pdeffendol/spatial_adapter#35 (doesn't work with mysql instead of mysql2, left comment there)
yields:
uninitialized constant ActiveRecord::ConnectionAdapters::MysqlColumn
on line 52 of /vendor/plugins/spatial_adapter/lib/spatial_adapter/mysql.rb
now trying the (default) gem instead.
More info here: https://github.com/fragility/spatial_adapter