try fixing mysql utf8mb4 issues

config/initializers/mysql_modern_column_type_defaults.rb

@@ -0,0 +1,17 @@
+require 'active_record/connection_adapters/abstract_mysql_adapter'
+
+ActiveRecord::ConnectionAdapters::AbstractMysqlAdapter::NATIVE_DATABASE_TYPES.tap do |defaults|
+  # Decrease default string length from 255 -> 191 chars so *_type columns
+  # can be indexed with utf8mb4 without blowing 767 byte max key length.
+  defaults[:string][:limit] = 191
+
+  # Use microsecond precision for all timestamps. High-precision times are
+  # first available in MySQL 5.6, but default to second precision for
+  # backward compatibility.
+  defaults[:datetime][:limit] = 6
+  defaults[:time][:limit] = 6
+end
+
+
+# thanks @jeremy
+# https://github.com/rails/rails/pull/23009#issuecomment-171406595