[{ key: 3 }, { key: 2 }, { key: 1 }].should be_sorted_by(:key, :asc)

custom_matchers.rb

RSpec::Matchers.define :be_sorted_by do |key, order|
  match do |array|
    operator = order == :asc ? :<= : :>=
    array.each_cons(2).all?{ |cons| cons[0][key].send(operator, cons[1][key]) }
  end

  failure_message_for_should do |array|
    "expected #{array} to be sorted in #{key} #{order.to_s}"
  end

  failure_message_for_should_not do |array|
    "expected #{array} to not be sorted in #{key} #{order.to_s}"
  end

  description do
    "expected sorting in #{key}"
  end
end