I solved an interesting problem at work recently, so I wanted to discuss that here.

At Rhapsody, we handle millions of tracks and tracks are categorized under hundreds of genres. Genres are related to other genres by being a parent or a child to another genre. A genre may have up to 1 parent genre and have as many children genres. Some top level genres such as rock, pop, and hip hop do not have any parents. It looks like a tree. The figure below is an simplified example of how a genre tree looks like.

Some genres have tags associated with them. Tags are objects that describes what kind of quality a piece of content has. Drake’s latest album, Views, feels summery, catchy, soulful and reflective. Then Views will have tags like summer, pop, soulful and reflective.

Now there was a problem where we had a way to retrieve genres from tags, but we could not retrieve tags from genres. As a temporary solution to the problem, we decided that we would build and store the mapping of going from genres to tags as a static JSON file.

Genre tags are similar to genres themselves, and genre tags also have a tree data structure. Each tag has a parent tag and can have many children tags.

In order to go through the entire genre tags tree, I decided that I would implement recursive depth first traversal method in ruby to make API calls to retrieve data on all genre tags and its children.

There are different parts to the code to accomplish this, but let’s just look at the meatest part of the code.

def self.get_children(tag, array)
  if tag['childrenTagList'].nil? || tag['childrenTagList'].empty?
    array << { 'id' => tag['id'], 'genreIds' => tag['genreIds'] || [] }
    return array
  end

  array << { 'id' => tag['id'], 'genreIds' => tag['genreIds'] || [] }
  tag['childrenTagList'].each do |t|
    array << get_children(get_tag(CATALOG, t['id']), [])
  end

  array
end

The code accepts tag which is a string and array which contains pairs of tag_id and genre_id. I better check for the terminating condition. The terminating condition is to check to see if the tag has any children tags. If it doesn’t have any children tags, I proceed to populate the array with the tag-genre pair and return the array. Otherwise, I should still populate the array with the tag-genre pair. But in addition to that, I have to recursively call get_children for all children tags. get_children is responsible for requesting metadata of children tags given a tag ID, using Faraday. The response body in JSON format is parsed by Oj.