I want a method to reduce every value in a column (credit) by a specified amount. I have tried:

```
User.all.map! {|user| user.credit -= 50} 

```

which just maps the credit, rather than the user. I feel like the correct answer is to use

```
User.update_all(something)

```

but I don't know how to update a value relative to its previous value using this method.

This seems like a common requirement so I'm guessing there's a standard approach.