If you run into the following error when running a migration
in your legacy Rails app with MySQL 5.7 or above

rake aborted!
"Mysql2::Error: All parts of a PRIMARY KEY must be NOT NULL:"

the fix is by monkey patching the MySQL Adapter using the following code.

# lib/patches/abastract_mysql_adapter.rb
class ActiveRecord::ConnectionAdapters::AbstractMysqlAdapter
    NATIVE_DATABASE_TYPES[:primary_key] = 'int(11) auto_increment PRIMARY KEY'
end

and then require it from your environment.rb

require File.expand_path('../../lib/patches/abstract_mysql_adapter', __FILE__)

This issue is fixed in the latest version of Rails, but your legacy apps would need to be patched manually.

If you are interested in learning more about change that caused this issue, you can refer this link