Question

I have two tables: User, table1 and table2. User has_many table1. User has_many table2. table2 has 3 columns: column1, column2, column3. What I want to do is to multiply the value in column1 and column2 by a number and save it to column3.

In my table2.rb class file, I have defined this method:

def calculate(tablerow)
  tablerow.column3 = (tablerow.column1 * 1) + (tablerow.column2 * 2) 
end     

I have a method in table1Controller, which has a method do something. This method then calls the table2#calculate method above and passes an appropriate parameter.

When I call the calculate method, I get the following error message:

NoMethodError in table1Controller#dosomething
undefined method `*' for nil:NilClass

Now I know there is something not right about the calculate method. But I am not sure how to achieve what I am trying to achieve, which is just to do a calculation on the value of 2 columns and save the sum to a third column. My app uses SQLite3.

Answer 1

undefined method * for nil:NilClass means that one of the values you are trying to multiply is empty. In your example tablerow.column3 is probably nil. I think you either have a typo in your code or gave the example code wrong because the code does not match what you say you are trying to achieve.

Give this a shot, it should also eliminate the need to call this action from your controller and just save the value of column3 each time you save or create a record in table2

class Table2 < ActiveRecord::Base
  before_save :calculate_column3

  def calculate_column3
   self.column3 = (self.column1 * 1) + (self.column2 * 2) 
  end
end