N+1 query issues

Question

I have 2 models User and Order . I need to generate a chart where it will retrieve monthly balance in array. In my Order table, I have earnings and costs . Here is what I have come up some far:

user.rb

class User < ActiveRecord::Base

def baltime(time) #This will return balance based on time
  orders.where("created_at < ?", time).map(&:earning).compact.inject(:+).to_f -
  orders.live.where("created_at < ?", time).map(&:costs).compact.inject(:+).to_f
end

def group_by_months
  result = []
  forteenmonths = 14.times.map { |i| (Date.today - (i).month)}.reverse
  forteenmonths.each do |d|
    result << self.baltime(d)
  end
  result #This will return an array of the order balances
end

The above method is working, however, it will call 14 queries from the database. Are there any better way I can do this in order to tackle N+1 issues? Thanks in advance

Answer 1

Here is my way doing this with 2 queries but the code is a bit more complex. First I calculate all earnings for 14 months and store them to an array, then subtract costs from these months to get the final balance of each month.

def group_by_months
  earnings = orders.order(:created_at).pluck(:earning, :created_at)
  costs = orders.live.order(:created_at).pluck(:cost, :created_at)

  result = [0]
  i = 0
  date = Date.today - 13.months

  earnings.each do |earning, created_at| 
    if created_at > date
      i += 1
      result[i] = result[i-1]
      date = date + 1.month
    end
    result[i] += earning
  end

  i = 0
  date = Date.today - 13.months

  costs.each do |cost, created_at| 
    if created_at > date
      i += 1
      result[i] = result[i-1]
      date = date + 1.month
    end
    result[i] -= cost
  end

  result
end