unnecessary check with datamapper.rb

class Foo
  include DataMapper::Resource
  
  property :id, Serial
	property :foo, Integer, :unique => true
	property :desc, String
end

Foo.auto_migrate!

 Foo.create(:desc => "first", :foo => 1)
 f = Foo.first 
 # SELECT "id", "foo", "desc" FROM "foos" ORDER BY "id" LIMIT 1
 f.desc = "second"
 f.save
 # SELECT "id" FROM "foos" WHERE "foo" = 1 ORDER BY "id" LIMIT 1 <--- unnecessary check 
 # UPDATE "foos" SET "desc" = 'lol' WHERE "id" = 1