Sum on multiple columns with Activerecord

Question

I am new to Activerecord. I want to do sum on multiple columns of a model Student. My model student is like following:

 class Student < ActiveRecord::Base
   attr_accessible :class, :roll_num, :total_mark, :marks_obtained, :section
 end

I want something like that:

 total_marks, total_marks_obtained = Student.where(:id=>student_id).sum(:total_mark, :marks_obtained)

But it is giving following error.

NoMethodError: undefined method `except' for :marks_obtained:Symbol

So I am asking whether I have to query the model two times for the above, i.e. one to find total marks and another to find marks obtained.

Answer 1

You can use pluck to directly obtain the sum:

Student.where(id: student_id).pluck('SUM(total_mark)', 'SUM(marks_obtained)')
# SELECT SUM(total_mark), SUM(marks_obtained) FROM students WHERE id = ?

You can add the desired columns or calculated fields to pluck method, and it will return an array with the values.