Resolved ticket #212.

 * Made Property hashable by providing #hash and #eql? methods.
 * Made @dirty_attributes of Resource a Set to let users replace
   old dirty attributes with new ones.

lib/data_mapper/property.rb

@@ -230,6 +230,18 @@ def field
       @field ||= @options.fetch(:field, repository.adapter.field_naming_convention.call(name))
     end
 
+    def hash
+      return @model.hash + @name.hash
+    end
+
+    def eql?(o)
+      if o.is_a?(Property)
+        return o.model == @model && o.name == @name
+      else
+        return false
+      end
+    end
+
     def lazy?
       @lazy
     end

lib/data_mapper/resource.rb

@@ -1,3 +1,4 @@
+require 'set'
 require __DIR__ + 'support/string'
 require __DIR__ + 'property_set'
 require __DIR__ + 'property'
@@ -103,7 +104,7 @@ def attribute_loaded?(name)
     end
 
     def dirty_attributes
-      @dirty_attributes ||= []
+      @dirty_attributes ||= Set.new
     end
 
     def dirty?

spec/unit/property_spec.rb

@@ -18,6 +18,16 @@ class Tomato
     end
   end
 
+  it "should evaluate two similar properties as equal" do
+    p1 = DataMapper::Property.new(Zoo, :name, String, { :size => 30 })
+    p2 = DataMapper::Property.new(Zoo, :name, String, { :size => 30 })
+    p3 = DataMapper::Property.new(Zoo, :title, String, { :size => 30 })
+    p1.eql?(p2).should == true
+    p1.hash.should == p2.hash
+    p1.eql?(p3).should == false
+    p1.hash.should_not == p3.hash
+  end
+
   it "should create a String property" do
     property = DataMapper::Property.new(Zoo, :name, String, { :size => 30 })
 

spec/unit/resource_spec.rb

@@ -115,6 +115,17 @@ class Planet
     pluto.attribute_dirty?(:age).should be_true
   end
 
+  it 'should overwite old dirty attributes with new ones' do
+    pluto = Planet.new(:name => 'Pluto', :age => 500_000)
+    pluto.dirty_attributes.size.should == 2
+    pluto.attribute_dirty?(:name).should be_true
+    pluto.attribute_dirty?(:age).should be_true
+    pluto.name = "pluto"
+    pluto.dirty_attributes.size.should == 2
+    pluto.attribute_dirty?(:name).should be_true
+    pluto.attribute_dirty?(:age).should be_true
+  end
+
   it 'should provide a key' do
     Planet.new.should respond_to(:key)
   end