Provides #calculate_all method on your Active Record models, scopes and relations. It's a little addition to Active Record's #count, #maximum, #minimum, #average and #sum. It allows to fetch all of the above and any other aggregate functions results in one request, with respect to grouping.

Tested only with Postgres and MySQL only right now. It relies on automatic values type-casting of underlying driver.

stats = Order.group(:department_id).group(:payment_method).calculate_all(
  :count,
  :count_distinct_user_id,
  :price_max,
  :price_min,
  :price_avg,
  price_median: 'percentile_cont(0.5) within group (order by price desc)'
)
#
#   (2.2ms)  SELECT department_id, payment_method, percentile_cont(0.5) within group (order by price desc),
#      COUNT(*), COUNT(DISTINCT user_id), MAX(price), MIN(price), AVG(price) FROM "orders" GROUP BY "department_id", "payment_method"
#
# => {
#   [1, "cash"] => {
#     count: 10,
#     count_distinct_user_id: 5,
#     price_max: 500,
#     price_min: 100,
#     price_avg: #<BigDecimal:7ff5932ff3d8,'0.3E3',9(27)>,
#     price_median: #<BigDecimal:7ff5932ff3c2,'0.4E3',9(27)>
#   },
#   [1, "card"] => {
#     ...
#   }
# }

Active Record allows to use most common DB aggregate functions, COUNT(), MAX(), MIN(), AVG(), SUM() really easy. But there's a whole world of wonderful other functions in Postgres which I can't recommend enough if you going to have any work with statistics and BI on your data, though MySQL has something too.

Also, in many cases you'll need several metrics at once, and database often has to perform a full scan on the table for each metric, but it as well can calculate them all in one scan and one request.

#calculate_all to the rescue!

#calculate_all accepts a list of expression aliases and/or expression mapping. It could be either one string of SQL,

  Model.calculate_all('SUM(price) / COUNT(DISTINCT user_id)')

a hash of expressions with arbitrary symbol keys

  Model.calculate_all(total: 'COUNT(*)', average_spendings: 'SUM(price) / COUNT(DISTINCT user_id)')

or a list of one or more symbols without expressions, in which case #calculate_all tries to guess what you wanted from it.

  Model.calculate_all(:count, :average_price, :sum_price)

It's not so smart right now, but here's a cheatsheet:

#calculate_all tries to mimic magic of Active Record's #group, #count and #pluck so result type depends on arguments and on groupings.

If you have no group() on underlying scope, #calculate_all will return just one result.

# same as Order.distinct.count(:user_id), so, probably useless example
# but you can have any expression with aggregate functions there.
Order.calculate_all('COUNT(DISTINCT user_id)')
# => 50

If you have one group, it will return hash of results, with simple keys.

# again, Order.group(:department_id).distinct.count(:user_id) would do the same
Order.group(:department_id).calculate_all(:count_distinct_user_id)
# => {
#   1 => 20,
#   2 => 10,
#   ...
# }

If you have two or more groupings, each result will have an array as a key.

Order.group(:department_id).group(:department_method).calculate_all(:count_distinct_user_id)
# => {
#   [1, "cash"] => 5,
#   [1, "card"] => 15,
#   [2, "cash"] => 1,
#   ...
# }

If you provide just one argument to #calculate_all, its calculated value will be returned as is. Otherwise results would be returned as hash(es) with symbol keys.

so, Order.calculate_all(:count) will return just a single integer, but

Order.group(:department_id).group(:payment_method).calculate_all(:min_price, expr1: 'count(distinct user_id)')
# => {
#   [1, 'cash'] => {min_price: 100, expr1: 5},
#   [1, 'card'] => {min_price: 150, expr2: 15},
#   ...
# }

You can pass block to calculate_all. Rows will be passed to it and returned value will be used instead of row in result hash (or returned as is if there's no grouping)

Order.group(:country_id).calculate_all(:count, :avg_price) { |count:, avg_price:|
  "#{count} orders, #{avg_price.to_i} dollars average"
}
# => {
#   1 => "5 orders, 120 dollars average",
#   2 => "10 orders, 200 dollars average"
# }

Order.group(:country_id).calculate_all(:avg_price) { |avg_price| avg_price.to_i }
# => {
#   1 => 120,
#   2 => 200
# }

Order.calculate_all(:count, :max_price, &OpenStruct.method(:new))
# => #<OpenStruct max_price=500, count=15>

calculate-all should work with groupdate too:

Order.group_by_year(:created_at, last: 5).calculate_all(:price_min, :price_max)
=> {
  Sun, 01 Jan 2012 => {},
  Tue, 01 Jan 2013 => {},
  Wed, 01 Jan 2014 => {},
  Thu, 01 Jan 2015 => {},
  Fri, 01 Jan 2016 => {:price_min=>100, :price_max=>500}
}

Add this line to your application's Gemfile:

gem 'calculate-all'

And then execute:

$ bundle

Or install it yourself as:

$ gem install calculate-all

After checking out the repo, run bin/setup to install dependencies. Then, run rake test to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to rubygems.org.

Bug reports and pull requests are welcome on GitHub at https://github.com/codesnik/calculate-all.

The gem is available as open source under the terms of the MIT License.