#ruby-202001101424.rb

require 'csv'
require 'date'
require 'json'
require 'ostruct'

json = <<~JSON
  [{
    "key" : "foo",
    "doc_count" : 3916075,
    "interval" : {
      "buckets" : [
        { "month" : "2019-02-01", "doc_count" : 186904 },
        { "month" : "2019-03-01", "doc_count" : 196961 }
      ]
    }
  }, {
    "key" : "bar",
    "doc_count" : 168089,
    "interval" : {
      "buckets" : [
        { "month" : "2019-01-01", "doc_count" : 8876 },
        { "month" : "2019-04-01", "doc_count" : 8694 }
      ]
    }
  }]
JSON

topics = JSON.parse(json, object_class: OpenStruct)

months = topics.flat_map { |item| item.interval.buckets }.map(&:month)
months.uniq!
months.sort_by! { |month| Date.iso8601(month) }

csv = CSV.generate do |csv|
  csv << ['topic', *months]
  topics.each do |topic|
    buckets = topic.interval.buckets
    doc_count = buckets.map(&:month).zip(buckets.map(&:doc_count)).to_h
    doc_count.default = 0
    
    csv << [topic.key, *months.map(&doc_count)]
    # newest feature used Hash#to_proc ^ introduced in 2.3.0
  end
end

puts csv
# prints:
#
#     topic,2019-01-01,2019-02-01,2019-03-01,2019-04-01
#     foo,0,186904,196961,0
#     bar,8876,0,0,8694
#