Defer class code until a db connection is established

deferred_connection_handling.rb

# Enable class level code to lazily access database information.
# Often ActiveRecord class definitions will rely upon the table schema
# information.  For example the following snippets would acquire a database
# connection:
#   ignore_columns :foo
#   # or
#   attribute_names.each ...
# This results in rails accessing the database when booting, resulting in
# ridiculous behavior like this:
#   bin/rake db:setup
#     ActiveRecord::NoDatabaseError: Unknown database 'development'
#
# ...well yeah, duh. This module extends ActiveRecord to enable the lazy
# definition of class code - i.e. the code will execute the moment a
# connection is establish "naturally" rather than forcing one.  e.g.
#  once_connected { ignore_columns :foo }
module DeferredConnectionHandling
  def self.extended(klass)
    klass.class_eval do
      @@database_has_connected = Concurrent::AtomicBoolean.new
    end
  end

  def retrieve_connection
    res = super
    if @@database_has_connected.false?
      @@database_has_connected.make_true
      ActiveSupport.run_load_hooks(:database_connected)
    end
    res
  end

  def once_connected(&block)
    ActiveSupport.on_load(:database_connected, self) do
      class_eval(&block)
    end
  end
end

ActiveRecord::Base.extend(DeferredConnectionHandling)