How can I run updates in batches in Rails 3/4?

Question

I need to mass-update many thousands of records, and I would like to process the updates in batches. First, I tried:

Foo.where(bar: \'bar\').find_in_batches.upda         


        

Answer 1

This is 2 years late, but the answers here are a) very slow for large data sets and b) ignore the builtin rails capabilities (http://api.rubyonrails.org/classes/ActiveRecord/Batches.html).

As the offset value increases, depending on your DB server, it will do a sequence scan until it reaches your block, and then fetches the data for processing. As your offset gets into the millions, this will be extremely slow.

use the "find_each" iterator method:

Foo.where(a: b).find_each do |bar|
   bar.x = y
   bar.save
end

This has the added benefit of running the model callbacks with each save. If you don't care for the callbacks, then try:

Foo.where(a: b).find_in_batches do |array_of_foo|
  ids = array_of_foo.collect &:id
  Foo.where(id: ids).update_all(x: y)
end