b/lib/cancan/active_record_additions.rb
index 2afa33d..eb0729e 100644
--- a/lib/cancan/active_record_additions.rb
+++ b/lib/cancan/active_record_additions.rb
@@ -20,7 +20,7 @@ module CanCan
       # Here only the articles which the user can update are returned. This
       # internally uses Ability#conditions method, see that for more information.
       def accessible_by(ability, action = :read)
-        conditions = ability.conditions(action, self) || {:id => nil}
+        conditions = ability.sql_conditions(action, self) || {:id => nil}
         joins = ability.association_joins(action, self)
         if respond_to? :where
           where(conditions).joins(joins)
diff --git a/lib/cancan/can_definition.rb b/lib/cancan/can_definition.rb
index 9b3b945..0124c6a 100644
--- a/lib/cancan/can_definition.rb
+++ b/lib/cancan/can_definition.rb
@@ -1,7 +1,7 @@
 module CanCan
   # This class is used internally and should only be called through Ability.
   class CanDefinition # :nodoc:
-    attr_reader :conditions, :block
+    attr_reader :conditions, :block, :base_behavior, :definitive
     
     def initialize(base_behavior, action, subject, conditions, block)
       @base_behavior = base_behavior
@@ -23,18 +23,19 @@ module CanCan
     
     def can?(action, subject, extra_args)
       result = can_without_base_behavior?(action, subject, extra_args)
+      return :_NOT_MATCHED if result == :_NOT_MATCHED || !result
       @base_behavior ? result : !result
     end
 
     def association_joins(conditions = @conditions)
       joins = []
-      conditions.each do |name, value|
+      (conditions || []).each do |name, value|
         if value.kind_of? Hash
           nested = association_joins(value)
           if nested
             joins << {name => nested}
           else
-            joins << name
+            joins << {name => []}
           end
         end
       end
@@ -54,8 +55,10 @@ module CanCan
     def can_without_base_behavior?(action, subject, extra_args)
       if @block
         call_block(action, subject, extra_args)
-      elsif @conditions && subject.class != Class
-        matches_conditions? subject
+      elsif @conditions.kind_of?(Hash) && subject.class != Class
+        matches_conditions?(subject)
+      elsif [true, false, :_NOT_MATCHES].include? @conditions
+        @conditions
       else
         true
       end
diff --git a/spec/cancan/ability_spec.rb b/spec/cancan/ability_spec.rb
index 169c040..26c2b76 100644
--- a/spec/cancan/ability_spec.rb
+++ b/spec/cancan/ability_spec.rb
@@ -16,13 +16,27 @@ describe CanCan::Ability do
     @ability.can?(:foodfight, String).should be_false
   end
   
-  it "should return what block returns on a can call" do
+  it "should return what block returns on a can call, except for nil and false" do
+    @ability.can :read, :all
+    @ability.can :read, Symbol do |sym|
+      [ sym ]
+    end
+    @ability.can?(:read, Symbol).should == [ nil ]
+    @ability.can?(:read, :some_symbol).should == [ :some_symbol ]
+  end
+  
+  it "should pass to previous can definition, if block returns false or nil" do
     @ability.can :read, :all
     @ability.can :read, Symbol do |sym|
       sym
     end
-    @ability.can?(:read, Symbol).should be_nil
+    @ability.can :read, Integer do |i|
+      i > 0
+    end
+    @ability.can?(:read, Symbol).should be_true
     @ability.can?(:read, :some_symbol).should == :some_symbol
+    @ability.can?(:read, 1).should be_true
+    @ability.can?(:read, -1).should be_true
   end
   
   it "should pass class with object if :all objects are accepted" do
@@ -121,6 +135,30 @@ describe CanCan::Ability do
     @ability.can?(:read, 123).should be_false
   end
   
+  it "should pass to previous can definition, if block returns false or nil" do
+    #same as previous
+    @ability.can :read, :all
+    @ability.cannot :read, Integer do |int|
+      int > 10 ? nil : ( int > 5 )
+    end
+    @ability.can?(:read, "foo").should be_true
+    @ability.can?(:read, 3).should be_true
+    @ability.can?(:read, 8).should be_false
+    @ability.can?(:read, 123).should be_true
+    
+  end
+  
+  it "should pass to previous cannot definition, if block returns false or nil" do
+    @ability.cannot :read, :all
+    @ability.can :read, Integer do |int|
+      int > 10 ? nil : ( int > 5 )
+    end
+    @ability.can?(:read, "foo").should be_false
+    @ability.can?(:read, 3).should be_false
+    @ability.can?(:read, 10).should be_true
+    @ability.can?(:read, 123).should be_false
+  end
+
   it "should append aliased actions" do
     @ability.alias_action :update, :to => :modify
     @ability.alias_action :destroy, :to => :modify
@@ -174,27 +212,48 @@ describe CanCan::Ability do
     @ability.can?(:read, [[4, 5, 6]]).should be_false
   end
   
-  it "should return conditions for a given ability" do
+  it "should return array of behavior and conditions for a given ability" do
     @ability.can :read, Array, :first => 1, :last => 3
-    @ability.conditions(:show, Array).should == {:first => 1, :last => 3}
+    @ability.conditions(:show, Array).should == [[true, {:first => 1, :last => 3}]]
   end
   
-  it "should raise an exception when a block is used on condition" do
+  it "should raise an exception when a block is used on condition, and no" do
     @ability.can :read, Array do |a|
       true
     end
     lambda { @ability.conditions(:show, Array) }.should raise_error(CanCan::Error, "Cannot determine ability conditions from block for :show Array")
   end
   
-  it "should return an empty hash for conditions when there are no conditions" do
+  it "should return an array with just behavior for conditions when there are no conditions" do
     @ability.can :read, Array
-    @ability.conditions(:show, Array).should == {}
+    @ability.conditions(:show, Array).should == [ [true, nil] ]
   end
   
   it "should return false when performed on an action which isn't defined" do
     @ability.conditions(:foo, Array).should == false
   end
   
+  it "should return appropriate sql conditions" do
+    obj = Class.new do
+      def self.sanitize_sql(hash_cond)
+        case hash_cond
+        when Hash then hash_cond.map{|name, value| "#{name}=#{value}"}
+        when Array
+          hash_cond.shift.gsub('?'){"#{hash_cond.shift.inspect}"}
+        when String then hash_cond
+        end
+      end
+    end
+    @ability.can :read, obj
+    @ability.can :manage, obj, :id => 1
+    @ability.can :update, obj, :manager_id => 1
+    @ability.cannot :update, obj, :self_managed => true
+    
+    @ability.sql_conditions(:update, obj).should == 'not (self_managed=true) AND ((manager_id=1) OR (id=1))'
+    @ability.sql_conditions(:manage, obj).should == {:id=>1}
+    @ability.sql_conditions(:read, obj).should == 'true=true'
+  end
+  
   it "should has eated cheezburger" do
     lambda {
       @ability.can? :has, :cheezburger
diff --git a/spec/cancan/can_definition_spec.rb b/spec/cancan/can_definition_spec.rb
index 697dc74..3a4cb24 100644
--- a/spec/cancan/can_definition_spec.rb
+++ b/spec/cancan/can_definition_spec.rb
@@ -17,7 +17,7 @@ describe CanCan::CanDefinition do
 
   it "should return single association for joins" do
     @conditions[:foo] = {:bar => 1}
-    @can.association_joins.should == [:foo]
+    @can.association_joins.should == [{:foo=>[]}]
   end
 
   it "should return multiple associations for joins" do
@@ -28,6 +28,6 @@ describe CanCan::CanDefinition do
 
   it "should return nested associations for joins" do
     @conditions[:foo] = {:bar => {1 => 2}}
-    @can.association_joins.should == [{:foo => [:bar]}]
+    @can.association_joins.should == [{:foo => [{:bar=>[]}]}]
   end
 end