Aggregate records in memory by

created_by_week.rb

puts records.inspect
# #=> [
#   {"id": 1, "created_at": 2014-06-04 06:07:31 -0700},
#   {"id": 2, "created_at": 2014-06-07 09:03:11 -0700},
#   {"id": 3, "created_at": 2014-06-09 07:47:18 -0700}
# ]

records.each_with_object(Hash.new { 0 }) do |record, hash|
  hash[record['created_at'].to_date.beginning_of_week] += 1
end

#=> { Mon, 02 Jun 2014 => 2, Mon, 09 Jun 2014 => 1 }