Decrementing db value of every row in rails

Question

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.

sufleR · Accepted Answer · 2015-01-19 21:49:31Z

5

You can pass string to update_all like in this example:

User.update_all("credit = credit - 50")

SQL which will be exectued:

UPDATE "users" SET credit = credit - 50

answered Jan 19, 2015 at 21:49

sufleR

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1 Comment

Note that validations and callbacks are not invoked in this case.

Grych · Accepted Answer · 2015-01-19 21:48:47Z

1

You can update User.all, just iterate on all the rows and don't forget to save each record:

User.all.each do |user|
  user.credit = user.credit - 50
  user.save!
end

but the more efficient way would be updating all the rows in the database. You can issue an SQL like:

UPDATE users SET credit = credit - 50;

answered Jan 19, 2015 at 21:48

Grych

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find_each is generally preferably when iterating over all records, since it batches the fetches.

Andrey Deineko · Accepted Answer · 2015-11-30 19:09:32Z

1

You should use either update_all, as suggested by @sufleR, or find_each:

User.find_each{ |user| user.update!(credit: uuser.credit - 50) }

answered Jan 19, 2015 at 21:48

Andrey Deineko

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