Solving ActiveRecord::ReadOnlyRecord in Rails 3

When you pass an SQL fragment to a finder, join or named scope, ActiveRecord returns read-only results by default.

Use readonly(false) in your queries to force that results are writable. Ex:

(Rails 3)
User.joins("INNER JOIN `cars` ON `cars`.`user_id` = `users`.`id` AND `cars`.`colour`  = 'electric blue'").readonly(false)