Updated: Preload associations in loaded records

Sometimes you want to fetch associations for an ActiveRecord that you already loaded, e.g. when it has deeply nested associations.

Edge Rider gives your models a static method preload_associations. The ...

Remember why preloading associations "randomly" uses joined tables or multiple queries?

If you don't like the cleverness of thi...

Bad

# Given the following models

class Image < ActiveRecord::Base
  has_many :album_images
  has_many :albums, through: :album_images
end

class Album < ActiveRecord::Base
  has_many :album_images
  has_many :images, through: :album_i...

An association defined with has_many :through will return the same record multiple times if multiple join models for the same record exist (a n:m relation). To prevent this, you need to add ->{ uniq } as second argument to has_many (below Ra...

Note: Instead of using the method in this card, you probably want to use ActiveType's nested attributes which is a much more refined way of doing this.

The attach...

Imagine these models and associations:

class Deck < ApplicationRecord
  has_many :cards
end

class Card < ApplicationRecord
  belongs_to :deck, optional: true
end

Now you want to find all Decks without any Card or all Cards without a...

If this happens to you:

user.avatar = Avatar.new
user.avatar  # => nil

(where avatar is a belongs_to), you probably declared your association incorrectly.

Always do

class User < ActiveRecord::Base
  belongs_to :avatar

...

This is quite an edge case, and appears like a bug in Rails (4.2.6) to me.

Update: This is now documented on Edgeguides Ruby on Rails:

If you set the :va...

Rewritten.

When you eagerly load an association list using the .include option, and at the same time have a .where on an included table, two things happen:

1. Rails tries to load all involved records in a huge single query spanning multiple database tab...