exactly what does rake db:migrate do?

Question

Does rake db:migrate only add new migrations, or does it drop all migrations/changes and build everything new?

I think rake is throwing an error because it is trying to

Answer 1

Everytime you create a migration using scripts (like script/generate model ...) a new migration is added to the correct directory ready to be synched with the real database.

Actually rake db:migrate just checks which missing migrations still need to be applied to the database without caring about the previouse ones.

Of course if you modify the database using other ways is common to obtain out-of-synch things because as you said you can find yourself applying a migration to something that is changed underneath.