gistfile1.rb

$related_tags = {}

qestion_tags = [
     %w(ruby rails ide),
     %w(rails db),
     %w(ruby rails db mysql),
     %w(mysql sql),
     %w(rails mysql),
     %w(ruby rails postgres)
    ]

qestion_tags.each do |tags|
  tags.each do |t|
    $related_tags[t] ||= Hash.new
    related = tags - [t]
    related.each do |r|
      $related_tags[t][r] ||= 0
      $related_tags[t][r] += 1
    end
  end
end

require 'pp'
pp $related_tags
# {"ruby"=>{"rails"=>3, "ide"=>1, "db"=>1, "mysql"=>1, "postgres"=>1},
# "rails"=>{"ruby"=>3, "ide"=>1, "db"=>2, "mysql"=>2, "postgres"=>1},
# "ide"=>{"ruby"=>1, "rails"=>1},
# "db"=>{"rails"=>2, "ruby"=>1, "mysql"=>1},
# "mysql"=>{"ruby"=>1, "rails"=>2, "db"=>1, "sql"=>1},
# "sql"=>{"mysql"=>1},
# "postgres"=>{"ruby"=>1, "rails"=>1}}

def get_related_tags(tag)
  $related_tags[tag]
end

p get_related_tags("rails")
# {"ruby"=>3, "ide"=>1, "db"=>2, "mysql"=>2, "postgres"=>1}
p get_related_tags("rails").sort_by{|tag,count| count}.reverse.map(&:first)
# ["ruby", "db", "mysql", "ide", "postgres"]