use_utf8mb4_characterset.rb

class UseUtf8mb4Characterset < ActiveRecord::Migration
  def up
    # The migration has been done following:
    # - http://mathiasbynens.be/notes/mysql-utf8mb4#utf8-to-utf8mb4
    # - http://dev.mysql.com/doc/refman/5.5/en/charset-unicode-upgrading.html
    database = Rails.configuration.database_configuration[Rails.env]["database"]

    # Stop checking foreign keys
    execute "SET foreign_key_checks = 0"

    # Update the db
    execute "ALTER DATABASE #{database} CHARACTER SET utf8mb4 COLLATE = utf8mb4_unicode_ci"

    ActiveRecord::Base.connection.tables.each do |table|
      ActiveRecord::Base.connection.columns(table).each do |column|
        if [:string, :text].include?(column.type)
          type = (column.type == :text) ? 'TEXT' : 'VARCHAR(191)'
          execute "ALTER TABLE #{table} CHANGE #{column.name} #{column.name} #{type} CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;"
        end
      end

      execute "ALTER TABLE #{table} CHARACTER SET = utf8mb4, COLLATE = utf8mb4_unicode_ci;"

      execute "REPAIR TABLE #{table};"
      execute "OPTIMIZE TABLE #{table};"
    end

    execute "SET foreign_key_checks = 1;"
  end

  def down
    raise ActiveRecord::IrreversibleMigration
  end
end

Note that after that you'll have to reset the NOT NULL in any column that uses them.