Only validate a dirty resource when saving

* Prevents multiple validations when saving parent and children, since
  previously every child that was saved would tell the parent to
  save too.  Now the parent will be saved, and will not be be dirty
  any longer, so the unnecessary validation will be skipped.

[#1154 state:resolved]

dm-validations/lib/dm-validations.rb

@@ -57,7 +57,7 @@ def update(attributes = {}, context = default_validation_context)
 
     chainable do
       def save_self(*)
-        return false unless validation_context_stack.empty? || valid?(current_validation_context)
+        return false unless !dirty_self? || validation_context_stack.empty? || valid?(current_validation_context)
         super
       end
     end

dm-validations/spec/fixtures/barcode.rb

@@ -2,6 +2,7 @@ module DataMapper
   module Validate
     module Fixtures
       class Barcode
+        attr_accessor :valid_hook_call_count
 
         #
         # Behaviors
@@ -26,6 +27,13 @@ class Barcode
         def self.valid_instance
           new(:code => "3600029145")
         end
+
+        # measure the number of times #valid? is executed
+        before :valid? do
+          @valid_hook_call_count ||= 0
+          @valid_hook_call_count += 1
+        end
+
       end # Barcode
     end
   end

dm-validations/spec/public/resource_spec.rb

@@ -64,4 +64,42 @@
       end
     end
   end
+
+  describe '#save' do
+    before :all do
+      @resource.code = 'a' * 10
+      @resource.save
+    end
+
+    describe 'on a new resource' do
+      it 'should call valid? once' do
+        @resource.valid_hook_call_count.should == 1
+      end
+    end
+
+    describe 'on a saved, non-dirty resource' do
+      before :all do
+        # reload the resource
+        @resource = @resource.model.get(*@resource.key)
+        @resource.save
+      end
+
+      it 'should not call valid?' do
+        @resource.valid_hook_call_count.should be_nil
+      end
+    end
+
+    describe 'on a saved, dirty resource' do
+      before :all do
+        # reload the resource
+        @resource = @resource.model.get(*@resource.key)
+        @resource.code = 'b' * 10
+        @resource.save
+      end
+
+      it 'should call valid? once' do
+        @resource.valid_hook_call_count.should == 1
+      end
+    end
+  end
 end