Question

I have two tables: one called Projects, and another called Documents (projects has many documents, documents belong to projects).

The Projects table has a “limit” integer column wherein the user can specify the number of Documents they need.

I want to add up all of the limits and get a single value. So for example, if project A has a limit of 5, project B has a limit of 10, and project C has a limit of 8, I want to be able to display “23”.

What’s the proper Ruby command for this?

Answer 1

ActiveRecord::Base exposes the SQL SUM function as the sum method, so

Project.sum(:limit)

should do the trick. This is aware of scopes and so on, so you could do

Project.where(:active => true).sum(:limit)

to calculate the sum only for rows where active was true or

person.projects.sum(:limit)

to sum the limits of the projects associated with a person (assuming person has_many or habtm projects)

MIN, MAX and AVERAGE are exposed in similar ways.