rails 3.2
Invoice.sum(:amount)
Accurately returns the sum of all values in the amount column.
If I do the following:
invoices = Invoice.all
And then:
invoices.sum(:amount)
I get the following error:
NoMethodError: undefined method `+'
Is there a way to do this?
If you are certain that there is not a single amount == nil then what you tried, should work. If you have a single nil column this would result in
NoMethodError: undefined method '+' for nil. In this case you can either do invoices.map(&:amount).compact.sum or invoices.sum { |i| i.amount || 0 }
This is because #sum is an ActiveRecord method. What it is essentially doing is building the aggregation into your database query like so (in SQL):
SELECT SUM(name_of_column_to_sum) FROM table_name;
If you want to return the values and store them in a variable (#all),
SELECT * FROM table_name;
and THEN sum them, you are no longer working with an ActiveRecord query, but rather an ActiveRecord relation (an array-like object that is returned from the ActiveRecord query). So, at that point you would need to use vanilla Ruby to sum up. You could use #map and then #sum, but it would be faster and cleaner to just use #inject, like so:
invoices.inject(0){ |sum, invoice| sum + invoice.amount }
.all is building a whole bunch of activerecord objects in memory just to iterate over one value. It's smarter to use the .sum activerecord method in this case than to use .inject. For more on that check out this postYou have to use map function, like this:
invoices.map(&:amount).sum
invoices.sum(:amount) work. It's because the OP is mixing up activerecord and vanilla ruby. See my answer for details.