lazy_load_hooks allows Rails to lazily load a lot of components and thus making the app boot faster. Because of this feature now there is no need to require ActiveRecord::Base at boot time purely to apply configuration. Instead a hook is registered that applies configuration once ActiveRecord::Base is loaded. Here ActiveRecord::Base is used as example but this feature can be applied elsewhere too.

Here is an example where on_load method is called to register a hook.

initializer 'active_record.initialize_timezone' do
  ActiveSupport.on_load(:active_record) do
    self.time_zone_aware_attributes = true
    self.default_timezone = :utc
  end
end

When the entirety of ActiveRecord::Base has been evaluated then run_load_hooks is invoked. The very last line of ActiveRecord::Base is:

ActiveSupport.run_load_hooks(:active_record, ActiveRecord::Base)
Methods
O
R
Instance Public methods
on_load(name, options = {}, &block)

Declares a block that will be executed when a Rails component is fully loaded.

Options:

  • :yield - Yields the object that #run_load_hooks to block.

  • :run_once - Given block will run only once.

Source: show | on GitHub 

# File activesupport/lib/active_support/lazy_load_hooks.rb, line 39
def on_load(name, options = {}, &block)
  @loaded[name].each do |base|
    execute_hook(name, base, options, block)
  end

  @load_hooks[name] << [block, options]
end
run_load_hooks(name, base = Object)

Source: show | on GitHub 

# File activesupport/lib/active_support/lazy_load_hooks.rb, line 47
def run_load_hooks(name, base = Object)
  @loaded[name] << base
  @load_hooks[name].each do |hook, options|
    execute_hook(name, base, options, hook)
  end
end