# This migration creates the `versions` table, the only schema PT requires.
# All other migrations PT provides are optional.
class CreateVersions < ActiveRecord::Migration[4.2]
	# Class names of MySQL adapters.
	# - `MysqlAdapter` - Used by gems: `mysql`, `activerecord-jdbcmysql-adapter`.
	# - `Mysql2Adapter` - Used by `mysql2` gem.
	MYSQL_ADAPTERS = [
		"ActiveRecord::ConnectionAdapters::MysqlAdapter",
		"ActiveRecord::ConnectionAdapters::Mysql2Adapter"
	].freeze

	# The largest text column available in all supported RDBMS is
	# 1024^3 - 1 bytes, roughly one gibibyte.  We specify a size
	# so that MySQL will use `longtext` instead of `text`.  Otherwise,
	# when serializing very large objects, `text` might not be big enough.
	TEXT_BYTES = 1_073_741_823

	def change
		create_table :versions, versions_table_options do |t|
			t.string   :item_type, null: false
			t.integer  :item_id,   null: false
			t.string   :event,     null: false
			t.string :whodunnit
			t.string :ip
			t.string :user_agent
			t.integer :usuario_id
			t.json  :object
			t.json :object_changes
			# Known issue in MySQL: fractional second precision
			# -------------------------------------------------
			#
			# MySQL timestamp columns do not support fractional seconds unless
			# defined with "fractional seconds precision". MySQL users should manually
			# add fractional seconds precision to this migration, specifically, to
			# the `created_at` column.
			# (https://dev.mysql.com/doc/refman/5.6/en/fractional-seconds.html)
			#
			# MySQL users should also upgrade to rails 4.2, which is the first
			# version of ActiveRecord with support for fractional seconds in MySQL.
			# (https://github.com/rails/rails/pull/14359)
			#
			t.datetime :created_at
		end
		add_index :versions, [:item_type, :item_id]
	end

	private

	# Even modern versions of MySQL still use `latin1` as the default character
	# encoding. Many users are not aware of this, and run into trouble when they
	# try to use PaperTrail in apps that otherwise tend to use UTF-8. Postgres, by
	# comparison, uses UTF-8 except in the unusual case where the OS is configured
	# with a custom locale.
	#
	# - https://dev.mysql.com/doc/refman/5.7/en/charset-applications.html
	# - http://www.postgresql.org/docs/9.4/static/multibyte.html
	#
	# Furthermore, MySQL's original implementation of UTF-8 was flawed, and had
	# to be fixed later by introducing a new charset, `utf8mb4`.
	#
	# - https://mathiasbynens.be/notes/mysql-utf8mb4
	# - https://dev.mysql.com/doc/refman/5.5/en/charset-unicode-utf8mb4.html
	#
	def versions_table_options
		if MYSQL_ADAPTERS.include?(connection.class.name)
			{ options: "ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_general_ci" }
		else
			{}
		end
	end
end