Active Record: Eager load association with parameter

Question

I have the following models:

class Rating < ActiveRecord::Base
  belongs_to :item
  belongs_to :user
end

class Item < ActiveRecord::Base
  has_many :ratings
end

I want to fetch all items, and the ratings made by a specific user to show the current user's rating (if it exists!) next to each item.

I've tried...

Item.includes(:ratings).where('ratings.user_id = ?', user_id)

...but that don't give me the items with no ratings.

My first thought was a has_many association with an argument, and then pass that argument with the includes method. But that doesn't seem to exist.

How do I get all posts and eager loaded association filtered on a parameter without doing N+1 queries or loading all entities into memory?

Answer 1

Step 1

Make the current_user accessible in the model layer (one technique is outlined here: https://stackoverflow.com/a/2513456/163203)

Step 2

Add an association with a condition that gets evaluated during run time.

Rails 2

class Item < ActiveRecord::Base
  has_many :ratings
  has_many :current_user_ratings, 
           :class_name => "Rating", 
           :conditions => 'ratings.user_id = #{User.current_user.try(:id)}'
end

Rails 3.1 and above

class Item < ActiveRecord::Base
  has_many :ratings
  has_many :current_user_ratings, 
           :class_name => "Rating", 
           :conditions => proc { ["ratings.user_id = ?", User.current_user.try(:id)] }
end

Step 3

Item.includes(:current_user_ratings)

Answer 2

As I recently wrote in this blog post, I'd suggest the following in your case:

items = Item.all
ActiveRecord::Associations::Preloader.new.preload(items, :ratings, Rating.where(user_id: user_id))

you can use the custom scopes of the preloader and access items.each { |i| i.ratings } already scoped by the user.

Answer 3

It looks like you're basically modeling a has_many :through relationship: Item has_and_belongs_to_many User, and Rating is the join model. You can read about :through relationships in the Rails Guide to Active Record Associations.

If that is the case, I would recommend structuring your model relationships using has_many :through as follows:

class Rating < ActiveRecord::Base
  attr_accessible :item_id, :user_id
  belongs_to :item
  belongs_to :user
end

class User < ActiveRecord::Base
  has_many :ratings
  has_many :rated_items, :through => :ratings
end

class Item < ActiveRecord::Base
  has_many :ratings
  has_many :rated_by_users, :through => :ratings, :source => :user
end

Then, say you have the following records in the DB:

$ sqlite3 db/development.sqlite3 'SELECT * FROM items';
1|2013-03-22 03:21:31.264545|2013-03-22 03:21:31.264545
2|2013-03-22 03:24:01.703418|2013-03-22 03:24:01.703418

$ sqlite3 db/development.sqlite3 'SELECT * FROM users';
1|2013-03-22 03:21:28.029502|2013-03-22 03:21:28.029502

$ sqlite3 db/development.sqlite3 'SELECT * FROM ratings';
1|1|1|2013-03-22 03:22:01.730235|2013-03-22 03:22:01.730235

You could request all Items, along with their associated Rating and User instances, with this statement:

items = Item.includes(:rated_by_users)

This executes 3 SQL queries for you:

Item Load (0.1ms)  SELECT "items".* FROM "items" 
Rating Load (0.2ms)  SELECT "ratings".* FROM "ratings" WHERE "ratings"."item_id" IN (1, 2)
User Load (0.2ms)  SELECT "users".* FROM "users" WHERE "users"."id" IN (1)

And trying to access the User(s) that rated each Item can be done by calling the #rated_by_users association method on each Item:

> items.map {|item| item.rated_by_users }
=> [[#<User id: 1, created_at: "2013-03-22 03:21:28", updated_at: "2013-03-22 03:21:28">], []]