Rails model results to hash - variable as a key issue

Question

I've tried to put results from my vote model in a hash for further usage, but I don't know how to create a hash key from a variable in Ruby. See example below:

    def create_hash_array(campaign_votes)
        target_hash = Hash.new
        campaign_votes.each_with_index do |cv,i|
          target_hash[cv.content_id] = {} if i == 0

          if target_hash[cv.content_id].member?(cv.vote_button_id)
            target_hash[cv.content_id][cv.vote_button_id] = (target_hash[cv.content_id][cv.vote_button_id]).to_i + 1
          else
            target_hash[cv.content_id] = {cv.vote_button_id => nil}
          end

        end
        target_hash
    end

Usually I got an error:

undefined method `member?' for nil:NilClass

but it comes from unrecognized target_hash[cv.content_id], how can I make does variable to be recognized target_hash[cv.content_id] ??

The error is obvious as `target_hash[cv.content_id]` tries to fetch value from hash with key `cv.content_id` which is not set yet and is nil and you calling `member` on it. Basically, do u want to nest your hash with `cv.vote_button_id` key if it exists right? — Md. Farhan Memon, Apr 24 '17 at 21:34

score 3 · Accepted Answer · answered Apr 24 '17 at 21:37

I think your code can be boiled down to this:

def create_hash_array(campaign_votes)
  target_hash = Hash.new { |h,k| h[k] = Hash.new(0) }

  campaign_votes.each do |cv|
    target_hash[cv.content_id][cv.vote_button_id] += 1
  end

  target_hash
end

There's multiple problems here, many to do with getting all tangled up in the process. You initialize the element of the target_hash structure only on the 0 index position, yet each campaign_vote could have different content_id values, meaning you're missing out on those.

This approach creates a single auto-generating Hash that will populate keys with counter hashes, that is hashes defaulting to 0. That means you can always navigate them and += 1 will work because of the default.

This approach is pretty common in Ruby, especially Hash.new(0), which is super handy for doing simple counters of arbitrary objects.

Rails model results to hash - variable as a key issue

1 Answers1