Memoize serializer classes

[amcaplan]

These should be memoizable, and it avoids (in most cases, though an explicit jsonapi_serializer_class_name can't benefit here since we can't know it'll be reliably the same) the need for an expensive #constantize operation.

Monkeypatching in this change in a production application I'm working with shaved ~5% off total request time when dealing with large numbers of included models. I also threw together a quick Rails app to give an accurate-ish benchmark (I believe #constantize performs worse when there are more constants/namespaces to choose from, but don't quote me on it): https://github.com/amcaplan/jsonapi-serializer-benchmark/blob/master/lib/tasks/benchmark.rake

Using benchmark-ips, on the master branch, I saw 5.92/sec with a high of 5.949/sec over 3 runs, as opposed to 6.31/sec with a low of 6.276/sec over 3 runs on my branch, meaning it's a 6.5% performance increase (in that particular case) and the variance of the two sets of figures didn't overlap (so it's pretty clearly a win).

[djones]

Thanks for this PR @amcaplan. I did a quick benchmark to understand what impact this PR has more than 2 years on.

This is what I ran:

require 'benchmark'
require 'factory_girl'

require './lib/jsonapi-serializers'
require './spec/support/serializers'
require './spec/support/helpers'
require './spec/support/factory.rb'

include FactoryGirl::Syntax::Methods

user = create(:user)
long_comments = create_list(:long_comment, 1000, user: user)
post = create(:post, :with_author, long_comments: long_comments)

# Make sure each long-comment has a circular reference back to the post.
long_comments.each { |c| c.post = post }

Benchmark.bm do |benchmark|
  benchmark.report("serialize") {
    100000.times do
      JSONAPI::Serializer.serialize(post)
    end
  }

  benchmark.report("serialize with author and long-comments includes") {
    100.times do
      JSONAPI::Serializer.serialize(
        post,
        {include: ['author', 'long-comments']}
      )
    end
  }

  benchmark.report("serialize with recursive loading of relationships") {
    100.times do
      JSONAPI::Serializer.serialize(
        post,
        {include: ['long-comments.post.author', 'author', 'long-comments']}
      )
    end
  }
end

This PR definitely improves performance.

(source)

It clearly has a positive impact of at least 5%. With a gain of closer to 9% when simply running serializer 100,000 times. These measurements aren't too far off yours... although I would guess I'm using a newer version of Ruby than you did at the time.

[djones]

🍍 LGTM

[amcaplan]

It's reassuring to know my work has withstood the test of time! 😂

In all seriousness, thanks for accelerating this. I'm delighted that the performance work I did 2 years ago can start making other people's apps faster.