Use Rails model association!

It is always recommended to use the defined model association instead of querying directly. And there are many reasons, why you should follow that advice. Example.
The models:

class Task < ActiveRecord::Base
  belongs_to :person
end

class Person < ActiveRecord::Base
  has_many :tasks
end

The Controller:

class TasksController < ApplicationController
  @task = Task.find_by_id params[:id]
  @person = Person.find_by_id @task.person_id
end

It generates the queries:

Task Load (6.6ms)  SELECT `tasks`.* FROM `tasks` WHERE `tasks`.`id` = ? LIMIT 1  [["id", 1]]
 => Task id: 1, ...
Person Load (0.8ms)  SELECT `people`.* FROM `people` WHERE `people`.`id` IS NULL LIMIT 1
 => nil

The task @task is not associated with any person. Querying for the tasks person directly is bad practice, because it is a waste and even generates an accidential SQL statement (The condition "WHERE people.id IS NULL" is simply horrible). Refactor it with:

@person = @task.person
 => nil

Rails does not fire a SQL statement, because it checks, if the task object is associated at all.

It saves you a SQL query (if not associated).
It prevents you from duplicating a class name (Person).
It is more readable.

Supported by Ruby 1.9.3 and Ruby on Rails 3.2.1