ignoring_diff.rb

module IgnoringDiff
  def ignore_difference_for(key)
    self.extend(Module.new do
      define_method :diff do |*arguments|
        returning(super(*arguments)) do |difference|
          difference.delete(key)
        end
      end
    end)
  end
end

usage_example.rb

require 'ignoring_diff'

Hash.send(:include, IgnoringDiff)

h = {:foo => :bar}
h.diff({})
# => {:foo => :bar}
h.ignore_difference_for(:foo)
h.diff({})
# => {}

h.diff({:bar => :baz})
# => {:bar => :baz}

h.ignore_difference_for(:bar)
h.diff({:bar => :baz})
# => {}