aggregate arbitrary column on time interval for charts using Postgres and ActiveRecord/Arel

count_by_interval.rb

module CountByInterval
  def count_by_interval(aggregate_range = 'day', column = "#{self.table_name}.created_at", aggregate_operation = 'count(*)')
    aggregate_on = "date_trunc('#{aggregate_range}', #{column})"
    self.select_values = ["coalesce(#{aggregate_operation}, 0) as value", "#{aggregate_on} as date"]
    ActiveRecord::Base.connection.execute(self.group(aggregate_on).to_sql).collect {|r| r}
  end                     
end                       
                          
ActiveRecord::Relation.send(:include, CountByInterval) #EWWWWwww

usage.rb

User.where(:in_space => true).count_by_interval('month', 'launched_at', 'sum(body_mass)')
#=> [{"value"=>"7637", "date"=>"2012-04-01 00:00:00"}, {"value"=>"963", "date"=>"2012-05-01 00:00:00"}, {"value"=>"662", "date"=>"2012-06-01 00:00:00"}]