Reordering an array in the same order as another array was reordered

Question

I have two arrays a, b of the same length:

a = [a_1, a_2, ..., a_n]
b = [b_1, b_2, ..., b_n]

When I sort a using sort_by!, the elements of a will be arranged in different order:

a.sort_by!{|a_i| some_condition(a_i)}

How can I reorder b in the same order/rearrangement as the reordering of a? For example, if a after sort_by! is

[a_3, a_6, a_1, ..., a_i_n]

then I want

[b_3, b_6, b_1, ..., b_i_n]

Edit

I need to do it in place (i.e., retain the object_id of a, b). The two answers given so far is useful in that, given the sorted arrays:

a_sorted
b_sorted

I can do

a.replace(a_sorted)
b.replace(b_sorted)

but if possible, I want to do it directly. If not, I will accept one of the answers already given.