Calculate Percentage of the total number of records with scope and display virtual attributes (Rails / Active Record)

Question

Create a scope, perhaps something like this.

scope :mv, select('*,quantity*market_price as market_value, quantity*market_price/sum(quantity*market_price) as percent') 

which creates two virtual attributes, market_value and percent. The problem I am facing is creating a percentage with the included amount (). If I add sum (), the region returns a single record.

I need to calculate the percentage of market value relative to the total market value of a set of registered species.

Example:

 1, market_value: 100, percent: .10 2, market_value: 100, percent: .10 3, market_value: 100, percent: .10 4, market_value: 100, percent: .10 5, market_value: 100, percent: .10 6, market_value: 500, percent: .50 Total is 1000 

however, if I find it in the place where market_value <6, I should see

 1, market_value: 100, percent: .20 2, market_value: 100, percent: .20 3, market_value: 100, percent: .20 4, market_value: 100, percent: .20 5, market_value: 100, percent: .20 Total 500 

How can i do this?

I created a self.pct method, but the problem with the self.pct method is that it should run after all areas. if repocoded, the solution is wrong

Still

 class Position < ActiveRecord::Base attr_accessible :account_id, :account_type, :market_price, :quantity, :report_date, :symbol scope :long_only, where(:account_type => 'L') scope :short_only, where(:account_type=>"S") scope :equity_only, :conditions => ["symbol <> 'USD'"] scope :mv, select('*,quantity*market_price as market_value, quantity*market_price/sum(quantity*market_price) as percent') scope :mv1, lambda{|total| select(total) } #the problem with the self.pct method is that it needs to be run after all the scopes. if rescoped, the solution is wrong def self.pct string="*,(quantity*market_price) as market_value, (market_price*quantity/#{sum_market_value}) as percent" mv1(string) end def market_value self.market_price*self.quantity end def self.sum_market_value sum('quantity*market_price') end end 

Answer 1

I do not know if there is a way to do this in one request, but we can get it in two requests:

 require 'active_record' ActiveRecord::Base.establish_connection adapter: 'sqlite3', database: ':memory:' ActiveRecord::Schema.define do self.verbose = false create_table :positions do |t| t.integer :quantity t.integer :market_price end end class Position < ActiveRecord::Base def self.with_market_value select "*, quantity*market_price as market_value, quantity*market_price/#{total.to_f} as percent" end def self.total select('sum(quantity*market_price) as sum_of_market_values').first.sum_of_market_values end end Position.create! quantity: 25, market_price: 4 Position.create! quantity: 25, market_price: 4 Position.create! quantity: 25, market_price: 4 Position.create! quantity: 25, market_price: 4 Position.create! quantity: 25, market_price: 4 Position.create! quantity: 25, market_price: 20 Position.with_market_value.map { |p| [p.market_value, p.percent] } # => [[100, 0.1], [100, 0.1], [100, 0.1], [100, 0.1], [100, 0.1], [500, 0.5]] Position.where('market_price < 10').with_market_value.map { |p| [p.market_value, p.percent] } # => [[100, 0.2], [100, 0.2], [100, 0.2], [100, 0.2], [100, 0.2]] # ** NOTE THAT IT EXECUTES EAGERLY ** Position.with_market_value.where('market_price < 10').map { |p| [p.market_value, p.percent] } # => [[100, 0.1], [100, 0.1], [100, 0.1], [100, 0.1], [100, 0.1]]