Dynamically sorting an array of hashes in Ruby

Question

I would like to sort an array of hashes by several dynamic criteria. Let's say I have this array

persons = [
  {
    id: 1,
    first_name: "Bill",
    last_name: "Zamora",
    age: 37
  },
  {
    id: 2,
    first_name: "Alexia",
    last_name: "Reyes",
    age: 70
  },
  {
    id: 3,
    first_name: "Anthony",
    last_name: "Nelson",
    age: 25
  }
]

I know that you can easily sort an array by multiple criteria with the following code

persons.sort_by!{ |p| [p[:age], p[:first_name]] }

However, in this example the number and the order of the fields by which the array is sorted is hard-coded. In my case, this is determined dynamically at runtime. So I do not know by how many fields the array is to be sorted nor what fields are sorted in which order.

I am looking for an elegant solution to sort my array using a configuration object that I don't know before. Such a configuration could look like this:

sort_settings = [
  {
    field: "first_name",
    order: "asc"
  },
  {
    field: "age",
    order: "desc"
  }
]

I am very grateful for any help on this!

Answer 1

It's quite challenging to sort strings in desc order using sort_by, it'd be better to use "lower-level" sort method, which sorts by specified comparator using <=> operator. A quick solution for this looks something like:

persons.sort do |a, b|
  comparator = 0

  sort_settings.each do |s|
    a_field = a[s[:field].to_sym]
    b_field = b[s[:field].to_sym]

    comparator = a_field <=> b_field

    comparator = -comparator if s[:order] == "desc"

    break unless comparator == 0
  end

  comparator
end

The block must implement a comparison between a and b, and return -1, when a follows b, 0 when a and b are equivalent, or +1 if b follows a.

Thus, we iterate over sort_settings and compare specified fields using <=>, which returns 1, 0 or -1. If the specified order is desc, we invert the value. If comparator returns something different to zero, we don't need to continue iterations.