change_to_utf8mb_encoding.rb

# rails generate migration change_to_utf8mb_encoding

class ChangeToUtf8mbEncoding < ActiveRecord::Migration
  # def execute(text)
  #   puts text
  # end

  def db
    ActiveRecord::Base.connection
  end

  def up
    execute "ALTER DATABASE `#{db.current_database}` DEFAULT CHARACTER SET utf8mb4 DEFAULT COLLATE utf8mb4_unicode_ci;"
    db.tables.each do |table|
      execute "ALTER TABLE `#{table}` CHARACTER SET = utf8mb4;"
      db.columns(table).each do |column|
        case column.sql_type
        when "text"
          execute "ALTER TABLE `#{table}` CHANGE `#{column.name}` `#{column.name}` TEXT CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;"
        when /varchar\((\d+)\)/i
          # InnoDB has a maximum index length of 767 bytes, so for utf8 or utf8mb4
          # columns, you can index a maximum of 255 or 191 characters, respectively.
          # If you currently have utf8 columns with indexes longer than 191 characters,
          # you will need to index a smaller number of characters.
          length = $1.to_i
          length = 191 if length > 191 && db.indexes(table).any? { |index| index.columns.include?(column.name) }
          execute "ALTER TABLE `#{table}` CHANGE `#{column.name}` `#{column.name}` VARCHAR(#{length}) CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;"
        end
      end
    end
  end
end