Case 1: If you run only this command to generate scaffold -
rails generate scaffold MODEL_NAME FIELD_NAME:DATATYPE
Ex - rails generate scaffold User name:string address:text
but till now you did not run any command for migration like
rake db:migrate
then you should need to run only this command like -
rails destroy scaffold User name:string address:text
Case 2: If you already run(Scaffold and Migration) by below commands like -
rails generate scaffold User name:string address:text
rake db:migrate
Then you should need to run first rollback migration command then destroy scaffold like below -
rake db:rollback
rails destroy scaffold User name:string address:text
So In this manner, we can undo scaffolding. Also we can use d for destroy and g for generate as a shortcut.