对于您的向上迁移：
execute "CREATE TABLE cars_users2 AS SELECT DISTINCT * FROM cars_users;"
drop_table :car_users
rename_table :car_users2, :car_users
而下来：
raise ActiveRecord::IrreversibleMigration
完全迁移：
class TheMigration < ActiveRecord::Migration
def up
execute "CREATE TABLE cars_users2 AS SELECT DISTINCT * from cars_users;"
drop_table :car_users
rename_table :car_users2, :car_users
end
def down
raise ActiveRecord::IrreversibleMigration
end
end
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