count how many callbacks per model in your Rails app

callback_counter.rb

require 'csv'

# ensure all classes have been laoded
Rails.application.eager_load!

def calc_callback_counts
  models = ApplicationRecord.descendants

  class_callback_counts = {}

  models.each do |model|
    counts = {}
    class_callback_counts[model.class_name] = counts

    ActiveRecord::Callbacks::CALLBACKS.each do |callback_type|
      kind, name = callback_type.to_s.split("_")

      chain = model.__callbacks[name.to_sym]&.send(:chain)
      filtered = chain&.select { |chain_callback| chain_callback.kind == kind.to_sym }
      counts[callback_type] = filtered ? filtered.length : 0
    end

  end

  class_callback_counts
end

class_callback_counts = calc_callback_counts

csv_array = []
csv_header = (['classname'] << ActiveRecord::Callbacks::CALLBACKS).flatten
csv_array << csv_header

class_callback_counts.each do |class_name, callback_counts|
  row  = ([class_name] << callback_counts.values).flatten
  csv_array << row
end

CSV.open('callback_counts.csv', 'wb') do |csv|
  csv_array.each do |row|
    csv << row
  end
end