In Rails 3.2, how to "pluck_in_batches" for a very large table

Question

I have a massive table Foo from which I need to pluck all values in a certain field, Foo.who.

The array has millions of rows, but only a few thousand different values in the who column.

If the table was smaller of course I'd simply use Foo.pluck(:who)

If I use Foo.find_in_batches do |a_batch| each set is an Array of Foo records, rather than an activerecord collection of Foo records, so I cannot use .pluck() and AFAIK the only way to extract the who column is via something like .map(&:who) that iterates over the array.

Is there a way to pluck the who column from Foo in batches that does not require then iterating over each element of each batch to extract the who column?

Answer 1

In Rails 5 you can use:

Foo.in_batches do |relation|
  values = relation.pluck(:id, :name, description)
  ...
end

Upd: for prevent memory leaks use:

Foo.uncached do
  Foo.in_batches do |relation|
    values = relation.pluck(:id, :name, description)
    ...
  end
end