Question

When we call

bundle exec db:migrate

Does it always create a new table? Or just create new columns that are not in the previous table?

And what does rollback do? How does it know which column to delete and is this deleting thing reversible? For example, if I accidentally deleted a useful column, can I cancel the previous action and get it back?

Answer 1

I assume you mean, bundle exec rake db:migrate

1. Active Record tracks which migrations have already been run. Example: If you are using sqlite3 as Database you can look at the table – schema_migrations which has these migration details. It mostly has entries like

    row id          version
       1            "20120926105202" #matched migration timestamp
   

2. This rake task just runs what you have in the migration files under <app>/db/migrate/.
   So, if you have creation of tables in that migration file, it would do that.

3. To Rollback the last migration you can always run rake db:rollback

4. For a better understanding, I request you to read this Guide on Rails Migrations. Reading through it would resolve all your queries w.r.t migrations.