Migrate entire MySQL database to utf8mb4 encoding

migrate_to_utf8mb4.rb

class MigrateToUtf8mb4 < ActiveRecord::Migration
  def change
    execute "ALTER DATABASE `#{ActiveRecord::Base.connection.current_database}` CHARACTER SET utf8mb4;"

    ActiveRecord::Base.connection.tables.each do |table|
      execute "ALTER TABLE `#{table}` CHARACTER SET = utf8mb4 COLLATE utf8mb4_unicode_ci;"
      ActiveRecord::Base.connection.columns(table).each do |column|
        if column.sql_type =~ /varchar\((\d+)\)/
          limit = $1.to_i
          limit = 191 if ActiveRecord::Base.connection.indexes(table).any?{|i| i.columns.include? column.name} && limit > 191
          execute "ALTER TABLE `#{table}` CHANGE `#{column.name}` `#{column.name}` VARCHAR(#{limit}) CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;"
        elsif column.sql_type == "text"
          execute "ALTER TABLE `#{table}` CHANGE `#{column.name}` `#{column.name}` TEXT CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;"
        end
      end
    end
  end
end