Memoized reflections accessor

Its value never change since associations are defined at class load time
so there is no need to build the hash everytime the method is called.

Before this change:

    Calculating -------------------------------------
             reflections   804.000  i/100ms
    -------------------------------------------------
             reflections      8.213k (±26.2%) i/s -     36.180k

After this change:

    Calculating -------------------------------------
             reflections    24.548k i/100ms
    -------------------------------------------------
             reflections      1.591M (±25.7%) i/s -      7.364M

Benchmark script:

    require 'bundler/setup'

    require 'active_record'
    require 'benchmark/ips'

    ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ':memory:')
    ActiveRecord::Migration.verbose = false

    ActiveRecord::Schema.define do
      100.times do |i|
        create_table "users#{i}", force: true
      end

      create_table :cars, force: true do |t|
        100.times do |i|
          t.references "users#{i}"
        end
      end
    end

    class Car < ActiveRecord::Base
      100.times do |i|
        belongs_to "users#{i}".to_sym
      end
    end

    Benchmark.ips do |x|
      x.report('reflections') { Car.reflections }
    end

0 files changed, 0 insertions, 0 deletions