Code review

I wanted to give you some input on what you have so far. 
 * Provide example config files for other developers (.env, config/database.yml).
 * Having a unique index on city name will be problematic.
 * Keep your factories simple. Don't use `rand(n)`. Drop your test db after making these changes.
 * Avoid using `_id or .id`. Use the active record object instead.
This line takes care of validating presence