With Rails 2/3 it was possible and easy to export (dump) a database to YAML using one of a few plugins (see Best way to export a database table to a YAML file?).
Howeve
The method indicated in the post linked in the question is still valid, but the code is not anymore a plugin: it's the yaml_db gem.
The repository is at https://github.com/yamldb/yaml_db
To dump the db, add the gem to the Gemfile, issue
bundle update
and then issue:
rake db:data:dump
the dump file is:
db/data.yml
If, like me, you hit the error
undefined method `[]=' for #
it can be fixed by following the instructions given in this ticket:
https://github.com/ludicast/yaml_db/pull/45
All you should have to do is add
gem 'yaml_db', github: 'jetthoughts/yaml_db', ref: 'fb4b6bd7e12de3cffa93e0a298a1e5253d7e92ba'
to the Gemfile