Metaprogramming the metrics

coach.rb

# Usage:
# coach = Coach.first
# coach.sessions_count(:started_at)
# coach.coachees_count(:created_at)
# coach.sessions_sum(:price, :started_at)

class Coach < ActiveRecord::Base
  include Metric

  metrics_for :sessions, :coachees
end

metric.rb

module Metric
  extend ActiveSupport::Concern

  module ClassMethods
    def metrics_for(*tables)
      tables.each do |table|
        define_method("#{table}_count") do |column, options = {}|
          send(:count, table, column, options)
        end

        define_method("#{table}_sum") do |field, column, options = {}|
          send(:sum, table, column, field, options)
        end
      end
    end
  end

  extend ClassMethods

  private

  def count(table, column, options)
    aggregate = 'count(id)'

    find(table, column, aggregate, options)
  end

  def sum(table, field, column, options)
    aggregate = "sum(#{field})"

    find(table, column, aggregate, options)
  end

  def find(table, column, aggregate, options = {})
    period, quantity = format_period(options[:period])

    options = options.merge(id:       id,
                            period:   period,
                            quantity: quantity,
                            where:    "#{self.class.name.downcase}_id")

    query = sql(table, column, aggregate, options)

    ActiveRecord::Base.connection.execute(query)
  end

  def format_period(period)
    if period =~ /\A(\d+\s(week|month|year))\z/
      [$2, $1]
    else
      ['month', '1 month']
    end
  end

  def sql(table, column, aggregate, options)
    %Q{
      WITH results AS
      (
        SELECT
        #{aggregate} AS amount,
        date_trunc('#{options[:period]}', #{column}) AS date
        FROM #{table}
        WHERE #{options[:where]} = #{options[:id]}
        GROUP BY date
      )
      SELECT date, COALESCE(sum(amount), 0) AS amount
      FROM
      (
        SELECT
        generate_series(
          min(date), max(date), '#{options[:quantity]}'
        )
        AS date
        FROM results
      ) x
      LEFT JOIN results
      USING (date)
      GROUP BY date
      ORDER BY date DESC
    }
  end
end

# maybe I can suggest:
def find(table, column, aggregate, options)
  fail ArgumentError, 'First argument (column) must be present' if column.nil?
  where = "#{self.class.name.downcase}_id"
  query = sql(table, column, aggregate, where)
  model(table).find_by_sql([query, {id: id, period: 'month', quantity: 1}.merge(options))])
end

There are two lines that contradict each other. Line 8 states that |column = nil| while line 47 states that 'First argument (column) must be present'. Get the conditional lines out and just remove the = nil part from line 8.

Pues lo voy a sacar a una gema, no?