The ActiveRecord migration DSL includes a method remove_column that can be used to remove an existing column from a table.

It can be used like so, to remove the sign_in_count column from the users table.

def change
  remove_column :users, :sign_in_count
end

Though that will work fine, you'll run into an IrreversibleMigration error if you try to rails db:rollback. It usually a good bet to make migrations reversible when it is easy to do so.

All we need in order to make this migration reversible is to add the column type.

def change
  remove_column :users, :sign_in_count, :integer
end

Now you can rollback (or migrate up-down-up) no problem.

Keep in mind that only the structural changes are reversible. When you remove the column, all of the data goes with it, and that cannot be undone with a simple rollback.