CODEWITHSUNDEEP
ruby-on-railsruby
JamesJY
JamesJY

Reputation: 135

Rails - How to add values in seperate objects together

You have an array of objects which contain a key-value reference. 

[{booking_ref: 'w578383', foo: 'bar', price1: 500, price2: 30],
{booking_ref: 'w578383', foo: 'bar', price1: 600, price2: 40},
{booking_ref: 'r123523', foo: 'bar', price1: 699, price2: 4}]

I want to:

I am thinking:

objects.group_by(&:booking_ref).each {|group|
  group.merge {|key, value1, value2| value1 + value2 if key == price1 || price2}
}

Does that work and if so how do I then return them back out of the group_by state?

Upvotes: 0

Views: 106

Answers (4)

shiladitya
shiladitya

Reputation: 2310

First, group_by is fine, then use a map to iterate the hash and then reduce the value from a list to a sum using inject

objects.group_by(&:booking_ref)
       .map{|ref, list| {booking_ref: ref, 
                        price1:  => list.inject(0){|sum,h| sum + h.price1},
                        price2: => list.inject(0){|sum,h| sum + h.price2}
                        }
           }

Upvotes: 0

Cary Swoveland
Cary Swoveland

Reputation: 110725

Whenever you can use Enumerable#group_by you can use some form of Hash#merge or Hash#update (aka merge!), and vice-versa. Others have used group_by, so here's a hash-merge answer.

Letting the variable bookings equal your array of hashes, you can write the following.

keys_to_aggregate = [:price1, :price2]

bookings.each_with_object({}) { |g,h| h.update(g[:booking_ref]=>g) { |_,o,n|
  keys_to_aggregate.reduce(o) { |f,k| f.merge(k=>o[k] + n[k]) } } }.values
  #=> [{:booking_ref=>"w578383", :foo=>"bar", :price1=>1100, :price2=>70},
  #    {:booking_ref=>"r123523", :foo=>"bar", :price1=>699, :price2=>4}]

Note that before Hash#values at the end of the expression is evaluated we have the following.

bookings.each_with_object({}) { |g,h| h.update(g[:booking_ref]=>g) { |_,o,n|
  keys_to_aggregate.reduce(o) { |f,k| f.merge(k=>o[k] + n[k]) } } }
  #=> {"w578383"=>{:booking_ref=>"w578383", :foo=>"bar", :price1=>1100, :price2=>70},
  #    "r123523"=>{:booking_ref=>"r123523", :foo=>"bar", :price1=>699, :price2=>4}}

This uses the form of Hash#update that employs a block to determine the values of keys that are present in both hashes being merged. See the doc for details, particularly the definitions of the value-determining block's three variables (k, o and n). (I've substituted _ for k [the key] to signify that it is not used in the block calculation.)

Upvotes: 1

Eric Duminil
Eric Duminil

Reputation: 54253

With hash objects, you could calculate the sums and merge them back to the first hash in each group :

bookings = [
{booking_ref: 'w578383', foo: 'bar', price1: 500, price2: 30},
{booking_ref: 'w578383', foo: 'bar', price1: 600, price2: 40},
{booking_ref: 'r123523', foo: 'bar', price1: 699, price2: 4}
]

grouped_bookings = bookings.group_by{ |h| h[:booking_ref] }.map do |ref, hs|
  sums = hs.each_with_object(Hash.new(0)) do |h, sum|
    %i(price1 price2).each do |price|
      sum[price] += h[price].to_i
    end
  end
  hs.first.merge(sums)
end

p grouped_bookings
# [{:booking_ref=>"w578383", :foo=>"bar", :price1=>1100, :price2=>70},
# {:booking_ref=>"r123523", :foo=>"bar", :price1=>699, :price2=>4}]

Upvotes: 1

gwcodes
gwcodes

Reputation: 5690

Taking an object-oriented approach here, you can get this looking quite neat and elegant. The key is to define the + method on the object. Full example below:

class Booking
  attr_accessor :booking_ref, :foo, :price1, :price2

  def initialize(params={})
    params.each { |key, value| send "#{key}=", value }
  end

  # add up prices, and return new Booking object
  def +(other)
    new_obj = self.dup
    new_obj.price1 += other.price1
    new_obj.price2 += other.price2
    new_obj
  end
end

# set up example bookings
bookings = [
  Booking.new(booking_ref: 'w578383', foo: 'bar', price1: 500, price2: 30),
  Booking.new(booking_ref: 'w578383', foo: 'bar', price1: 600, price2: 40),
  Booking.new(booking_ref: 'r123523', foo: 'bar', price1: 699, price2: 4)
]


# grouping becomes very simple - potentially a one-liner
bookings.group_by(&:booking_ref).map { |_, values| values.reduce(&:+) }
# => [
        #<Booking:... @booking_ref="w578383", @foo="bar", @price1=1100, @price2=70>,
        #<Booking:... @booking_ref="r123523", @foo="bar", @price1=699, @price2=4>
     ]

Upvotes: 0

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