Preload your ActiveRecord objects!

Eager loading associated ActiveRecord::Base model objects is always a good choice. A lot Ruby on Rails developer take advantage of the ActiveRecord::QueryMethods#includes and do not care anymore. A better approach is to care.
For example the original associations:

class Category < ActiveRecord::Base
  has_many :foods
end

class Food < ActiveRecord::Base
  belongs_to :category
end

and eager loading (aka preloading) the categories and their associated foods:

@categories = Category.includes(:foods).all

generates a SQL like:

SELECT "categories".* FROM "categories"
SELECT "foods".* FROM "foods" WHERE "foods"."category_id" IN (1, 2, 3)

Please note that ActiveRecord::QueryMethods#includes generates 2 SQL queries. The database tables categories and foods are not joined together.

And that is exactly the same what ActiveRecord::QueryMethods#preload does. That is why:

@categories = Category.preload(:foods).all

also generates the SQL:

SELECT "categories".* FROM "categories"
SELECT "foods".* FROM "foods" WHERE "foods"."category_id" IN (1, 2, 3)

But beware, the selection conditions are not allowed to refer to the preloaded association table attributes.
So something like:

@categories = Category.preload(:foods).where("foods.name LIKE '%corn%'").all

will raise an SQL exception:

SQLException: no such column: foods.name: SELECT "categories".* FROM "categories"  WHERE (foods.name LIKE '%corn%')

But as long as the query is simple and is intended to keep on being simple, it totally makes sense to prefer ActiveRecord::QueryMethods#preload to ActiveRecord::QueryMethods#includes.
Furthermore it is more intentional about what the code is doing. And that is always a win.
Further articles of interest:

• Load your ActiveRecord objects eagerly!

Supported by Ruby 2.1.1 and Ruby on Rails 3.2.17