Rails has now added a benchmark generator which is pretty helpful.
rails g benchmark some_benchmark
it creates the file at
script/benchmarks/some_benchmark.rb
It should automatically add the ips-benchmark gem to your gemfile after running bundle but if it doesn't just add it under the development section in your gemfile.
run it in the terminal by doing
ruby script/benchmarks/some_benchmark.rb
since it loads config/environment.rb in the benchmark file all models are accessible.